February 15, 2017
Soil samples in North Dakota indicate low levels of overwintering wheat midge larvae in cocoons for the 2017 season, according to Janet Knodel, North Dakota State University Extension Service entomologist.
But there are two hot spots.
“Only 2% of the soil samples had economic population densities of wheat midge (greater than 500 midge larvae per square meter) this past year,” Knodel says. “These higher populations were located in east-central Divide and southeastern Burke counties in northwestern North Dakota.”
OVERWINTERING LARVAE: Soil samples showed low levels of wheat midge larvae overwintering in nearly all of North Dakota. The majority of the soil samples, 68%, had zero wheat midge cocoons.
“This is good news for North Dakota wheat producers as it will reduce the likelihood that insecticide will be needed for wheat midge control in wheat in 2017,” she says.
Wheat midge populations ranged from zero to 2,071 midge larvae per square meter, with an average of 42 larvae per square meter, in 2016. In 2015, wheat midge populations were slightly lower, ranging from zero to 429 midge larvae per square meter, with an average of 25 larvae per square meter.
“Other areas with low wheat midge populations (200 to 500 larvae per square meter) occurred in small, localized areas in northeast Bottineau, southeast Burke, central Divide, central McLean, northeastern Mountrail, northwestern Renville, northwestern Towner and central Ward counties,” says Knodel. “These population levels are still considered noneconomic and low risk for wheat midge.”
Knodel recommends scouting any at-risk wheat fields from heading to early flowering (more than 50% flowering), when wheat midge is emerging. A wheat midge degree-day model predicts the emergence of wheat midge and helps producers determine when to scout.
The wheat midge degree-day model is available on the North Dakota Agricultural Weather Network at ndawn.ndsu.nodak.edu/wheat-growing-degree-days.html.
Select your nearest NDAWN station and enter your wheat planting date. The output indicates the expected growth stage of the wheat and whether it is susceptible to midge infestation, as well as how far along the wheat midge emergence is, such as 50% females emerged.
Scout at night
Scout for the orange adult flies at night when temperatures are greater than 59 degrees F and the winds are less than 6 mph. Use a flashlight and slowly scan the heads of wheat plants for wheat midge adults, counting the number of flies per head.
The economic thresholds for wheat midge are: one or more midge observed for every four or five heads on hard red spring wheat, or one or more midge observed for every seven or eight heads on durum wheat.
Some bad news
The bad news for 2017 is that the beneficial parasitic wasp Macroglenes penetrans, which kills wheat midge eggs and larvae, also was low, with an average of 4.8% parasitism rate in 2016, according to Knodel.
PERCENT PARASITISM: Soil samples also showed low levels of a wheat midge parasite that keeps the midge populations under control.
Eighty-nine percent of the larval cocoons had zero incidence of parasitism in 2016, similar to the level in 2015 with 91%.
The highest parasitism rates were found in Burke, Bottineau and McLean counties. Because the parasitic wasp is dependent on its host, wheat midge, its populations are usually higher in areas where midge populations also have been high the past year.
“We need to continue to conserve parasitic wasp populations when possible by spraying insecticides only when wheat midge populations are at economic threshold levels, and avoiding any late insecticide applications to minimize the negative impacts on parasitic wasps that are active at that time. This tiny, metallic wasp does an excellence job keeping the wheat midge in check by providing free biological control of wheat midge in wheat fields,” Knodel says.
NDSU Extension Service agents collected the soil samples. The North Dakota Wheat Commission supports the wheat midge survey.
Source: NDSU Extension Communications
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