Drought Reduces Wheat Midge Populations For Second Year In A Row

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 (NDSU) Extension entomologist.

A total of 2,070 soil samples were collected from 22 counties in the fall of 2021 to estimate the statewide risk for wheat midge in the 2022 wheat-growing season. The distribution of wheat midge is based on unparasitized cocoons found in the soil samples.

"The majority of the soil samples had zero wheat midge cocoons in the soil for the past two years," says Knodel. "The percentage was 95% with no midge cocoons in 2022 and 86% in 2021. This is the record low since the wheat midge larval survey for overwintering cocoons started in 1995. In 2018, we had another low year with 84% of the soil samples with no cocoons."

Knodel states that only about 5% of soil samples were positive for wheat midge cocoons, the density ranging from 36 to 71 cocoons per square meter. This is a low risk for wheat midge infestation, which is classified as one to 200 midge cocoons per square meter.

"Low risk areas were scattered in eight counties throughout the state, including the northwest area (Divide, Mountrail and Renville Counties), north-central area (Bottineau, Benson and Pierce Counties), the west-central area (McLean County) and the northeast area (Ramsey County)," says Knodel.

"No soil samples had moderate or high cocoon densities of wheat midge (201 to over 800 midge larvae per square meter)," says Knodel.

"This dramatic decrease in wheat midge populations since 2019 is probably due to drought in 2020 and 2021," adds Knodel. "Drought can cause wheat midge to overwinter for two years instead of the typical emergence during the following season. Larvae also 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. Comparing precipitation from May through August with wheat midge cocoon densities for each surveyed county over the past 11 years shows a strong positive correlation between precipitation and wheat midge populations."

With the very low populations of wheat midge for two years in a row, producers may not have to scout for adult midges this year unless the field is continuous wheat, and/or favorable moist weather in late June to early July occurs during emergence. These two factors can cause rapid increases in the numbers of emerging adult wheat midges, especially in areas that did receive adequate precipitation last year.

Knodel recommends that producers still 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.

If wheat midge is detected, 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.

"This forecast is good news for growers since the risk for yield loss and reduced grain quality from wheat midge is low," says Knodel. "Unfortunately, the bad news is that the beneficial parasitic wasp can't survive without its host, wheat midge. To my surprise, no parasitized cocoons were found in 2021. This is the first time that no parasitic wasps were observed."

"Parasitic wasps play an important role in natural control of wheat midge and parasitize the eggs or larvae," she adds. "In contrast, the parasitism rate was 15% in 2020, 36% in 2019 and 9% in 2018."

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