Low Forecast For Wheat Midge Continues Into 2023

Soil samples from North Dakota counties indicate low levels of overwintering wheat midge larvae (cocoons) for the 2023 season, says Janet Knodel, North Dakota State University Extension entomologist.

A total of 2,040 soil cores were collected from 22 counties in the fall of 2022 to estimate the statewide risk for wheat midge for the 2023 spring wheat growing season. The risk for 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 three years," Knodel says. "The percentage was 97.5% with no midge cocoons in 2023, 95% in 2022 and 86% in 2021. This is the record low since the wheat midge larval survey for overwintering cocoons started in 1995."

Only about 2.5% of soil samples were positive for wheat midge cocoons, with density ranging from 36 to 143 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 (Burke County), north-central area (Benson, McHenry and Rolette Counties), and the west-central area (McLean County)," Knodel says.

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

Knodel adds, "This dramatic decrease in wheat midge populations since 2019 is probably due to drought in 2020 through 2022. 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. In some locations, wheat midge larvae remained in the wheat heads during harvest due to the dry environment and ended up in the harvest trucks instead of the soil. Comparing precipitation from May through August with wheat midge cocoon densities for each surveyed county over the past 12 years shows a strong positive correlation between precipitation and wheat midge populations."

Another reason wheat midge infestation risk is so low could be due to the late spring wheat planting in 2022 due to the cool, wet conditions in early May. Late planting dates reduce the risk of infestation due to the wheat heading after peak emergence of wheat midge.

With the very low populations of wheat midge for the third year in a row, night scouting for adult midges in spring wheat fields is not pressing, 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 spring 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 and percent of wheat midge emerged.

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.

Knodel says, "This forecast is favorable for growers since the risk for yield loss and reduced grain quality from wheat midge is low. Unfortunately, the bad news is that the beneficial parasitic wasp that attacks and kills wheat midge can't survive without its host. No parasitized cocoons were found in 2022 and 2021. This is the second 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 and Extension entomology specialists extracted the larvae from the soil samples. The North Dakota Wheat Commission supports the wheat midge survey.