Control of Walnut Husk Fly
I own one walnut tree. Last year, and this year the nuts have been devastated by the Walnut Husk Fly. I used some of the crop, tho the nuts were brown. I sprayed the ground under the tree with Neem Oil in August as suggested by the Master Gardener. The affected nuts are more numerous than last year. 75% of the crop. Do the grubs live over the winter in/on in the discarded husks? Do they bore into the fruit? Should all the husks be gathered and burned? Is there any more effective product to use than Neem Oil? How many applications should be made? Thank You, Judith Goldmann of Hillsboro
After the larvae finish feeding on the green husks making them black and blotchy they drop to the ground. The flies pupate in the soil within a protective case called the puparium which is the hardened dead exoskeleton of the last larval stage. This makes it difficult for a ground insecticide treatment such as you have tried to work effectively. The adult flies emerge from the soil the following season or may delay emerge until two winters have elapsed.
You are correct that a very good cultural practice is to frequently rake and destroy the fallen infested nuts. This kills the larvae/maggots before they get into the ground.
Killing of adult flies will prevent egg laying on the husks. The adult flies are easy to kill as they are very attracted to bait sprays. The commercial formulation is GF-120 Naturalyte fruit fly bait, which contains an organic insecticide (spinosad) and sweet feeding attractants (http://www.harmonyfarm.com/gf-120-nf-naturalyte-fruit-fly-bait/). You then spray it out on your tree when the adult flies are active, but it will need to be re-applied every 7 days or so. Because the flies are attracted to the bait droplets coverage of the tree is less important. You can also try to formulate your own version of GF-120 with molasses mixed with spinosad marketed to homeowners such as Monterey Garden Spray. No guarantees there.
You can determine adult activity from traps. Essentially they are yellow sticky cards with ammonium bait. See : http://jenny.tfrec.wsu.edu/opm/displaySpecies.php?pn=160.
More conventional sprays against the adults can be used to reduce the population, but it is nearly impossible for homeowners to achieve good spray coverage on a mature walnut trees. More information on walnut husk fly can be found here: https://pnwhandbooks.org/insect/nut/walnut/walnut-walnut-husk-fly
You could also try a fall or early spring soil treatment of entomopathogenic nematodes (http://www.arbico-organics.com/category/beneficial-nematodes?gclid=Cj0KEQjw4fy_BRCX7b6rq_WZgI0BEiQAl...) Good luck!
Thank you for your response. It is complicated action to take; it appears that the husk fly is a difficult problem . I will try the remedies that you have suggested. sincerely, Judy Goldmann