Updated April 2, 2020:
The timing of emergence and source of soybean gall midge adults was conducted in 27 fields across four midwestern states (NE, IA, MN, and SD) during the spring, summer and fall of 2019. In late March, adult emergence cages were deployed in the southern region of the monitoring network near Eagle, NE with the last cage deployed in Minnesota by mid-April. Due to the proximity and distance between cages, most states were able to check for adults every 4-7 days during the growing season. In east-central Nebraska, the close proximity of field sites allowed for monitoring of emergence cages every 2-3 days through late May followed by daily monitoring from late May until early September. A few exceptions were made during the growing season as a result of extreme or dangerous weather.

Foliar insecticide studies were conducted on-farm with growers in east-central and northeast Nebraska, and southwestern Minnesota with different insecticide products. Applications of insecticides were expected to occur at first emergence, however, low levels of adult activity led to a delay in insecticide applications for most research sites. In east-central Nebraska, a foliar insecticide study was conducted on small plots with four separate timings, with the first application occurring two days prior to first adult emergence, the second at first adult emergence, the third application at peak emergence, and last application at 4 days after peak emergence. Each timing was applied to a different plot. At each timing, two chemistry classes (pyrethroid and organophosphate) were tested in separate plots. In eastern Nebraska and western Iowa, additional insecticide studies were conducted on seed, in-furrow, and foliar treatments encompassing a total of 27 treatments.

Standardized methods for assessing soybean gall midge injury to plants have not been determined, as a result research was collected on different evaluation methods in each of the states. In Nebraska, two samples of 20 plants were taken from each plot and dissected for the presence or absence of soybean gall midge larvae approximately one month after the first emergence of adults. A subset of 10 plant samples were taken from each plot for the timing of insecticide study to determine the total number of larvae per plant. A second evaluation of larval presence was taken on all plots in late August. In Iowa, plots were evaluated weekly from soybean emergence to senescence on rating scale from 1-5 to document the severity of injury for each plot. Random plants were assessed in the field to determine the presence of larvae. In Minnesota, larval counts were taken on plants at different distances from the field edge.

To determine the role of plant injury for successful soybean gall midge infestation, adults were collected from emergence cages in the field and exposed to soybean plants in pots in the greenhouse equipped with time-lapse cameras that monitored adult activity over a 3-day period. A second study was conducted in the field where plants were injured using a simulated hail machine. Individual plant samples were collected from the field and larval presence relative to points of injury were assessed and compared to plants without hail injury.

On June 14, soybean gall midge adults were collected in east-central Nebraska and northwest Iowa. These sites are at geographic distinct latitudes, raising concern for future modeling of adult emergence. By June 21, adult emergence had occurred at 23 of the 27 sites with the total number of overwintering adults collected ranging from 1 to 98 per site. The number of cages at each of these sites ranged from 6 to 36 with more cages at sites where the landscape source of adult emergence was monitored. The duration of adult emergence across these overwintering sites varied between 3 and 23 days for each site. For sites with more than one adult capture, an average duration of 14 days of adult overwintering emergence was recorded. Over the entire season, a total of 2,625 soybean gall midge adults were collected from emergence cages across the 23 sites. Of that total, 2,076 adults were collected in east-central Nebraska, followed by 271 in southwest Iowa, 156 in northeast Nebraska, 102 in northwest Iowa, 19 in southwest Minnesota and one adult in eastern South Dakota. Nearly all soybean gall midge adults emerged from last year’s soybean fields with exception of a few that were collected from cages just off the field edge. No adults were collected from ditches on either side of the road between fields or in fields that were planted to corn the previous year. In mid-July, soybean gall midge adults were collected from this year’s soybean fields in east-central Nebraska. A second adult activity period was observed from this year’s soybean in mid-August. Daily monitoring of adults in east-central Nebraska shows that there were only two times during the season where adult emergence ceased for more than five consecutive days.

Of the insecticides tested, none provided complete control of soybean gall midge with the best results providing a 50% increase in soybean yield when compared to untreated plots. Significant injury to soybean plants from soybean gall midge was observed at all sites. With no treatment providing complete control, it is difficult to determine the yield potential of soybean in the absence of soybean gall midge pressure at these sites. A number of the sites where insecticide efficacy was tested were soybean-soybean rotations, raising concern about the interpretation of these studies relative to a corn-soybean rotation.

Five growers in east-central and northeast Nebraska initiated insecticide applications along field edge based on the alert system and achieved yield results that were similar to small plot studies. Insecticide timing studies found a significant yield difference for an organophosphate applied at peak emergence or 4 days after peak emergence. In contrast, pyrethroids treated plots were significantly greater for all application timings with the average yield difference of 12 to 15 bushels per acre. Numerical reductions in larval counts and for the percentage of infested plant were observed, however, none were found to be statistically different. This lack of difference may be due to the timing of when samples were pulled from the field.

Adults collected from emergence cages in the field were exposed to soybean plants in pots in the greenhouse equipped with time-lapse cameras that monitored activity over a 3-day period. Cameras captured a female adult around areas at the base of plant where stem expands causing stretch marks in the tissue. Time-lapse cameras placed in the field on V1 stage plants documented the symptomology of soybean plants over a 25-day period. These plants had no unnatural or physical wounds yet they became infested with soybean gall midge. Darkening of plant stems occurred soon after the V3 stage. These observations provide some evidence that females are likely laying eggs into natural openings in the plant during the early vegetative stage (~V3). Plants with no signs of physical damage almost exclusively showed signs of infestation near the soil surface below the cotyledon node. When simulated hail was applied to soybean plants, infestations were observed up to 15-cm from the soil surface in areas where hail stones had made contact with the plant. Such observations suggest that soybean gall midge may take advantage of these wounds.

Soybean gall midge is a potentially serious emerging insect pest. Soybean gall midge maggots in low numbers have the potential to cause major losses, especially near the edges of soybean fields. The insect seems to be spreading east and west from the Missouri River valley. Off-cycle, emergency funding from the NCSRP enabled field emergence monitoring and the development of a communicated warning system for farmers, insecticide timing and efficacy evaluations, and greenhouse studies that gave some insight about infestation timing and mechanisms. All of this work helped provide researchers with meaningful improvements in their understanding of the insect pest to assist farmers in the short term, and critical information that will be used for experiments and the development of management strategies for the long term.