2 Hot Spots for N.D. Wheat Midge Populations in 2017

Soil samples in North Dakota indicate low levels of overwintering wheat midge larvae (cocoons) for the 2017 season, according to Janet Knodel, North Dakota State University Extension Service entomologist.

A total of 201 soil samples were collected from 21 counties in the fall of 2016 to estimate the regional risk for wheat midge in 2017. The distribution of wheat midge is based on unparasitized cocoons found in the soil samples.

"Only 2 percent 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. Fortunately, the majority of the soil samples, 68 percent, had zero wheat midge cocoons."

Knodel adds, "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."

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 wheat fields that are at risk from heading to early flowering (more than 50 percent 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.

Producers can access the wheat midge degree day model on the North Dakota Agricultural Weather Network (NDAWN) at https://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 is susceptible to midge infestation, as well as how far along the wheat midge emergence is, such as 50 percent females emerged.

Scouting for the orange adult flies is conducted at night when temperatures are greater than 59 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.

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 percent parasitism rate in 2016, according to Knodel. Eighty-nine percent of the larval cocoons had zero incidence of parasitism in 2016, similar to the level in 2015 with 91 percent.

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.

Knodel adds, "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."

NDSU Extension Service agents collected the soil samples. The North Dakota Wheat Commission supports the wheat midge survey.