Wheat Head Army Worm – a new grain pest?
Recently the Washington state grain graders contacted Davenport Union Warehouse about some of their wheat samples that contained about 2% broken kernels and would only gain Sample Grade. Initially it looked as though grain borers or grasshoppers were causing the damage, but Kevin Reed of the McGregor Company in Davenport did some excellent scouting in affected fields and found pupae that are most likely from the Wheat Head Army Worm. WSU entomologists Dave Bragg and Rich Zack will maintain the pupae until moths emerge so that they can confirm that the insect is the wheat head army worm, Faronta diffusa.
You may view photographs of damaged grain and the life stages of the insect at the Spokane County Extension website, http://www.spokane-county.wsu.edu/smallfarms/index.htm and look under “What’s New”.
This confirmation will be first record of the wheat head army worm (WHAW) causing noticeable crop damage in Washington, and the insect has seldom even been found in the state. According to reports from Kansas, Colorado, and Alberta, the WHAW is a minor grain pest but occasionally it does cause noticeable crop injury. This is usually detected only at harvest and may be confused with damage from grain storage insects, as occurred in this situation.
According to Davenport and Odessa Unions, the infestation is localized and sporadic within a 10-mile radius of Davenport. Winter wheat is affected primarily though there is a little evidence of the insect in some spring grains. Please contact Diana Roberts, WSU Extension, if you find infestations beyond this area.
Small plots of barley at the WSU variety test site at Reardan suffered heavy damage, but John Burns (WSU Extension Agronomist) believes this is due to their date of grain development and he could see no obvious varietal differences.
The WHAW overwinters in the pupa stage. Adults emerge in the spring and lay eggs on many grass species, but wheat is the preferred host. The larvae that hatch from the eggs feed on maturing grain heads and chew directly into the developing kernels. They drop to the ground and pupate by harvest time. The pupae are about one inch long, dark brown, and shiny. They lie at the base of affected plants right where the crop residue layer meets the soil surface (about 1 inch deep).
The best news is that up to one third of the pupae we found near Davenport are parasitized by another insect, probably a wasp species. Tiny white larvae devoured and then broke out of one of the pupae I had. They have pupated also – see the photos on the website listed above.
In Alberta the WHAW only completes one generation per year, but further south in the USA (Kansas and Colorado) a second, fall flight of the moths may occur. Warm, dry weather favors the survival of the insect. It is possible that reduced tillage or direct seeding allows more of the pupae to survive and develop into moths.
As infestations in other states have been infrequent, researchers have not developed economic thresholds and there are no insecticides labeled specifically for WHAW. Farmers applying pesticides for WHAW close to harvest may have to delay harvest and risk yield loss to meet preharvest label requirements.
I recommend scouting for WHAW in the affected areas next season starting about two weeks after grain anthesis. As the larvae are most active at night and tend to hide during the heat of the day it is best to sweep early in the morning. Infestations are usually heaviest around field borders so check field interiors to get a realistic idea of the problem. I am optimistic that variable weather conditions and the insect predators occurring naturally in the area will keep the wheat head army worm populations below damaging levels so they will be only occasional pests, as in other states.
For further information on the wheat head army worm or to report an infestation, contact Diana Roberts at 509-477-2167 or e-mail <robertsd@wsu.edu>.
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