Citrus mealybug infestations continue to increase in the San Joaquin Valley (SJV) making this species an emerging concern for citrus growers. Although known to be present in California for more than 50 years, it was kept in check by natural enemies (Ebeling 1959) and was not a pest of concern until recently. As such, citrus mealybug is an understudied pest in California citrus systems, and little is known about its biology and field ecology, monitoring and threshold, and control options, making management challenging. In 2022, we initiated research studies to improve knowledge about field ecology, monitoring and management of mealybug in SJV citrus.
Citrus mealybug, Planococcus citri, is a soft-bodied, oval, flat and distinctly segmented insect covered with white mealy wax (Figure 1). This small polyphagous sap-sucking insect prefers to live in between fruit clusters or under the thick leaf canopy, therefore avoiding detection when population is low. Infestation causes chlorosis, leaf drop and stunted growth, while direct feeding damage results in tissue discoloration around feeding spots, fruit deformity and contamination of foliage and fruit with honeydew and sooty mold (Figure 2), resulting in loss of yield and marketability of fruit.
Adult female citrus mealybugs are ~3 millimeters long and have distinctly segmented bodies covered with white mealy wax. A female can lay up to 600 eggs in a lifetime, in a group of 5 to 20, deposited and protected within cottony ovisacs (Figure 1). These eggs hatch in 3 to 10 days depending on temperature and crawlers disperse to different parts of the plant looking for feeding sites. All life stages are mobile, and they spread by crawling from tree to tree, carried by ants, blown by wind and picked up by birds, machinery or labor crews. Immatures and adult females feed on leaves and fruits. Males have wings, are short-lived and do not feed.
Field Ecology for Monitoring
Citrus mealybug overwinters as eggs or crawlers in protected areas of the tree including fruit. Greenhouse research has shown eight complete generations in one year (Myers 1932). To generate information on seasonal activity and the number of generations in the SJV, we monitored citrus mealybug in 13 trees in a block known to have mealybug infestations in 2021. Double-sided sticky tapes were wrapped around the trunk and four main inner branches. Traps were replaced weekly, and the number of mealybugs caught on the traps was counted every week. In addition, males were monitored using a pheromone trap.
We found that mealybugs were present within the tree canopy throughout the season, but the areas where they aggregated changed as the season progressed. Overwintering eggs/crawlers serve as an inoculum for a new season. The first-generation activity starts in March as eggs hatch and crawlers move from the overwintering sites (cracks and crevices on trunk, leaves protected under thick canopy, fruit or from the ground) to new leaves and then to fruit. When fruit is present, most of the infestation was found on fruit. Males caught on the trap card showed six distinct peaks that preceded the crawler emergence, suggesting six complete generations of mealybug in the valley (Figure 3). This seasonal occurrence pattern can be used to scout for mealybug.
January to March: fruits or leaves. Mealybugs like protected areas. Check inside the tree canopy, fruit (navels and under the calyces) or leaves (underside) for overwintering adults, egg sacs and crawlers. Presence of sooty mold and white mealy wax is an indicator (Figure 2).
April to June: crawlers emerge and start moving into new flush or fruit. Egg masses, crawlers on the trunk and inner branches.
July to December: on fruit, feed and multiply. Overlapping generations can be present.
Monitoring can also be done by using a pheromone lure and traps. Place lure and traps by March 15, before the first-generation activity starts and monitor throughout the season. If you catch males on trap, scout trees for infestation and make pesticide applications when needed.
Management Options
Citrus mealybug is naturally regulated by natural enemies and predaceous insects. But recent outbreaks suggest that this ecological balance is shifting. Although very little is known about mealybug management in citrus systems, incorporating cultural, biological and chemical tools is encouraged as researchers continue to study the seasonal occurrence, efficacy and timing of pesticide applications for improved management.
Cultural control
Female and nymphal mealybugs are wingless and spread to new areas by humans, equipment, wind or birds. Sanitize equipment before moving to new areas of the grove. Pruning to open tree canopy and hedging trees to prevent canopy overlap will help slow the crawler movement and increase natural enemy activities.
Biological control
Several natural enemies are identified as effective biocontrol agents for controlling mealybug (Grafton-Cardwell et al. 2021).
Mealybug destroyer, Cryptolemus montrouzieri, is known to be a highly effective predator and is most effective when population pressure is high. This predator does not survive cold winters. It is readily available from commercial suppliers and can be released in orchards where mealybug was a problem in the previous year.
Anagyrus spp. and Coccidoxenoides have been found in mealybugs collected from the SJV, suggesting natural infestations. Future research will investigate presence of natural enemies and their impact on mealybug.
Chemical control
Although UC IPM guidelines have only one product recommended for mealybug control in citrus systems, several insecticides are recommended for controlling mealybug in grapes, pistachios and greenhouses (Haviland et al. 2019). Different species of mealybug may respond differently to insecticide applications. Moreover, different seasonal phenology of pests in separate cropping systems warrants crop-specific pesticide efficacy trials. Two field trials conducted in summer 2022 showed that one application significantly reduced mealybug infestation but did not provide long-term solution (Gautam 2022). If your block was infested, plan on an early season control, targeting the first-generation crawlers in late March/early April. Monitor populations and make a second application using a different group of insecticides within two weeks of the first application, targeting the newly hatched crawlers. Continue monitoring and use mealybug-effective materials.
Pheromone lure is not available as a management choice but is available for monitoring.
Other Factors
Natural enemies are relatively more susceptible to broad-spectrum insecticides such as organophosphates, carbamates, pyrethroids and neonicotinoids in groves with mealybug infestations, so reduce/limit the use of these chemicals. Spirotetramat is relatively non-toxic to mealybug destroyers (Grafton-Cardwell 2021).
Ant control: ants help in mealybug dispersion within the tree canopy and between trees and groves. They also defend mealybug colonies from natural enemies (Figure 4). Use effective ant control methods.
Prevent the spread of mealybug by informing the pruning/harvesting crew.
Strip and dispose of the fruit infested with mealybug as fruits may harbor mealybug for next season’s infestation.
Talk to neighboring growers about this threat.
The University of California and Fresno State University researchers are working on a citrus research board-funded project to improve knowledge on biology, seasonal phenology and management of citrus mealybug. Stay informed on research outcomes by participating in grower seminars, newsletter publications and UC IPM guidelines updates.
References
Ebeling, W. 1959. Subtropical fruit pests: Biology and control of citrus pests. Pp. 135-229. University of California, Division of Agricultural Sciences
Gautam, S.G., 2023. Core IPM Research Update: Working toward citrus mealybug IPM. Citrograph, 14: 32-36.
Grafton-Cardwell E.E., Baldwin R.A., Becker J.O., Eskalen A., Lovatt C.J., Rios S., Adaskaveg J.E., Faber B.A., Haviland D.R., Hembree K.J., Morse J.G., Westerdahl B.B. 2021. Revised continuously. UC IPM Pest Management Guidelines: Citrus. UC ANR Publication 3441. Davis, CA.
Haviland D.R., Bettiga L.J., Varela L.G., Baldwin R.A., Roncoroni J.A., Smith R.J., Westerdahl B.B., Bentley W.J., Daane K.M., Ferris H., Gubler W.D., Hembree K.J., Ingels C.A., Wunderlich L.R., Zalom F.G., Zasada I. 2019. Revised continuously. UC IPM Pest Management Guidelines: Grape. UC ANR Publication 3448. Davis, CA.
Myers, L.E. Two Economic Greenhouse Mealybugs of Mississippi, Journal of Economic Entomology, Volume 25, Issue 4, 1 August 1932, Pages 891–896, doi.org/10.1093/jee/25.4.891
