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Soil samples collected in North Dakota wheat fields indicate the lowest populations of overwintering wheat midge larvae (cocoons), since the inception of the survey in 1995, says Janet Knodel, North Dakota State University Extension entomologist. For the fifth year in a row, low populations of wheat midge were present, potentially reducing farmer's inputs for wheat midge management, she said. ADVERTISEMENT “Wheat midge can still increase in pockets where timely spring rains occur in 2025," Knodel said.
"So, it is always good insurance to scout wheat fields during the susceptible crop stages of heading through mid-flowering.” NDSU Extension agents collected a total of 1,920 soil core samples (10 cores per field) from 192 fields in 21 counties from August to October 2024. The distribution of wheat midge is based on unparasitized cocoons found in the soil samples.
Historically, wheat midge has caused significant loss in yield and quality in spring wheat and durum wheat in the northern tier of North Dakota counties. For 2024, wheat midge was positive at only three field sites (2% of the sites sampled) in three counties including Towner in the northeast, Pierce in the north-central region, and Ward in the northwest. These sites had low levels of wheat midge cocoons (1-200 cocoons per square meter), which does not result in yield loss in spring wheat the following year.
No soil samples had moderate or high cocoon densities of wheat midge (201 to over 800 midge larvae per square meter). “The majority of the soil samples had zero wheat midge cocoons for the past five years (98% in 2024, 90% in 2023, 97.5% in 2023, 95% in 2022 and 86% in 2021),” Knodel said.
“We believe that the populations of wheat midge are low due to the lengthy drought in North Dakota. Drought also caused wheat midge emergence to be more irregular and out of sync with susceptible crop stages for infestation, resulting in low populations. Dry conditions will delay when wheat midge larvae drop out to the soil for overwintering in late summer.
" Larvae will remain in the wheat head and are often harvested with the grain, ending up in the grain truck or bin, she said. Dry soil conditions also increase wheat midge mortality by making it difficult for the larvae to dig into the compacted soil for overwintering and by exposing them to predators on the soil surface. “With the very low populations of wheat midge for the fifth year in a row, scouting for wheat midge will be most important in continuous wheat fields," Knodel said.
"Wheat midge emergence and survival will be favored if North Dakota wheat growing areas receive over 1 inch of spring rains in May. Wheat midge cocoons also can remain dormant for several years and adults then emerge in future years when soil moisture is adequate. However, if more favorable environmental conditions, especially spring rains, return to North Dakota, it could favor the development of overwintering larvae in the soil during spring and adult emergence from late June to mid-July.
These factors can cause rapid increases in the numbers of emerging adult wheat midge.” IPM scouts and insect trappers also monitored for wheat midge using sex pheromone trapping during the field season, Knodel said. Trap monitoring is more sensitive to low densities of wheat midge in fields and alerts farmers to potential economic infestations.
Pheromone traps are used as an "early warning" system to trigger field scouting if the crop is in the susceptible stage. If more than 10 midges per trap are observed then field scouting should be initiated to determine if a field is at an economic threshold for wheat midge, she said. Trapping serves to document the distribution of the wheat midge and its parasitoids, as well as guiding scouting efforts and IPM practices.
ADVERTISEMENT A total of 20 pheromone traps were monitored in 18 counties in North Dakota in 2024. The past three years show a general decline in the total number of adult midges captured on traps: 2,287 adults in 2024, 8,446 adults in 2023, and 22,952 adults in 2022. The highest wheat midge counts were concentrated in the northwest and northeast counties of North Dakota.
Knodel said the beneficial parasitoid wasp that attacks and kills wheat midge is dependent on its host for its survival. It also continues to decline to low levels along with the wheat midge decline. No field sites had parasitic wasps in the 2024 midge soil samples.
Past parasitism rates were low only 5% in 2023, and 0% in 2022 and 2021. Higher parasitism rate were observed in 2020 (15%), 36% in 2019 and 9% in 2018. However, the pheromone trapping found wasps at 35% of the 2024 trap sites, 37% of the trap sites in 2023 and 50% of the trap sites in 2022, she said.
Dissecting wheat midge cocoons and trap monitoring for parasitoids help identify parasitoid distribution and overall success against wheat midge in North Dakota. In summary, soil cocoon sampling and trapping results show that wheat midge populations have declined significantly over the past years, Knodel said. However, pheromone trapping indicates that wheat midge is still widespread throughout the wheat-growing areas of the state.
As expected, the presence of parasitoid wasps also has declined recently. NDSU Extension county agents collected the soil samples and larval cocoons were extracted at the NDSU Extension Entomology laboratory. The North Dakota Wheat Commission supports the wheat midge larval soil survey and pheromone trapping.
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