By Jan Knodel
This is the second year in a row with good news for North Dakota wheat producers.
Wheat midge likely will be less of a problem this year, reducing the need for insecticide applications.
Soil samples in North Dakota indicate low levels of overwintering wheat midge larvae (cocoons) for the 2018 season, according to Janet Knodel, North Dakota State University Extension entomologist.
A total of 2,000 soil samples were collected from 21 counties in the fall of 2018 to estimate the regional risk for wheat midge in the 2019 field season. The distribution of wheat midge is based on unparasitized cocoons found in the soil samples.
“Less than one percent of the soil samples had economic population densities of wheat midge (greater than 500 midge larvae per square meter) this past year,” Knodel says. “The hot spot was located in one soil sample in the central area of both Burke and Ward counties in North Dakota. The majority of the soil samples, 84 percent, had zero wheat midge cocoons, which is a new record low for the wheat midge larval survey since it has been conducted in 1995.”
Knodel adds, “This is the second year in a row with good news for North Dakota wheat producers as it will reduce the likelihood that insecticide will be needed for wheat midge control in wheat in 2019.”
Wheat midge populations ranged from zero to 607 larvae per square meter, with an average of 17 larvae per square meter in 2018. Wheat midge populations were slightly higher in 2017 and 2016, ranging from zero to 1,321 in 2017 and 2,071 larvae per square meter in 2016.
“Other areas with low wheat midge populations (200 to 500 larvae per square meter) occurred in one small pocket in west-central Ward County,” says Knodel. “These population levels are still considered noneconomic and low risk for wheat midge.”
Knodel believes that the drought in the northwestern and north-central areas of North Dakota caused high mortality to wheat midge for the last two years. Larvae are susceptible to dryness and require dew or rain to drop out of the wheat heads and dig into the soil to overwinter as cocoons.
Knodel notes, “It is always good integrated pest management to scout for wheat midge adults during emergence, especially if wheat fields are in the heading to early flowering (less than 50 percent flowering) crop stage, the susceptible time period for wheat midge infestation.”
A wheat midge degree-day model predicts the emergence of wheat midge, and helps producers to determine when to scout and if their wheat crop is at risk. Producers can access the online wheat midge degree-day model on the North Dakota Agricultural Weather Network (NDAWN) at
https://ndawn.ndsu.nodak.edu/wheat-growing-degree-days.html.
To use the model, select your nearest NDAWN station and enter your wheat planting date. The output indicates the expected growth stage of the wheat and whether it is susceptible to midge infestation, as well as how far along the wheat midge emergence is, such as 50 percent females emerged.
Scouting for the orange adult flies should be conducted at night when temperatures are greater than 59 F and the winds are less than 6 mph. Use a flashlight and slowly scan the heads of wheat plants for wheat midge adults, counting the number of flies per head.
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, “The sad news for 2018 is that the beneficial parasitic wasp, Macroglenes penetrans, which naturally controls wheat midge eggs and larvae, continues to be at record lows with only a 4.8 percent parasitism rate in 2018. The parasitism rate has been low since 2015 - 4.2 percent in 2017, 4.8 percent in 2016 and 3.7 percent in 2015. Ninety-one percent of the larval cocoons had zero incidence of parasitism in 2018.”
2018 Wheat Midge Larval Survey - Midge Per Square Meter
2018 Wheat Midge Larval Survey - Percent Parasitism
The highest parasitism rates were observed in Divide, McLean and Nelson counties. Because the parasitic wasp is dependent on its host, wheat midge, its populations decreased as midge populations also decreased during the past few years.
Knodel emphasizes, “We need to continue to conserve parasitic wasp populations by scouting for wheat midge and spraying insecticides only when wheat midge populations are at economic threshold levels. Parasitic wasps fly later than wheat midge, so avoiding any late insecticide applications also will reduce the negative impacts on these ‘good’ insects. This tiny, metallic wasp does an excellence job keeping the wheat midge in check by providing free biological control of wheat midge in wheat fields."