“Soil samples in North Dakota indicate low populations of overwintering wheat midge larvae (cocoons) for the 2024 season,” says Janet Knodel, North Dakota State University Extension entomologist. “This is good news for North Dakota wheat farmers, reducing their inputs for wheat midge management.”
A total of 2,060 soil cores were collected from 22 counties in the fall of 2023 to estimate the statewide risk for wheat midge in the 2024 wheat growing season. The distribution of wheat midge is based on unparasitized cocoons found in the soil samples.
Wheat midge was positive at only 20 field sites (10% of the sites) in eight counties including Cavalier, Nelson and Towner Counties in the northeast, Bottineau, Pierce and McHenry Counties in the north-central region, McLean County in the west-central region, and Ward County in the northwest. These sites had low levels of wheat midge cocoons (1-200 cocoons per square meter) which does not cause yield loss in spring wheat. No soil samples had moderate or high cocoon densities of wheat midge (201 to over 800 midge larvae per square meter) which could cause economic losses in spring wheat.
“The majority of the soil samples had zero wheat midge cocoons in the soil for the past four years (90% in 2023, 97.5% in 2023, 95% in 2021 and 86% in 2020),” says Knodel. “We believe that the populations of wheat midge are low due to the extended drought in northern North Dakota over the past several years. Populations could return to higher levels once the drought subsides and we return to more normal rainfall.”
Dry conditions will delay when wheat midge larvae drop out to the soil for overwintering in late summer. During drought, larvae will remain in the wheat head and are often harvested with the grain, ending up in the grain truck or bin. Dry soil conditions also increase wheat midge mortality by making it difficult for the larvae to dig into the soil for overwintering and by exposing them to predators on the soil surface.
“With the very low populations of wheat midge for the fourth year in a row, scouting for wheat midge will be most important in continuous wheat fields, and/or when moist weather occurs in late June to early July which favors wheat midge emergence and survival,” advises Knodel. “Wheat midge cocoons also can remain dormant for several years and adults then emerge when soil moisture is adequate. These factors can cause rapid increases in the numbers of emerging adult wheat midge. The most critical time to scout spring wheat for adult wheat midge is from heading- through the early-flowering stages.”
Knodel recommends that farmers 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/.
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 points out that the beneficial parasitic wasp that attacks and kills wheat midge is dependent on its host, wheat midge, for its survival. Since few wheat midge cocoons were detected, only one site had parasitic wasps in the 2023 soil samples with 100% parasitism in Nelson County. No parasitized cocoons were found the previous two years (2022 and 2021).
“Parasitic wasps play an important role in natural control of wheat midge and parasitize the eggs or larvae,” adds Knodel. “In contrast, the parasitism rate was 15% in 2020, 36% in 2019 and 9% in 2018.”
NDSU Extension county agents collected the soil samples and larval cocoons are extracted by the NDSU Extension Entomology laboratory. The North Dakota Wheat Commission supports the wheat midge larval soil survey.
Source : ndsu.edu