A pest of walnuts in North America and Europe, walnut husk fly (Rhagoletis completa) adults can be identified by their distinct, triangular wing pattern and yellowish-white scutellum. As old insecticide tools are phased out, walnut growers seek new options for managing walnut husk fly. A new guide in the Journal of Integrated Pest Management details current research into walnut husk fly monitoring and management. (Photo by Jack Kelly Clark, courtesy University of California Statewide IPM Program)

By Moet Takata

In the fall, walnuts become ready to harvest when the green hull splits to reveal the nut shell. However, some hulls can become blackened and shriveled when infested by walnut husk fly (Rhagoletis completa), a major pest of walnuts in North America and Europe. This can cause shell staining and kernel damage, reducing the value of the walnut itself.

A New, Old Pest

The walnut husk fly is a yellowish-brown fly with a dark, triangular band on its wings. Though this fly has just one generation per year, adults can emerge across multiple months. Depending on the climate, this period can range from as early as mid-May to as late as October. Due to this prolonged emergence, managing this pest relies on intensive monitoring and well-timed insecticide sprays and often requires multiple applications throughout the season.

While native to the central United States, this fly has been a prevalent pest in California walnut orchards since the 1920s. Until the early 2000s, walnut husk fly was secondarily controlled by organophosphate sprays applied for codling moth (Cydia pomonella), another walnut pest. However, these sprays have largely been phased out by regulations, due to their harmful effects on the environment. Currently, the options are primarily pyrethroids and neonicotinoids, which have their own side effects on beneficial insects, or spinosad, which is slow-acting and has a shorter efficacy period.

Due to the shifting regulations, non-target pesticide effects, and compounding cost of multiple applications, walnut growers need alternative options to manage this pest. To explore this, the nonprofit Community Alliance with Family Farmers and researchers from the University of California Cooperative Extension and UC Berkeley, compiled a review on the biology, ecology, and management of walnut husk fly. This profile, published in the open-access Journal of Integrated Pest Management, covers some of the current research into walnut husk fly monitoring and management that may prove promising.

As old insecticide tools are phased out, walnut growers seek new options for managing walnut husk fly (Rhagoletis completa), a major pest than can cause shell staining and kernel damage. A new guide in the Journal of Integrated Pest Management details current research into walnut husk fly monitoring and management. As shown here, larval feeding on the walnut husk can cause external damage (top); early-season infestations can also cause significant internal damage (bottom). (Photos by Jhalendra Rijal, Ph.D.)

Honey, Vinegar, or Semiochemicals?

Since well-timed insecticides are crucial for effective control, there have been many studies on the efficacy of different monitoring traps and lures. The current standard is a flat, sticky panel trap with an ammonium carbonate (AC) lure, which catches a large number of flies but is not particularly selective toward walnut husk fly. To improve walnut husk fly monitoring tools, a research team led by Landry Sarles, Ph.D., at the University of Liège’s Gembloux Agro-Bio Tech identified two aggregation pheromones released by the husk fly males. Testing of these pheromones, δ-hexalactone and δ–heptalactone, is ongoing in California walnut orchards. Initially led by Steven J. Seybold, Ph.D., with the USDA Forest Service, and now by Robert Van Steenwyk, Ph.D., with UC Berkeley, field trials of these pheromones show promising results.

Both pheromones boost the trap counts when used in conjunction with the standard AC lure, though δ–heptalactone performs better than δ-hexalactone. The δ–heptalactone lure, alone or in combination with the AC lure, is more effective early in the season or in orchards with low walnut husk fly pressure. In addition, ongoing field trials show that adding δ–heptalactone increases the attractiveness of GF-120, a pre-mixed insecticide/bait mixture commonly used in organic orchards. Species-specific attractants such as this pheromone allow for early detection and more precise treatment.

A majority of walnut husk flies (Rhagoletis completa) pupate within the top 4 inches of soil and emerge after the overwintering period, though a small proportion of pupae may stay dormant for two years. As old insecticide tools are phased out, walnut growers seek new options for managing walnut husk fly. A new guide in the Journal of Integrated Pest Management details current research into walnut husk fly monitoring and management. (Photo by Jack Kelly Clark, courtesy University of California Statewide IPM Program)

Going Below Ground

While there have been efforts to utilize parasitic wasps and natural predators for biological control of walnut husk fly, their effects have been too low to effectively manage this pest. Therefore, current research efforts have shifted to target the walnut husk fly during its overwintering period. Around harvest time, larvae will drop from the walnut husk to the orchard floor and burrow into the soil to begin pupating.

Researchers Jhalendra Rijal, Ph.D., and Sudan Gyawaly, Ph.D., with the UC Cooperative Extension, saw potential in using entomopathogenic nematodes and fungi to control walnut husk fly. In preliminary lab trials, soil treatments of nematodes Steinernema feltiae and Heterorhabditis bacteriophora showed higher mortality of walnut husk fly than untreated soil. Based on these findings, they conducted field trials of the fungus Beauveria bassiana and a nematode combo of Steinernema carpocapsae, S. feltiae, and H. bacteriophora. By applying once in fall to target burrowing larvae and once in summer to target emerging adults, they saw a reduction of adult populations by 23% with B. bassiana and 29% with the nematode combo. These results are promising, and further research on entomopathogens will optimize their application against walnut husk fly.

As walnut husk fly spreads and walnut production changes, walnut growers will need to rely on alternative management methods. Cultural methods like mulching or discing are largely unexplored in research, and developed degree-day models are underutilized. Further research and exploration are needed if farmers are to effectively respond to this increasingly challenging pest.

Moet Takata is the Ecological Pest Management program manager at the Community Alliance with Family Farmers, a California nonprofit dedicated to building sustainable food and farming systems. She is based in Sacramento, California. Email: [email protected].