ISSUE 2 May 11, 2006
WHEAT MIDGE OUTLOOK FOR 2006
Wheat midge was first detected in 1994 in the northeastern counties bordering Manitoba, Canada. Wheat midge continued to expand into the north-central region and eventually moved westward into northwestern North Dakota and Montana. Economic damage was common in 1995 through 2001. However, for the past several years there has been a general decline in wheat midge populations, causing less economic damage to wheat and durum fields. The annual soil survey conducted last year detected low levels of overwintering wheat midge larvae for the upcoming 2006 season (see wheat midge map). Wheat midge populations ranged from zero to less than 500 midge larvae per square meter, with most of the state having less than 200 midge larvae per square meter during 2005. Areas with 201 to 500 midge larvae per square meter included southern Mountrail County, central McLean County and northwestern Towner County. Fields with more than 1,200 midge larvae per square meter are considered high risk. At that point, some control tactic must be used to reduce midge populations.
There are several reasons for the decline in wheat midge populations. A majority of the 2005 wheat crop was planted early or late, avoiding the peak emergence period of adult wheat midge. Other factors included poor egg laying conditions, with night temperatures below 59 degrees and winds exceeding 6 mph, and a tiny, black parasitic wasp that helped control wheat midge populations. This wasp, Macroglenes penetrans, parasitizes the wheat midge larvae and eggs, and will emerge the following spring and kill the wheat midge larvae. Parasitism can range from 0 percent to 100 percent, with the higher rates occurring in areas where midge populations have been high the past few years (see parasitism map). With the wheat midge population being low for the past several years, the survey is starting to observe a decline in the parasitic wasp population. Weather conditions and the stage of wheat will be important in determining if sporadic outbreaks of wheat midge will cause economic damage this year. Environmental conditions favoring wheat midge development include moist soil conditions prior to emergence in late June to early July; warm, high humidity and light wind conditions during egg laying. If favorable environmental conditions exist for wheat midge populations, economic injury still can occur if wheat or durum is in the susceptible stage (heading to 50 percent flowering) during wheat midge emergence and egg-laying. Crop injury from wheat midge reduces yields and lowers the grade of harvested grains. The wheat midge survey is conducted by the NDSU Extension Service and supported by the North Dakota Wheat Commission.