Timing of adult emergence from the soil varies by geography with first adult emergence observed in mid-June in Nebraska and early July in Minnesota (Knodel, 2019). Adults have a long emergence window – overwintering generation adult emergence extended over a 17-day period in a Corteva Agriscience study in 2019 and as long as 37 days in a 2021 study (Figure 10). Adults live three to five days and do not feed on soybean plants (Calles-Torrez et al., 2020).
Females lay eggs in cracks and fissures in soybean stems. Females do not pierce the stem tissue when laying eggs. Larval infestation of soybean plants has not been observed prior of the V2-V3 growth stage. At this stage of soybean growth, the stem diameter expands creating small fissures allowing the overwintering generation adults to deposit eggs into the stem (McMechan et al., 2021c). Prior to V3, the soybean stems do not have these fissures.
Newly hatched larvae feed under the epidermis of the stem and go through three instars. Larvae drop off the plant to the soil, where they form larval cocoons and pupate (Calles-Torrez et al., 2020). Adults then emerge and repeat the cycle. Adults are not strong fliers, so are limited in their mobility. The effect of wind in dispersing adults is under investigation.
Based on observations so far, soybean gall midge appears to go through two or three overlapping generations per season. The substantial overlap between generations makes it difficult to detect discrete generations within the growing season, and larvae can be present in an infested field continually over the majority of the growing season. The timing of adult emergence cessation in the fall appears to be relatively consistent from year to year (McMechan et al., 2021a).
Two other host species for soybean gall midge have been identified – alfalfa and sweet clover. There is no apparent need for management in these alternate hosts. Populations observed in alfalfa have been relatively low (McMechan et al., 2021a).
Figure 10. Trap set up following soybean planting to measure soybean gall midge adult emergence from the soil in 2019.