Dry Weather Decreases Risk For Wheat Midge In 2021

Soil samples in North Dakota indicate lower levels of overwintering wheat midge larvae (cocoons) for the 2021 season, says Janet Knodel, North Dakota State University Extension entomologist.

A total of 2,050 soil samples were collected from 22 counties in the fall of 2020 to estimate the statewide risk for wheat midge in the 2021 field season. The distribution of wheat midge is based on unparasitized cocoons found in the soil samples.

"The majority of the soil samples, 86%, had zero wheat midge cocoons in the soil," Knodel says. "This is the record low since the wheat midge larval survey for overwintering cocoons started back in 1995. In 2018, we had another year with 84% of the soil samples with no cocoons."

Twelve percent of the soil samples were at low risk, with one to 200 midge larvae per square meter.

Scouting for adult midges always is recommended because wet weather can cause rapid increases in the number of midges during emergence.

The moderate risk level (201 to 500 midge larvae per square meter) was observed in 1.5% of the samples.

"Moderate risk areas were scattered in three counties throughout the state, including the central area (Wells County), the west-central area (Mountrail County) and the northwestern area (Divide County)," Knodel says.

"Only 0.5% of the soil samples had very high population densities of wheat midge (greater than 800 midge larvae per square meter)," Knodel says. "The hot spots were located in south-central Divide County, close to the Williams County line and southeastern Mountrail County.

"These areas will definitely need to be scouted for wheat midge to make effective management decisions," Knodel says. "Insecticides may be needed to reduce yield losses from economic populations of wheat midge."

Some tips for monitoring for wheat midge include scouting during the night, when wheat midge is most active, warm night temperatures about 59°F and winds less than 6 mph. Wheat is most susceptible to midge infestations from heading to early flowering (less than 50% flowering).

"Lots of mosquito repellent and a flashlight are needed to effectively count the number of orange-colored flies per wheat head," Knodel says.

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.

Knodel adds, "This dramatic decrease in wheat midge populations from 2019 is probably due to dryness in 2020, especially in August. Larvae are susceptible to dryness and require rain to emerge from the soil in late June through mid-July, and to drop out of the mature wheat heads and dig into the soil to overwinter as cocoons. Plotting precipitation from May through August against wheat midge cocoons for each surveyed county over the past 10 years shows a strong positive correlation between precipitation and wheat midge populations."

Knodel recommends that producers use the wheat midge degree-day model to predict the emergence of wheat midge and to determine when to scout, and if their wheat crop is at risk.

Producers can access the wheat midge degree-day model on the North Dakota Agricultural Weather Network (NDAWN) website 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 the crop is susceptible to midge infestation, as well as the timing of wheat midge emergence.

Knodel says, "Although the forecast is good for wheat growers to avoid the ravages of wheat midge, the bad news for 2021 is that the beneficial parasitic wasp is at a historic record low, with only 1.5% of the wheat midge cocoons parasitized.

"Parasitic wasps play an important role in natural control of wheat midge and parasitize the egg or larva," she adds. "In contrast, the parasitism rate was 15% in 2020, 36% in 2019 and 9% in 2018. Parasitism was only observed in Williams, Mountrail and Ward counties."

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