The Can Husk Fly is a significant pest of walnut trees, impacting both the quality and yield of the nuts. Understanding its life cycle, recognizing the signs of infestation, and implementing effective management strategies are crucial for walnut growers.
Adult walnut husk fly.
Identifying the Can Husk Fly
The adult can husk fly resembles a housefly in size but boasts distinctive markings. A yellow spot beneath the wing attachment and a dark triangular band at the wingtip differentiate it from other flies commonly found in orchards. Adult females are slightly larger than males, possess a pointed abdomen with an ovipositor (egg-laying organ), and have a straw-colored first leg segment, while the males’ is brown to black. A 10x hand lens can easily reveal these differences.
Walnut husk fly eggs.
Life Cycle and Behavior
Can husk flies have a single generation per year. They overwinter as pupae in the soil, emerging as adults from early June to early September, with peak emergence typically occurring between July and mid-August. In coastal regions, emergence can begin as early as mid-May. Female flies deposit clusters of approximately 15 eggs beneath the walnut husk’s surface. Within five days, these eggs hatch into white maggots that later turn yellow with black mouthparts. The maggots feed on the husk for three to five weeks, causing significant damage. Mature maggots then drop to the ground, burrow several inches into the soil, and pupate. Most emerge as adults the following summer, but some can remain in the soil for two years or more.
Damage Caused by Can Husk Fly
The can husk fly is a mid- to late-season pest affecting all walnut-growing areas. Black walnuts and all English walnut cultivars are susceptible, with some varieties like Tulare, Hartley, Serr, and Franquette being particularly vulnerable. The initial signs of infestation are small stings on the husk resulting from egg-laying. As maggots feed, the husk softens, blackens, and decays, although the outer skin usually remains intact. This decay stains the nutshell, rendering the nut unsuitable for in-shell sale. Early-season infestations (late July to mid-August) can lead to shriveled kernels, mold growth, and yield reduction. Early damage can result in a 30% loss in nut value. Late infestations cause less kernel damage but can stain shells and hinder hull removal.
Walnut husk damage caused by can husk fly maggots.
Managing Can Husk Fly Infestations
Not all orchards require annual insecticide treatment for can husk fly. When necessary, precise timing is critical and varies depending on the insecticide, monitoring method, and specific orchard conditions. Husk flies cease to be a problem after husk split. Growers facing consistent late-season damage may consider using ethephon to accelerate maturity and husk split.
Monitoring Techniques
Trapping: Utilizing unbaited yellow sticky traps (Pherocon AM NB) super-charged with ammonium carbonate is crucial for monitoring adult fly populations. These traps should be placed high in the tree canopy on the north side, preferably in areas known for previous infestations or near black walnut trees. Monitoring should begin by June 1st and continue at least twice weekly, ideally three times a week.
Egg Detection: Examining trapped female flies for eggs allows for more precise insecticide timing. Eggs appear as small, pearly white grains of rice. Insecticide application should occur upon finding the first female with eggs.
Sting Monitoring: While less effective for determining treatment timing, monitoring for stings on nuts can indicate the effectiveness of insecticide applications. Fresh stings, even after insecticide application, may suggest inadequate control.
Treatment Options
Several insecticides, often used in conjunction with baits, are effective against can husk fly. Baits attract flies to the insecticide, enhancing control. Choosing insecticides with different modes of action is important for resistance management. Consult your local agricultural extension for the most current recommendations on registered insecticides and their appropriate usage. Always follow label instructions carefully. Organically acceptable methods, such as GF-120 and Entrust (spinosad with an organic bait), are also available.
Conclusion
Effective can husk fly management requires a multifaceted approach that combines diligent monitoring with timely insecticide applications. By understanding the pest’s biology and utilizing appropriate control strategies, walnut growers can minimize damage and ensure the production of high-quality nuts. Regular monitoring and adherence to integrated pest management principles are essential for sustainable and effective can husk fly control.
