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(originally published January/February 2017)

Navel orangeworm (NOW) populations exploded in 2017, costing growers tens of millions of dollars in reduced quality and lost yields. In a year where double-digit damage estimates from nut processors were not uncommon, the question heading into the coming growing season (and future seasons beyond 2018)—how do we limit damage from this pest? Is our current arsenal of integrated pest management (IPM) tactics enough to keep damage in the desired 1 to 2 percent range given the two million (plus) acres of commercial nut crop habitat in California (not to mention the myriad other crop and non-crop plants that play host to NOW)? This article covers the “tried-and-true” strategies, as well as where we need to head in the future of NOW management to ensure clean, safe, and profitable nut crops for years to come.

A four-pronged approach to NOW management in nut crops has been suggested for years based on University research and field success stories. These include: sanitation, minimizing damage by other sources, timely (early) harvest to avoid late generation flights, and insecticide treatments as deemed necessary by monitoring pest activity and crop phenology.

Sanitation

By now you have certainly received the message—SANITIZE! This single activity is the absolute backbone of all pest management targeting NOW, no matter the nut crop. This practice results in direct destruction of overwintering worms, as well as destruction of spring habitat for any part of the population that survived the winter (those remaining in the orchard or those migrating into the orchard from external sources outside of your control). In many cases, it’s simply not enough to get nuts on the ground, but additional destruction of the nuts is required in order to achieve maximum reduction in emergence, population build-up, and damage (NOW will lay eggs on and develop in ground mummies if that is all that is available).

Plenty of research summarizes the effectiveness of sanitation practices (e.g., Higbee and Siegel 2009, California Agriculture, Volume 63). Research has also suggested that females prefer to oviposit (lay eggs) on nuts previously damaged by NOW, and that development rate and survival success are both also positively correlated with previous kernel damage (Hamby and Zalom 2013, Journal of Economic Entomology, Volume 106). Therefore, all mummies are not created equal. Clearly, a mummy with live overwintering worm(s) is a bigger threat in the coming season than one without live worm(s). Live moth(s) will emerge from these mummies and give rise to subsequent generations, which will ultimately target the in-season crop at/after hull split. But, even a mummy that no longer contains a live worm come late winter-early spring, but had previous NOW damage, may be a more desirable and hospitable “home” for oviposition and early generation development (leading to greater population development as the season progresses).

Encourage your growers not to underestimate the value of sanitation, particularly in a year with very high potential carry over based on elevated damage in the 2017 harvest. Sanitation to the University-standard guidelines (average 0.2 mummies/tree southern San Joaquin Valley; average 2 mummies/tree northern San Joaquin Valley and Sacramento Valley) may not possible due to prohibitive costs, labor shortages, or inclement weather limiting orchard access (as was the case this past season). In these cases, it may become necessary to target sanitation efforts to get the most bang-for-the-buck (i.e., emphasize mummy reduction in blocks with the biggest NOW threat).

Crack-Out

How to determine which areas these are? This can be determined based on block-specific estimates of a combination of total mummy load, mummy infestation, and kernel damage. Mummy samples can be collected and cracked-out after harvest and before sanitation efforts (ideally, post-sanitation there will be too few mummies left to collect a meaningful sample in a timely manner). Note the percent infestation and kernel damage as well as total number of viable worms (multiple NOW can emerge from a single mummy). Extrapolations can then be made based on these data combined with estimates of total mummy load in the block, indicating where the highest potential NOW pressure may be in the coming season. There are newly available mummy “crack-out” services to assist in evaluating these parameters and how to best use the information for site-specific orchard management strategies targeting NOW.

What information can be used to facilitate “decision-support”? In other words, is treatment necessary? If so, when are the ideal timing(s)? These are million-dollar questions, and no single piece of information (to date) can provide fail-safe treatment guidelines. In fact, correlating in-season arthropod (insect and mite) population estimates with ultimate harvest damage is the IPM holy grail—and one that largely continues to elude researchers. A recent retrospective analysis of six years of data (Rosenheim et al. 2017, Journal of Economic Entomology, Volume 110) suggested that population NOW estimates taken just prior to harvest were the best predictors of almond damage. However, as practitioners are aware, treatment decisions are needed earlier than this for NOW in California’s nut crop systems.

Monitoring and Risk-Assessment

Let not your heart be troubled, however. We do have a number of different monitoring and risk-assessment methods, that when taken as a whole, may provide a basis for decision-support and treatment recommendations. These include using egg traps for establishing population biofixes that mark the onset of activity of each generation, pheromone traps for tracking adult male flight activity and relative population abundance, kairomone (ground almond-pistachio bait bag) traps for tracking adult female flight activity and relative population abundance, degree-day models for predicting population cycles, crop phenology landmarks (e.g., hull split) and their coincidence with pest activity, estimates of population pressure based on mummy evaluation (as described above), previous season harvest damage, proximity to external sources of infestation, environmental conditions, and the list goes on.

Refining these puzzle pieces into a useable risk-assessment model that can be validated across cropping systems, geographic regions, and a multitude of other variables, will require an ecoinformatics (“big data”) approach. Luckily, we are entering (in fact, already in) an era of crop management where technology is increasingly affording researchers, crop advisors, growers, and land managers improved methods of record-keeping, data analysis, and anonymized sharing of information in order to work toward solving these difficult crop production issues.

Future of IPM Management

What about the future of integrated pest management for NOW? Mating disruption is becoming more widely adopted among nut crop producers in California (particularly almond and pistachio). With the increased nut crop footprint in California and the ubiquitous and unrelenting nature of a pest like NOW, this may well become the 5th pillar in our basic NOW management strategy in the near future (in addition to the four noted early in this article). Multiple products are now available from a number of companies, which provides options for use and adoption, and may drive costs down due to increased market competition. Population reduction using mass trapping, or attract-and-kill, approaches are possible. The pistachio industry has invested in sterile insect technology, in which moths are irradiated, rendering them sterile, and then released into the “wild-type” population in order to reduce the number of successful matings. This technique has been successfully used for other serious crop pests in the United States, including pink bollworm and screwworm, and releases for medfly in areas of detection are ongoing in California. Widespread adoption of the “tried-and-true” management methods, as well as these novel approaches (and others, as they become available and are validated), will be needed for long term and effective management of NOW.

 

By: Emily J. Symmes Sacramento Valley Area IPM Advisor University of California Statewide IPM Program and Cooperative Extension

Photo 1. NOW adult

Photo 1. Adult navel orangeworm. Credit: University of California Statewide IPM Program.

Photo 2.NOW eggs on almond mummy copy

Photo 2. Navel orangeworm eggs on an almond mummy. Credit: Jhalendra Rijal.

Photo 3. NOW damage walnut

Photo 3. Navel orangeworm larva and damage in walnut. Credit: University of California Statewide IPM Program.

Photo 4. NOW damage almond

Photo 4. Navel orangeworm damage in almond. Credit: University of California Statewide IPM Program.