Field studies in the Sacramento Valley compared the performance of a small six-rotor UAV drone sprayer versus a traditional manned airplane for applying insecticides for summer worm control in alfalfa hay fields, 2020 (photo by Ian Grettenberger, UC Davis.)
Drones are a viable option for aerial application of pesticides in alfalfa. UCCE Farm Advisor Rachel Long in Yolo County, along with Dr. Ken Giles, Dr. Xuan Li and Bill Reynolds from UC Davis, reported on field trials where drones were compared to fixed wing aircraft in pesticide applications for summer worm control in alfalfa.
The research team conducted two trials in Yolo County using the insecticide Prevathon. Fields were divided with one part sprayed by airplane and the other by a drone. Application rates were five gallons per acre for one field and ten gallons per acre for the second field.
Spray cards to assess coverage were set out in the alfalfa canopy prior to the pesticide application and plant samples were taken after the application to determine residue concentrations. Summer worm counts were also taken with a sweep net to compare efficacy of each spray application method.
In the UC ANR Alfalfa and Forage News, Long described the results of the trial. The spray cards showed that both the drone and airplane application had equivalent spray coverage. The drone application had more variability in deposition uniformity. This is due to refinement of airplane application technology over a 60-year period, Long noted, while drone applications need to be fine tuned for optimum pest control.
There were few differences in the residue concentrations of Prevathon between the airplane and drone for both spray rates.
Drones can be another tool for growers to use for aerial application of pesticides, Long wrote. California now has a specific UAV (unmanned) ag pilot license category where the pilot of the drone is not required to have a commercial pilot certificate, only the UAV certificate. Obtaining a California ag pilot unmanned license is now an established process.
Current limitation in the use of drones for aerial application on crops is a 55-pound weight limit on drone carrying capacity. Some drone companies have obtained certificates for handling more than 55 pounds in California, but Giles notes that only Yamaha has current certification for an over 55-pound spray application.
After three to four seasons of experience with the Yamaha carrying 220 pounds, Giles reports good deposition with insecticide in orchards for hull split sprays.
Drone technology is being used in China and other areas in Southeast Asia, but is not yet common in the US. Giles said use of UAV drones for pesticide applications is increasing.
Cal Poly, San Luis Obispo field experiments were carried out to determine if soil solarization can reduce weed and pathogen pressure and improve plant health and strawberry yields. Field trials also looked at the effect of sudangrass cover crops and if they enhanced effectiveness of soil solarization (photo courtesy Cal Poly, SLO.)
Solarization trials showed that 4 to 8 weeks of temperatures above 122 degrees F are necessary to kill the toughest weed seeds in soil.
Soil solarization uses clear, polyethelene material to cover irrigated ground and achieve high temperatures to kill soil pests including weed seeds. Research trials at the Cal Poly, San Luis Obispo organic farm were done to determine temperatures and time it took to kill weed seeds in the soil.
The field experiments were carried out to determine if soil solarization can reduce weed and pathogen pressure and improve plant health and strawberry yields in San Luis Obispo County. Field trial also looked at the effect of sudan grass cover crops and if they enhanced effectiveness of soil solarization.
Effectiveness of solarization is based on ambient soil and air temperatures and the intensity of solar radiation.
The time the tarp is left on the soil depends on the soil temperatures generated. In most cases, Cal Poly researchers report, solarization should raise the ambient soil temperatures between 10 and 20 degrees in the top six inches of the soil. Solarization is most effective when used during the summer months when solar radiation is high in sunny, warm climates.
Researchers also report that the best plastics to use are clear, one to three millimeters thick and UV-inhibited to prevent breakdown in sunlight. Cost for the material ranges from $150 to $300 per acre.
Summary of the solarization trial includes finding that solarization provided efffective weed control for three and a half months after tarp removal. Verticillium wilt populations were reduced 80.7% compared to non-solarized test plots. Solarized test plots experienced almost no disease pressure until late May when temperatures warmed. Solarized plants did experience disease pressure from charcoal rot, a warm season pathogen. Total plant mortality was higher in non-solarized plots with 35% compared to 16% in solarized plots. Solarized plots had higher strawberry yields.
Effectiveness of solarization depends not only on the temperatures achieved with tarping, but also on the weed seed and disease species present in the soil. In areas with cooler climates or frequent foggy or cloudy days during the summer, knowing temperature thresholds required to kill the pests in the field can be important in determining if solarization is a viable option.
Rusty red spores cover a Ganoderma conk on an almond tree trunk. Other signs on trees are white rot and mycelium at infection sites. The fungus causes heartwood and sapwood to decay, reducing structural stability (photo by Bob Johnson, UC Davis.)
UC plant pathologists are currently conducting spore surveys to assess the spread of Ganoderma root rot in Central Valley almond orchards.
This disease, Ganoderma adspersum, is much more prevalent in the southern Central Valley where it was likely first introduced, said Daisy Hernandez, a researcher in the UC Davis plant pathology department. The fungus has recently been found further north in the valley, a sign that it is spreading.
This wood-rotting and airborne fungi causes trees to rot from the inside out, leaving weakened trunks at the soil line and causing infected trees to fall. There is no known control once trees are infected. Fourth leaf trees can be infected. Average age of infected trees is 14.
Conks, the fungal fruiting bodies, grow at the soil line on exposed roots and are a sign of a Ganoderma infection. Live conks are a brilliant white color on the underside and release rust-colored spores which often accumulate on the upper surface of the conk making it appear red.
Other signs on trees are white rot and mycelium at infection sites. The fungus causes heartwood and sapwood to decay, reducing structural stability.
Trees with Ganoderma infections show signs of general decline. There may also be a flat strip on the tree trunk or clefting at the graft union. Many infected orchards are first-generation and have a high incidence of conks. These produce “astronomical” numbers of spores, 12 to 40 million a day.
Hernandez said spread of Ganoderma infections in orchards is attributed to airborne spores from mature conks. A common factor among infected almond orchards is the sandy loam soil that allows for proper drainage. Hernandez said it might be possible that this soil type contributes to the fungus’ ability to spread via spores that percolate up to 60 centimeters into the soil. When the trees are irrigated, the spores infect roots that have become damaged during shaking for harvest and sanitation. Sweeping also promotes spore dispersal throughout the orchard. There is no evidence the infection spreads from root to root. Each infection is spore-related.
Conks on trees may be removed and destroyed to delay infections, but care must be taken to not cause further spread of spores in the orchard. Delay is the keyword here, Hernandez noted, as conks can regrow at the same site in five to ten weeks.
All components of the smart sprayer retrofit kit are attached to the sprayer body without changing the sprayer structure, with the exception of connecting a variable-flowrate solenoid valve to each nozzle.
Fruit, nut, ornamental nursery, horticultural and greenhouse industries are among the fastest-growing enterprises in US agriculture. Application of pesticides and other production strategies have ensured their high-quality products meet stringent market requirements. However, low-efficiency, decades-old spray technologies are commonly used to treat these specialty crops and have caused an enormous amount of pesticide waste, additional costs in crop production and concerns around worker safety. The pesticide waste has also caused environmental contamination and ecosystem damage because pesticide sprays indiscriminately kill both pests and beneficial insects. Spray drift and off-target loss will likely remain a major problem as long as pesticides are applied using indiscriminate spray equipment.
Need for Efficient Technologies
Pesticide application is the most complicated operation in crop production because there are many variables affecting spray strategies and practices. In many cases, when decisions must be made to apply chemicals within a very narrow time window in response to escalating pest pressure, a simple “best guess” practice is often under vague labeling of pesticide rates to control pests that may result in excessive application of pesticides.
Given constrained environments for specialty crop production, an ideal spray management program for pests and diseases should include improved delivery systems that are flexible for spraying the amount of chemicals to match tree structures instead of acreage base. Such spray application will also produce minimum spray drift and off-target loss of pesticide on the ground and in the air.
To achieve this goal, a new automated universal intelligent spray control system was developed as a retrofit kit to attach on existing sprayers. With the intelligent control system, the conventional spraying systems can determine the presence, size, shape, and foliage density of target plants such as trees and grape vines, and then automatically apply the amount of pesticides as needed according to plant architectures in real time. With the control system, growers themselves can upgrade their own sprayers to precision sprayers with intelligent functions rather than buying new sprayers, and sprayer manufacturers do not need to change their current sprayer designs. The primary requirement for the upgrade action is to connect a variable-flowrate solenoid valve to each nozzle, and all other components are attached to the sprayer body without changing the sprayer structure.
This new system is the product of a decade of research and development by engineers at USDA-ARS at Wooster, Ohio in collaboration with researchers at The Ohio State University, Oregon State University, University of Tennessee, Clemson University, Texas A&M University, Iowa State University, Washington State University, Penn State University, University of Queensland and USDA-ARS.
Since 2013, the system has been tested as a retrofit on different types of the air-blast sprayers for pest control effectiveness, reliability and repeatability on real farm fields. Comparative field biological tests were also conducted to evaluate insect and disease control for the sprayers with and without the intelligent-decision control capabilities in commercial nurseries, apple orchards, peach orchards, pecan orchards, vineyards and small fruit productions as well as university research farms in Ohio, Oregon, Tennessee, South Carolina, Texas, California and Washington.
These activities were voluntarily held by UCCE in Napa County and University of Queensland in Australia.
Figure 1. Integration of universal intelligent spray control system as a retrofit kit into existing sprayers (photo courtesy S. Booher.)
System Features
Spray deposition uniformity insidecanopies, chemical usage and off-target losses were investigated for the plants at different growth stages in ornamental nurseries, apple orchards, peach orchards and vineyards. Multi-year field tests have demonstrated the intelligent spray system is reliable and can reduce pesticide use in a range between 30% and 90%, reduce airborne spray drift between 60% and 90%, and reduce spray loss to the ground between 40% and 80%, resulting in chemical savings in a range of $56 to $812 per acre annually. At the same time, the insect and disease control efficiencies are comparable or even better than standard sprayer practices. Because it uses less spray volume, it can spray more acres with the same amount of tank mixtures, thus reducing tank refilling times and reducing labor and fuel costs.
As a result, Smart Guided Systems, LLC commercialized the intelligent spray system, and a commercial version of the product has been developed with joint efforts between USDA-ARS and Smart Guided Systems, LLC. The new control system (See Figure 1) includes a new laser sensor, an Android Samsung tablet, a GPS navigator, an automatic flow rate controller, air filtration unit, a toggle switch box and a universal mounting kit.
The laser sensor is mounted between the tractor and the sprayer to “see” plants on both sides of the sprayer. It releases 54,000 detection signals per second with a 270-degree and 164-ft radial detection range. The laser signals bounced back from the plant canopies is used to determine the presence of a plant canopy and measure the canopy height, width, foliage density and canopy foliage volume (Figure 2). The GPS navigation device (or the radar speed sensor mounted at the bottom of the sprayer) measures sprayer travel speeds and location of each plant in the field. Based on the plant canopy foliage volume and the sprayer ground speed, the amount of spray for each nozzle is determined and then discharged to different parts of each plant in real time. Each nozzle is connected to a 10 Hz pulse width modulation (PWM) solenoid valve, and the nozzle flow rates are controlled by manipulating the duty cycle of the PWM waveforms with the flow controller. The flow controller consists of microprocessors to generate flow rate commands for each nozzle to discharge variable spray rates. Field data collected and processed with the intelligent spray system are synchronized through the tablet WiFi to the cloud.
Figure 2. Laser sensor signals are used to measure canopy architecture and then manipulate individual nozzle flow rates as the function of the sectional canopy foliage volume and travel speed in real time.
The Android tablet provides the information for operators to communicate with the spray control system. The screen displays the sprayer travel speed, total discharged spray volume, spray width, and active nozzles. The operator can use the touch screen to modify the spray parameters as needed. The tablet allows the operator to activate the sprayer output on one or both sides in manual or automatic mode. All the electronic devices are powered by a 12V DC tractor battery. Another precaution includes the air filtration unit to discharge filtered air to prevent the laser sensor surface from getting dust and droplets. The toggle switches on the switch box are used to turn on/off main power, turn on/off the air filtration unit manually and override the automatic controller to activate nozzles as needed. Because sprayer travel speeds are automatically measured and included in the spray output control, applicators do not need to specify how fast they drive the tractor. However, travel speeds are not suggested to be higher than 5 MPH for orchard spray applications.
Features in the commercial system also include tree counting, tree size, foliage density heat map comparison capability, liquid volume sprayed per plant, maps of sprayed plant locations, ability to turn nozzles on/off independently through the tablet screen, cloud sync feature, web portal for configuration settings and spray coverage report view, system log files, five different languages (English, Spanish, French, German and Italian), and options for choosing metric or imperial units. The commercial products have been used by growers in the US and other countries with crops including citrus, nursery, pecan, blueberry, peach, almond, apple and pear with pesticide usage reductions in the range between 30% to 85% depending on crop types and growth stages. John Deere also established an agreement with Smart Guided Systems to sell the commercial intelligent spray control system for use in high-value crop applications through their dealer network.
The intelligent spray control system advances conventional standard pesticide application systems with the flexibility to spray specific positions on the plants. It reduces human involvement in decisions on how much spray volume is needed because the spray volume applied in the field is automatically controlled by the plant foliage volume instead of the antiquated gallons per acre.
The conventional air-blast spray system has been used from generation to generation for almost 80 years because of its robustness. Growers have accumulated extensive experience on using it to control pests in accommodation with their own crops. After being retrofitted with the intelligent spray control system, the conventional sprayers are able to turn on and off each nozzle and will stop spraying non-target areas such as gaps between trees, on the ground and above trees while their capabilities of spray penetration, spray range and spray deposition quality on plants remain the same. This new generation of spray technology is anticipated to be a primary precision spray technology for future decades to save chemicals for growers and provide a sustainable and environmentally responsible approach to protecting crops.
Figure 1
Distribution of ease of questions in an academic exam compared to certification testing.
I never really understood what certification means until I heard it described by a psychometrician. I have been a Certified Crop Advisor (CCA) for nearly two decades and felt I knew what it means to be a certified professional. I recently attended the North American Certified Crop Advisors On-line Board Meeting. I listened to a presentation by Scott Thayn, Ph.D., CMS, a psychometrician, or a statistician specializing in distinguishing the differences between individuals. Thayn is the president of Certification Management Services, the third-party agency hired by the Agronomy Society to help develop and manage the testing required for certification. The presentation addressed a proposal to give rankings on how well a test taker did on the CCA exam. Board members were hearing from potential members who were unable to pass the exams and wanted more feedback to help them study for their next attempt. The proposal, and the way the statistician took it apart, were a revelation, and it got me thinking that the mechanics behind certification are not well understood.
Certification Exam Intricacies
The Home page for Certifications under the website Agronomy.org states, “Certification is the standard by which professionals are judged. The purpose of a certification program is to protect the public and the profession. It is a voluntary enhancement to a person’s career credentials. Being certified adds credibility and shows that you are serious about what you do.”
A prospective candidate digging deeper would find they need to meet certain criteria to be considered a CCA: academic, experience and examination. Simply speaking, certification indicates one has demonstrated the knowledge and experience to perform at a higher level than their peers.
Hearing the proposal to give test takers feedback on their performance was familiar to me as a board member who participates on the Exam Committee for the Western Region. I have heard from many colleagues who did not pass one or both certification exams and are frustrated by the lack of a score or indication where they underperformed. I struggled to explain to my friends why the exams were pass/fail and why they just had to keep trying. I believe the frustration lies in the expectations of an academic testing experience clashing with the reality of certification exams.
Data indicates most people who take the certification exams are recent college graduates. Having a college degree in agriculture is a requirement for becoming a certified crop advisor. College graduates have spent most of their lives with graded exams. Academic testing presents a broad range of questions to both examine a student’s proficiency and encourage them to improve. A student who gets a low grade on a test will hopefully review the questions marked incorrect and study the subject to raise their grade on the final exam. This familiar approach to testing is contrary to certification exams.
The distribution of difficulty of certification exam questions is quite narrow compared to an academic exam (See Figure 1). The certification exam begins by defining competency areas, the major subjects that define the everyday work of the crop advisor. Performance objectives rest under the competency areas. Each performance objective spawns several possible exam questions. Each exam question must be tied to a performance objective to accurately test one’s comprehensive knowledge of agronomy.
Where an academic exam contains a large variation in question difficulty, certification exam questions ask, “What is the minimum knowledge a professional must have to be proficient in this area.” This is determined by groups of volunteer CCAs, with guidance by the Agronomy Society’s excellent statistician Dawn Gibas, Ph.D., who reviews the performance of each exam question. Questions that nearly everyone gets right are eliminated as well as those that almost no one answers correctly. A complete exam review process takes place every four to five years.
An illustration of the difference between academic and certification exams can be given with a sports analogy. An academic exam is comparable to a high school physical education track and field program, where everyone is expected and encouraged to participate. A certification exam, on the other hand, is like the selection process for the Olympic high jumping team. The high school physical education program sets the bar low and gradually raises it to help students practice their technique and jump higher. But when the world competition is jumping over seven feet, the US team would set the bar at a level near that to select the most competitive team. During the selection process, if the bar is set too high, then they don’t have a team, but set the bar too low and the team has a poor chance of winning. When the psychometrician used this example, the proposal to classify the specific abilities of test takers was withdrawn.
The complexity of 21st-century agriculture practiced in the Western Region of the US supports the need for the most qualified field people providing the best recommendations for our growers so we can continue to deliver the highest-quality, safest agricultural products in the world.
Concerns of incomplete label and interference with fertilization and pesticide plans received 20 responses in the survey. (all photos by Z. Wang.)
Crop biostimulants are many things. First, they are claimed to be multi-functional, including enhancement of soil and crop health, acceleration of soil nutrient cycling and improvement of crop productivity and fruit quality, among other benefits. Second, they are categorized by the active ingredient, application method, cost and target crops. Third, they are popular among ever-greater numbers of vegetable farmers regardless of production system, scale and commodity. Finally, some of their performance/efficacy is variable, while many others are unknown.
According to Grand View Research Inc., the global revenue generated by biostimulants was $1.74 billion in 2016, dominated by European and North American companies. Projections indicate that the market value will reach $4.14 billion by 2025.
Though it is difficult to obtain an exact number of biostimulant products currently on the market, the estimate is over 600 with more becoming available each year. However, this grand prosperity of crop biostimulants is not shared equally among people who use them. Particularly to vegetable growers who operate farming at every scale that differs widely in climates, production timing, marketable portion, planting techniques, field preparation and maturity, it can be extremely complex to choose the right product from the long list and use it at the right time in the right way. The first and maybe the foremost step toward a more effective use of crop biostimulants among vegetable growers is to understand their current use, experience, concerns and hopes. To accomplish the task, a survey was sent out to collect the specific information from vegetable growers mainly in the San Joaquin Valley and other counties in California.
Tomato (46) and watermelon (16) each represented crops with more than 10 submitted responses (photos by Z. Wang.)
The Survey and Respondent
The survey was sent to approximately 648 vegetable growers in late October 2020 with the help of other UCCE advisors and commodity boards. The survey was then closed about two months thereafter before the responses were summarized. The original survey can be found at cestanislaus.ucanr.edu/Agriculture/Vegetable_Crops/Biostimulant_Survey/. The survey contains eight questions with the first four asking growers how they farm and the last four related to their experience and opinions to crop biostimulants. By the end of December 2020, we received a total of 83 responses (12.8%), with 74 of them being valid responses (11.4%). Nine responses were not included because there were two replies without an answer to any of the question, five responses from oversea, and two responses from counties outside California. Details about the composition and production of the 74 respondents are included below and in Table 1.
By production, there were 10, 27 and 37 growers claiming organic only, conventional only and mix of both.
By scale, there were 31, 7 and 36 growers with vegetable production scale below 100 acres, 100 to 500 acres and over 500 acres.
By commodity, crops with more than 10 responses included tomato (46), pepper (23), melon (21), summer/winter squash (21), leafy greens/herbs (21), cole crops (16), watermelon (16) and onion (13).
By production location, the 74 growers claimed to have their vegetable fields in 21 counties across California. For details, see Table 1.
Table 1. List of counties for the respondents’ vegetable field locations.
Responses Regarding Biostimulants
The responses indicated that over half of respondents (40) know some knowledge about biostimulants, of which 37 applied biostimulants to at least one or two of their vegetable crops. There were nine growers who claimed having the highest knowledge level (very well), but two of them did not use biostimulants to any of their vegetable crops. There is no surprise that the majority of growers who responded with just a little knowledge or not knowing anything about crop biostimulants did not apply any biostimulant to any of their vegetable crops. The respondents were almost equally distributed by the application level of biostimulants (Table 2). The survey also asked the previous experience or future impression regarding the efficacy of biostimulants on improving vegetable growth. From the results, 50 growers, representing 68% of total respondents, shared the experience or impression that biostimulants could conditionally confer their efficacy. Less than 20% of the respondents indicated a consistent, positive performance on improving their vegetable crops, while only 13% gave the negative impression on biostimulant efficacy (Figure 1).
Figure 1. Responses to previous experience or future impression regarding biostimulant efficacy.Table 2. Number of grower responses to the understanding/knowledge level of biostimulants and how much of vegetable crops growers apply biostimulants to.
Concerns and Hopes
I have received numerous questions in the past years from vegetable growers, their advisors and colleagues regarding the biostimulant selection, effect evaluation, quality control and incompatibility with other field activities. “Going in blind”, “Unable to identify the benefits”, and “Snake oil” are common complaints. One of the main objectives for the survey is to identify the biggest concerns of using biostimulants on vegetable crops among growers. The survey results showed that about half of the respondents identified the difficulty of choosing a proper product as one of the main concerns followed by the risk of low or no return on investment. In addition, concerns of incomplete label and interference with fertilization and pesticide plans received 20 responses (Figure 2). Lastly, the survey asked growers their agreement level to future actions of improving the use of crop biostimulants. For all future measures, an average of 89% of the respondents agreed/strongly agreed that they will be helpful and important to improve future use of crop biostimulants (Table 3). These responses reflect the hopes from growers, and their voices should be heard by academia, industry, extension and other sectors to outline future efforts aiding a practical and profitable use of these biologics.
Figure 2. Growers’ concerns regarding the use of crop biostimulants on vegetable crops.Table 3. Number of responses to the agreement on the importance of future measures in improving the use of biostimulants.
UCCE Biostimulant Testing Trials
The UCCE farm advisors from Stanislaus County are actively working with vegetable growers and biostimulant companies to conduct various testing trials each year. The main goal is to fill the data gap with more unbiased, statistically-viable product efficacy data on various vegetable commodities in the Central Valley. Since 2019, we have evaluated numerous biostimulants on processing tomato and watermelon productivity, fruit quality and plant health. Stay tuned to our newsletter and check for previous results (Veg Views: cestanislaus.ucanr.edu/news_102/Veg_Views/).
References
Biofertilizers Market Size, Share & Trends Analysis Report By Product, By Application, And Segment Forecasts, 2012 – 2022. grandviewresearch.com/industry-analysis/biofertilizers-industry.
Brussels sprouts field in Santa Maria (photo by S.K. Dara.)
Brassica crops such as broccoli, brussels sprouts, cabbage, canola, cauliflower, collards, kale, kohlrabi, turnip and mustards are important vegetable or oilseed crops. The value of brassica vegetables, also known as cole crops, is more than $1.2 billion in California, which is the leading producer of these crops. Among various arthropod pests that attack brassica crops, the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae), is of significant importance. Thought to be of European origin, now with worldwide distribution, DBM exclusively feeds on cultivated and weedy crucifers. DBM can have up to 12 generations per year, especially under warmer climate.
Female moths deposit 150 eggs on average. Four larval instars feed on foliage and growing parts of young plants or bore into heads or flower buds, resulting in skeletonization of leaves, stunting of the plants or failure of head formation in some hosts. Pupation occurs on the lower surface of leaves or in florets. Adult moths are grayish-brown, and when at rest, a light-colored diamond-shaped pattern can be seen on the upper side of the wings.
Farmers typically rely on synthetic and biological insecticidal applications for controlling DBM. Multiple species of parasitoids and predatory arthropods also provide some control. Due to a heavy reliance on insecticidal control, DBM resistance to several insecticides is a common problem. Resistance of DBM to Bacillus thuringiensis (Ferré et al., 1991), abamectin (Pu et al., 2009), emamectin benzoate, indoxacarb and spinosad (Zhao et al., 2006), pyrethroids and other insecticides (Leibee and Savage, 1992; Endersby et al., 2011) have been reported from around the world. Excessive use of any kind of pesticide leads to resistance problems (Dara, 2020) to an individual pesticide or multiple pesticides.
Diamondback moth larva and adult (photos by Jack Kelly Clark, UC IPM.)
Integrated pest management (IPM) strategy encourages the use of various control options for maintaining pest control efficacy and reducing the risk of resistance development (Dara, 2019). Regularly monitoring pest populations to make treatment decisions, rotating pesticides with different modes of action, exploring the potential of biocontrol agents, and other non-chemical control approaches such as mating disruption with pheromones are some of the IPM strategies for controlling the DBM. While sex pheromones are effectively used to manage several lepidopteran pests and are proven to be a critical IPM tool, mating disruption is not fully explored for controlling DBM. A study was conducted in Brussels sprouts to evaluate the efficacy of a sprayable pheromone against the DBM and to enhance current IPM strategies.
Methodology
The study was conducted on a 10-acre Brussels sprouts field in Santa Maria. Cultivar Marte was planted in early July for harvesting in December 2020. A typical diamondback control program includes monitoring DBM populations with the help of sticky traps and lures and applying various combinations of biological and synthetic pesticides at regular intervals. This study evaluated the efficacy of adding CheckMate DBM-F to the grower standard practice of monitoring the DBM populations with traps and lures and applying pesticides. Treatments included 1.) grower standard pesticide program (See Table 1) grower standard pesticide program with two applications of 3.1 fl oz of CheckMate DBM-F on August 9 and September 11. Treatment materials were applied by a tractor-mounted sprayer using a 100 gpa spray volume and necessary buffering agents and surfactants. Each treatment was five acres and divided into four quadrants representing four replications.
Table 1. Pesticides, buffering agents and surfactants, their active ingredients, rates/ac (along with the IRAC mode of action groups) and retail pricing for those applied in the grower standard diamondback moth control program. *Applied diamondback moth and aphid control
In the middle of each quadrant, one Suterra Wing Trap was set up with a Trécé Pherocon Diamondback Moth Lure. Lures were replaced once a month in early September and early October. Sticky liners of the traps were replaced every week to count the number of moths trapped. Traps were placed on Aug. 1, 12 and 24, Sept. 1, 11, 18 and 27, and Oct 6, and the moth counts were taken from respective traps on Aug. 8 and 20, Sept. 1, 11, 18 and 27, and Oct. 6 and 15. CheckMate DBM-F was applied at 3.1 fl oz/ac on Aug. 9 and Sept. 11. The number of larvae and their feeding damage on a scale of 0 to 4 (where 0=no damage, 1=light damage, 2=moderate damage, 3=high damage, 4=extensive/irrecoverable) were recorded from 25 random plants within each replication on Aug. 30 and Oct. 6 and 18. Data were subjected to analysis of variance using Statistix software and significant means were separated using Tukey’s HSD test. The retail value of various pesticides was also obtained to compare the cost of treatments.
When CheckMate DBM-F[(Z)-11-Hexadecenal (3) , (Z)-11-Hexadecen-1-yl Acetate (1)] was applied the first time on Aug. 9, Dibrom 8 Emulsive was replaced with Warrior II, the buffering agent Quest was not used, and the surfactant Dyne-Amic was replaced with Induce (dimethylpolysiloxane) to avoid potential compatibility issues. The impact of this substitution is expected to be negligible within the scope of this study. The retail cost of 3.1 fl oz CheckMate DBM-F is $45.60. The cost of lures and traps would be about $4 to $8 per acre for a six-month crop like Brussels sprouts.
Results and Discussion
Traps in replication 4 in both treatments on August 8 and replication 1 in the grower standard were missing, probably knocked down by a tractor. The day before CheckMate DBM-F was first applied, the mean number of adult DMB caught was 227 in the grower standard and 271 in the plots that would receive the pheromone application (Figure 1). There was a gradual decline in moth counts during the rest of the observation period in both treatments. However, the decline was higher in the plots that received CheckMate DBM-F. The number of moths per trap were about 19% higher in the pheromone-treated plots compared to the grower standard before the study but were nearly 98% lower by the end of the study (Figure 2). The reduction in moth populations from mating disruption was significant on September 18 (P =0.039) and October 15 (P = 0.006).
Figure 1. Mean number of diamondback moth adults found in the traps.Figure 2. Reduction in moth populations by adding pheromone for mating disruption.
The mean number of larvae per 25 plants in a replication was zero on all observation dates except for 0.01 on Aug. 30 in the plots that received CheckMate. Four insecticide applications made by the time the study was initiated and the remaining six applications effectively suppressed larval populations.
Larval feeding damage ratings were consistently low (P < 0.0001) in the plants that did not receive CheckMate DBM-F (Figure 3). The damage was limited to the older leaves at the bottom of the plants and must have been from early feeding before the initiation of the study. The lack of larvae and the evidence of new feeding damage also confirm that the crop remained healthy and pest-free.
Figure 3. Feeding damage by diamondback moth larvae.
Yield and Cost Comparisons
Since frequent pesticide applications effectively suppressed larval populations and prevented their feeding damage, the effectiveness of mating disruption on larval populations or their damage could not be determined in this study. Moths found in the traps probably developed from the larvae in the field or could have been those that flew in from other areas.
However, lower moth populations in CheckMate DBM-F treatment demonstrated the overall influence of mating disruption and pest suppression.
It is common to make about 10 to 12 pesticide sprays during the six-month crop cycle of Brussels sprouts. The cost of each application varied from about $73 to $192 depending on the materials used with an average cost of about $128 per application in this study. The cost of two CheckMate DBM-F applications is $91. If diamondback moth populations could be reduced with mating disruption, it is estimated that two to three pesticide applications could be eliminated. That results in $164 to $292 of saving for the pesticide costs and additional savings in the application costs per acre by investing $91 in the mating disruption. Since DBM can develop resistance to several chemical and natural pesticides, eliminating some applications as a result of mating disruption also contributes to resistance management along with potential negative impact of pesticides on the environment. Compared to other mating disruption strategies, a sprayable formulation compatible with other agricultural inputs is easier and more cost-effective to use.
The grower’s yield data showed 762 cartons/acre from the grower standard block with pesticides alone and 814 cartons/acre from the block that received pesticide and pheromone applications. Although there seems to be a 7% yield difference, since data from individual plots could not be collected for statistical analysis, the impact of DBM mating disruption on yield improvement is inconclusive.
This study demonstrated that mating disruption with CheckMate DBM-F will significantly enhance the current IPM practices by reducing pest populations, contributing to insecticide resistance management, and reducing pest management costs. Additional studies with fewer pesticide applications that allow larvae to survive and cause some damage might further help to understand the role of mating disruption where pest populations are not managed as effectively as in this field.
Thanks to the PCA and grower for their research collaboration, Tamas Zold for his technical assistance in data collection, Ingrid Schumann for market research of pesticide pricing and Suterra for the financial support.
Feeding damage in cauliflower (photo by S.K. Dara.)
References
Dara, S. K. 2019. The new integrated pest management paradigm for the modern age. J. Int. Pest Manag. 10: 12. Dara, S. K. 2020. Arthropod resistance to biopesticides. Organic Farmer 3 (4): 16-19. Endersby, N. M., K. Viduka, S. W. Baxter, J. Saw, D. G. Heckel, and S. W. McKechnie. 2011. Widespread pyrethroid resistance in Australian diamondback moth, Plutella xylostella (L.), is related to multiple mutations in the para soidum channel gene. Bull. Entomol. Res. 101: 393. Ferré, J., M. D., Real, J. Van Rie, S. Jansens, and M. Peferoen. 1991. Resistance to the Bacillus thuringiensis bioinsecticide in a field population of Plutella xylostella is due to a change in a midgut membrane receptor. Proc. Nat. Acad. Sci. 88: 5119-5123. Leibee, G. L. and K. E. Savage. 1992. Evaluation of selected insecticides for control of diamondback moth and cabbage looper in cabbage in Central Florida with observations on insecticide resistance in the diamondback moth. Fla. Entomol. 75: 585-591. Pu, X., Y. Yang, S. Wu, and Y. Wu. 2009. Characterisation of abamectin resistance in a field-evolved multiresistant population of Plutella xylostella. Pest Manag. Sci. 66: 371-378. Zhao, J-Z., H. L. Collins, Y-X. Li, R.F.L. Mau, G. D. Thompson, M. Hertlein, J. T. Andaloro, R. Boykin, and A. M. Shelton. 2006. Monitoring of diamondback moth (Lepidoptera: Plutellidae) resistance to spinosad, indoxacarb, and emamectin benzoate. J. Econ. Entomol. 99: 176-181.
Finding the right fumigation and herbicide program can help reduce long-term labor costs (photo courtesy L. Stoeckle.)
When methyl bromide was banned in 2005, California strawberry growers lost an effective tool in their crop care toolbox to control weeds, soilborne diseases, nematodes and symphylans. Special-use permits allowed them to continue using the fumigant through 2016, but growers feared final loss of the powerful soil fumigant might be the end of profitable production.
The California Strawberry Commission agreed, noting that elimination of methyl bromide fumigation brought forth several soilborne diseases for which there is no post-plant control. Of particular concern were Fusarium wilt (Fusarium oxysporum f.sp. fragariae), Verticillium wilt (Verticillium dahlia) and charcoal rot (Macrophomina phaseolina), according to the California Strawberry Commission.
But once again, necessity proved to be the “mother of invention,” said independent agronomist and PCA Lee Stoeckle, owner of Stoeckle Agricultural Consulting in Ventura, Calif.
Stoeckle has advised strawberry and caneberry growers for more than 30 years, and he noted that effective alternative fumigants have taken up the methyl bromide void and are now widely and successfully used. While California strawberry acreage has fallen, total production has actually increased thanks to innovation and application of new technology.
Stoeckle’s family-owned business provides recommendations on 3,000 acres of strawberries and 100 acres of blackberries in Santa Barbara County and San Luis Obispo County and 2,000 acres of strawberries in Ventura County. In addition, he consults on production of 800 acres of strawberry and 250 acres of raspberries in Baja Mexico. While he is primarily responsible for above-ground insect and disease control, he doesn’t write fumigation recommendations but instead advises based on what he learns about weeds and soil pathogens.
A strawberry field at the beginning of the growing season in Oxnard, California.
A One-Two Punch
Top problem soilborne diseases, according to Stoeckle, are Fusarium, Macrophomina, Phytophthora, Anthracnose and Verticillium wilt. Top problem above-ground pests include two-spotted spider mites, Lygus bug, Botrytis fruit rot/gray mold and powdery mildew, depending on varietal susceptibility. Other pests necessary to watch include worms and aphids.
“The optimum disease and weed control program is a one-two punch,” Stoeckle says. “Hit it at the end of the season as a burndown and at the beginning before next planting via drip irrigation. The backbone of our control program is Pic-Chlor 60 EC (1,3-dichloropropene plus chloropicrin) at the max recommended rate (350 lbs/ac), going after the heavy hitting ‘big boys’ (soilborne diseases) when applied in August or September before planting.” He recommends Vapam or K-Pam to get the additional benefits to control these diseases (Fusarium, Verticillium, Macrophomina phacelia) and eliminate the inoculum reservoir in the crown.
Stoeckle says emulsified formulations of Telone® C-35 and chloropicrin can be applied with irrigation water through drip irrigation systems. Metam sodium is the active ingredient in Vapam® HL™ and metam potassium in the active ingredient in K-Pam® HL™. Both are AMVAC® soil fumigants, which give off methyl isothiocyanate (MITC) when combined with water via drip or otherwise. If drip fumigation is planned, good results have been obtained with a sequential application of chloropicrin or 1,3-dichloropropene plus chloropicrin, followed 7 days later with metam sodium or metam potassium.
Stoeckle considers fumigants essential and offers the math: “If we equate the value of every plant to be $2 each and you lose 10% of your plants, that’s 2,500 plants on a population of 25,000 per acre. Your choice is to fumigate or take a $5,000 loss. That’s an easy decision. The return on investment on fumigation is huge. We expect to see a 20% to 30% return on fumigant investment.”
He is quick to add there are other management decisions that help maximize production, noting, “I could go on and on about using the best plastic mulch or optimum fertilization practices.”
Best Practice Weed Control Reduces Production Costs
Fifth-generation Santa Barbara County grower Brett Ferini, owner of Rancho Laguna Farms, grows 400 acres of strawberries (300 fall plant, 100 summer plant) and 20 acres (adding another 20 for 40 total) of blackberries. Plus, he grows 35 acres of organic blackberries. He used Vapam on blackberries in February 2020.
“Vapam worked out really well in controlling nutgrass/nutsedge,” he said. “Pic-Clor 60 has no effect.”
For weed and disease control, “we hit everything at fall pre-plant with Pic-Clor 60 through drip tape and seven days after that we follow with Vapam,” he said. “It does a terrific job on nutsedge and other weeds, as well as the soilborne diseases including Verticillium, Phytophthora and, anecdotally, Macrophomina.”
Ferini says the operation plans to add a Vapam burndown at the end of the 2020 growing season
“We definitely [have] cut weeding by 60% on summer plant,” he says. “Labor is our highest cost. We saw $1,050 savings per acre vs. our normal costs of $1,800/acre. The big test will be on the fall plant when we get more rain. Our fall weeding cost normally runs $2,800 to $3,000 per acre. Using Pic-Clor 60 followed by Vapam treatment, I expect weeding cost savings of $1,000 to $1,500 per acre.”
Also seeing results with soil fumigants is Santa Barbara grower Josh Ford. He is COO of Ocean Breeze Ag Management LLC in Ventura, which grows 450 acres of strawberries, 50 acres of blackberries, and 25 acres of raspberries. Ford’s biggest soilborne pests are Macrophomina, Fusarium, and Phytophthora cactorum. Nutgrass is his most difficult weed to control.
“We’ve been using soil fumigants for many years,” he says. “We were using methyl bromide, but now we apply chloropicrin once a year and K-Pam once to twice a season. If nut grass is a bad problem, we will knock it down at the end of the season with K-Pam and also pre-plant K-Pam. Our ROI is good when you consider the increased cost of labor to manually remove nut grass.”
Dr. Themis Michailides sampling symptomatic fig trees. (photo courtesy T. Michailides.)
Back in 2004, and again in recent years, there were concerns by fig growers mainly in Madera and Merced counties about an excessive killing of major branches of their fig trees (Figure 1). Visits to some orchards back then and recently indicated that indeed they had a major problem. Initial close examinations of the dead branches showed symptoms which were similar to another disease: branch wilt of walnut.
Figure 1. Left, Fig tree affected by severe limb dieback; top right, still active canker; bottom right, inactive canker (branch is dead) (all photos courtesy G. Gusella.)
The bark of dead fig branches had cracks and one could easily remove large pieces of the bark, exposing the woody tissues underneath which were covered by a black powder. Rubbing this black powder with your finger could easily remove masses of it (Figure 2). The inner surface of the broken and removed bark pieces were also black due to these powder masses. A lot of trees had many dead major branches while others had one or two dead along with other branches bearing chlorotic and thin canopy, distinct from the green and dense canopy of healthy branches.
Figure 2. Top, Neoscytalidium dimitiatum, the cause of limb dieback producing spores (arthrospores) as the mycelia dried up and separate to small segments under the bark; bottom, easily rubbing off the spores under the bark (photo courtesy Beth Teviotdale.)
Pathogen Activity
To collect samples, we cut some of the symptomatic branches close to the interface of dead and alive-looking (green) tissues. We noticed that in a cross section, the dead woody tissues were delineated from the healthy tissues by a dark brown line while the living woody tissues were white (Figure 1).
Slices of these woody tissues from the branches were taken, isolations were made in the laboratory and a fungus known to be a pathogen of woody tissues was consistently recovered. The name of this pathogen is Neoscytalidium dimidiatum, which is a new taxonomic name of Hendersonula toruloidea fungus, which represents the pathogen first reported to cause the branch wilt disease of walnut.
Checking the literature, the same fungus under a different name (i.e. Nattrassia mangifera) was reported in 1945 on commercial figs in California as well as on Ficus religiosa and Ficus bengalensis, causing dieback and trunk cankers. In addition to walnut branch wilt, which is a common disease of walnuts grown in the San Joaquin Valley, the same fungus was reported in causing branch wilt and dieback of poplar, eucalyptus and mango. In other reports, we found out that this pathogen can cause killing of major branches of walnut, ash trees and grapefruit. More recently, it has been reported causing cankers and hull rot of almond. On fig shoots, the pathogen grows and infects injured bark (i.e. mechanical wounds, wounds by hail or sunburn), invades the woody tissues and kills the branch. When the branch is killed, it dries and usually the bark cracks, exposing large masses of black spores. These are not true spores but are small segments of mycelia that become black as the tissues dry up and break down into small pieces, producing a layer of black powder under the bark. The fungus also produces pycnidia that protrude through small cracks of the bark (See Figure 3). However, it is the spores produced in masses by the breaking mycelia called arthrospores. that can be spread readily by air and/or splashing rain and can cause infections of pruning wounds and other injuries of branches.
Figure 3. Left, pycnidia protruding through bark cracks; right, arthrospores of Neoscytalidium dimidiatum causing limb dieback of fig (photos courtesy T. Michailedes.)
Survey of Affected Areas
Before doing pathogenicity studies with the Neoscytalidium fungus, we wanted to make sure that this fungus was found frequently throughout the area where fig trees showed similar symptoms to the ones we initially observed in Madera County. Therefore, a survey of 16 fig orchards with branch dieback symptoms, representing all the major fig varieties (Black Mission, Calimyrna, Conadria and the male trees (Roeding and Stanford caprifig varieties)) was done in Fresno, Madera and Kern counties. Neoscytalidium was isolated in all of these orchards.
Limb and branch samples from the majority of these orchards had 60% to 100% Neoscytalidium, while three had 7% to 11%, and two 26% to 32%. In 12 of these orchards, a second pathogen, Phomopsis spp., was isolated along with Neoscytalidium in the first year of the survey. Phomopsis sinarencis has been reported in California causing an epidemic on Kadota figs back in 1935 and in other countries as an important fig canker pathogen. By the third year of this survey, less Phomopsis was isolated, and, very recently, almost none was isolated, probably because the very susceptible Kadota variety is rarely now planted in California. Phomopsis is known as a pathogen fungus associated with canker diseases in many other crops around the world, but more investigations are needed to figure out its role in fig limb dieback.
Differences in Susceptibility
To determine if there were any differences in susceptibility to the limb dieback pathogen, we inoculated six cultivars directly in the field. We found that three months after inoculations, the cultivars Kadota, Black Mission and Sierra developed twice as long canker size than the cultivars Brown Turkey, Calimyrna and Conadria (Figure 4). Growers also reported that they see the problem to be more severe in Black Mission than other cultivars. Inoculations of six cultivars showed that Neoscytalidium is a plant pathogen that likes high temperatures. For instance, it cannot grow below 50 degrees F; its optimum temperature for growth is 90 to 95 degrees F, and it can even grow at 104 degrees F to some extent. Therefore, this fungus likes hot summer temperatures and prefers to infect sunburned branches and pruning wounds.
Figure 4. Susceptibility of various fig cultivars to limb dieback pathogen Neoscytalidium dimidiatum.
In experiments, we inoculated shoots of fig of different ages, including current growth (green) shoots, one-year, two-year and three-year-old shoots, by wounding and inoculating with either a mycelial plug or a spore suspension. Interestingly, the three-year-old shoots developed almost three-fold larger cankers than the cankers on current and the one-year-old shoots. This suggests that larger cuts in the field during pruning seem to be more susceptible to infection than cuts made in current or one-year-old shoots. Also, inoculations done in May, June and July resulted in larger cankers than those done from August to November. ‘
When we compared infection on pruning wounds done in winter vs. those done in summer, pruning wounds in the summer developed almost threefold larger cankers than those done during winter months. Therefore, it is recommended that pruning of figs should be done in winter when pruning wounds seem to be less susceptible. Figs can be protected from infections of the branch wilt pathogen if shoots are painted with whitewash to protect them from sunburn. Applying Surround® on shoots also protected the shoots from infection, and this is recommended to become a routine practice by fig growers. Figure 5 shows results of inoculation experiments done following various treatments in the field and artificial inoculation with the pathogen.
Although wounding by only mallet or only sunburn resulted in larger canker than the non-inoculated, un-wounded/un-treated shoots, the shoots that were damaged by mallet wounding and sunburn at the same time resulted in the longest cankers. White wash or spraying with Surround protected the shoots even after wounding with mallets and inoculation (Figure 5 and 6). Therefore, pruning that exposes the shoots to sunburn and or any other type of wounding should be avoided, and spraying with Surround will help protect the fig shoots from the limb dieback pathogen.
The authors thank the California Fig Institute for funding this research and a number of fig growers who allowed us to sample their orchards.
Figure 5. Effect of stress factors (mallet wounding and sunburn) and treatment with white wash affecting the severity of limb dieback of fig.
Figure 6. Effect of Surround® spray on the severity of limb dieback of fig.
Experimental plots of plastic mulches in fall-bearing raspberries in Wisconsin (photo by H. McIntosh.)
Spotted-wing drosophila (SWD), Drosophila suzukii, is an invasive vinegar fly and a major pest of soft-skinned fruit crops. The fly was first detected in the continental U.S. in 20081 and has quickly spread from its native range in Eastern Asia throughout the U.S. and into most major fruit-producing regions of the world2. For small-scale fruit growers, damage from this pest substantially reduces the yield of marketable fruit, making susceptible crops challenging to grow economically and sustainably3,4. For large-scale growers, the presence of SWD can lead to complete crop loss due to processors’ zero-tolerance policies for insect infestation5.
Biology
Vinegar flies typically lay their eggs in damaged or rotting fruit, but female SWD have a highly serrated ovipositor that allows them to saw through the skin of undamaged, ripening fruit6,7, which makes SWD an especially detrimental pest. Larvae emerge inside of and feed on the fruit, making it mushy and unmarketable. Recent research showed that around 80% of larvae drop from the fruit to pupate in the top layer of soil and that they are more likely to drop from the fruit if the fruit is overcrowded8,9. Larvae and pupae can also reach the ground when damaged fruit becomes mushy and falls to the ground.
SWD has a quick generation time, with many generations per year in most regions. The fly develops fastest in temperatures between 68 to 83 degrees F, but is unable to develop at temperatures above about 87 degrees F10,11. In temperate regions like the Upper Midwest or Pacific Northwest, SWD populations are highest during summer months. In hotter regions like California or Florida, fly populations are highest in the spring and fall and are much lower during their hot and dry summer months12.
SWD thrives in high humidity, developing fastest around 94% humidity13. Researchers found that females laid more eggs in the inner canopy of blackberry and blueberry plants, likely because the environment is more humid, cooler and darker14,15.
(Left) Spotted-wing drosophila adult female and male on a raspberry. Male flies are easy to identify by the large black spots on their wings. (Right) Spotted-wing drosophila females have a serrated ovipositor that allows them to lay eggs in undamaged, ripening fruit (photos courtesy Agri-Mag and Chris Thomas.)
Fruit crops that are the most susceptible to SWD include raspberries, blackberries, blueberries, strawberries, sweet and tart cherries, and some cultivars of wine grapes16-18. However, SWD can survive on alternative hosts like wild blackberries and apples, buckthorn and honeysuckle. It remains largely unknown how SWD survive in the winter and spring before fruit is available in the landscape and on farms, but one study found that SWD can develop on non-fruit hosts like mushrooms and bird manure19.
Traditional Management
Pest pressure from SWD is often very high due to the fly’s fast development time, optimal development conditions in the summer and high availability of host plants and food in the agroecosystem. Management relies heavily on chemical control in organic and conventional systems, which is costly to growers. In California, chemical controls for SWD cost around $470/acre for conventional and $1,210/acre for organic growers3.
Only a few insecticides approved for use in organic systems are effective at controlling SWD, limiting organic growers’ options for control20. Unfortunately, recent reports show evidence of insecticide resistance developing for some active ingredients in some regions, including spinosad (the main insecticide used to control SWD in organic systems)21,22.
Cultural practices can help reduce the fly’s population and are often used in tandem with chemical controls. Such practices include harvesting fruit promptly (every one to two days), frequent field sanitation, burial or composting of infested fruit and exclusion netting23,24. However, these methods are labor-intensive and expensive.
Since SWD is sensitive to temperature and humidity, cultural practices that modify the crop canopy microclimate have the potential to reduce infestation by deterring adults from laying eggs or disrupting larval development inside of fruit. Management strategies for SWD typically target adult flies in the canopy, but since the majority of SWD larvae fall to the ground before pupation, ground-based cultural management practices could also be important for reducing populations.
Spotted-wing drosophila life cycle (courtesy Jana Lee, USDA-ARS.)
Growers have used plastic mulches since the 1960s to modify the microclimate in fruit and vegetable agroecosystems. Plastic mulches are commonly used for weed control, promoting earlier ripening, improving fruit quality or color and increasing yield25,26. Different colors of plastic mulches have also been shown to successfully control insect pests including aphids, whiteflies, Asian citrus psyllid and Mexican bean beetles27-30.
Plastic Mulches for SWD
Based on the extensive body of literature reporting that plastic mulches can modify the crop microclimate, control some insect pests and provide other horticultural benefits, we tested the impact of three colors of plastic mulches on SWD adult and larval populations. Our study was conducted in 2019 and 2020 on a small fruit and vegetable farm in South Central Wisconsin in fall-bearing raspberries.
In this study, we tested black and white-on-black biodegradable plastic mulches (Organix Solutions AG film), metallic polyethylene mulch (Imaflex SHINE N’ RIPE) and a grower-standard control where grass filled in the space between the alleyway and the raspberry plants. We assessed the three mulches’ impact on SWD adult and larval populations in fall-bearing raspberry.
We laid the mulches by hand when the raspberry canes were just emerging from the soil in late April. We laid two mulch strips (25 feet long by 2.3 feet wide) along each side of the row, leaving a six-inch gap between the strips for the canes to grow. The edges of the mulches were secured with biodegradable sod stakes. All four treatments were randomly distributed in each of four rows of fall-bearing raspberries (cultivars “Polana” and “Caroline”), totaling 16 plots.
Starting when the first flies were detected in June, we measured the adult SWD populations passively using clear sticky cards placed in the fruiting zone, which were replaced weekly to estimate fly populations by week.
Larval infestation of fruit was evaluated by counting the number of larvae using the salt float method31. The evaluations were done two to four times per month starting in August.
Adult and larval populations were measured throughout the season until adult populations reached zero, usually in mid-October.
We also did a preliminary experiment to test whether plastic mulches could kill larvae that fell onto the mulch surface. We put lab-reared larvae into ‘corrals’ made from plastic sandwich containers and recorded their mortality and movement over three hours.
‘Corrals’ made from plastic sandwich containers used to test mortality of larvae on the mulch surface (photo by H. McIntosh.)
Population Reductions
In both years of our study, we found significantly lower SWD populations above all three plastic mulches compared to the control plots. Over the two-year period, the black and metallic mulches reduced the adult population of SWD by 51% and the white mulch reduced flies by 42% compared to the control.
Interestingly, the plastic mulches only reduced female fly populations and did not impact the number of male flies caught on the sticky cards. With fewer female flies in the canopy above the plastic mulches, it was unsurprising that we also found fewer larvae infesting the fruit in the mulched plots. Over the two-year study, the black mulch decreased the number of larvae in fruit by 72%, the metallic mulch by 61%, and the white mulch by 52% compared to the control.
Plastic mulches may be more effective than other types of mulches tested for managing SWD. In our study, we recorded the lowest adult fly populations and larval infestation of fruit above the black plastic mulch. A 2019 study tested black fabric weedmat as a cultural control for SWD in blueberry in several states and found no effect of the weedmat on SWD infestation of blueberries32. It is possible that some quality of the plastic mulch material (such as reflectivity or lack of permeability) makes it more deterrent to SWD than the weedmat.
In our preliminary experiment, larvae placed on the plastic mulches died quickly. Larvae on the black mulch died in less than one hour, and larvae on the white and metallic mulches died in less than three hours. We recorded high surface temperatures on the mulches, with all mulches heating up above 87 degrees F (SWD’s threshold for development) for two to four hours each day. On hot days, the black mulch got above 150 degrees F.
When placed on the mulch, we observed larvae struggling to crawl and visibly desiccating within minutes, making it unlikely that larvae could crawl off the mulch into the safety of the soil. We will collect more data in summer 2021 to confirm these promising results.
The results of our study provide evidence that black, white and metallic plastic mulches can reduce SWD adult and larval populations in fall-bearing raspberry in the Upper Midwest, showing promise for use of plastic mulches in sustainable pest management.
Combining plastic mulches with other cultural practices including short harvest intervals (every one to two days) and frequent field sanitation could have an additive effect on reducing SWD populations, potentially reducing the need for chemical controls in conventional and organic cropping systems.
Next Steps
Although the use of plastic mulches reduces SWD populations in the canopy of raspberry plants, the specific mechanisms causing this reduction are still unknown. We are still investigating how canopy light conditions, temperature and humidity are
influenced by the plastic mulches and whether these factors can explain the reduction in SWD populations we measured.
In the next two years of this project, we will conduct field experiments to determine whether the three plastic mulches we tested influence beneficial insects, including pollinators, and how the mulches impact soil health, raspberry plant growth, fruit quality and yield in Wisconsin’s climate.
Testing these mulches in other regions and fruit crops is warranted to determine if the reduction of SWD is maintained in different climates and other susceptible crops. Overall, plastic mulches are a promising new tool for more sustainable management of SWD in raspberry in the Upper Midwest.
References
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