Figure 1. Wheat stem maggot.
Courtesy: J. McMechan, University of Nebraska Extension
While scouting wheat fields throughout South Dakota, we have started noticing the presence of bleached heads scattered throughout many different fields. These discolored heads are the result of an infestation of the wheat stem maggot (Figure 1).
The symptoms associated with wheat stem maggot infestations can be confused with symptoms of wheat scab. The difference between wheat scab and wheat stem maggot symptoms is that plants infected by wheat scab will have partially bleached or completely bleached heads and the peduncle remains green or may be slightly discolored (Figure 2). Bleached spikelets by scab may also have a pinkish color. Plants infested with the wheat stem maggot will have entire heads and peduncles that are bleached (Figure 3). If the discolored head of an infested plant is pulled on, it will easily come off from the rest of the plant.
Fig. 2. Scab symptoms (bleached spikelets and florets) on wheat heads.
Fig. 3. Wheat stem maggot infested wheat plant with bleached head and the peduncle. Credit: A. Varenhorst
Lifecycle & Behavior
Wheat stem maggots have two or possibly three generations per year in South Dakota. The first generation of wheat stem maggot adults emerge in early June and lay their eggs on the leaves of grasses, including wheat. The maggots hatch in mid-June to July and will tunnel into the stems of the host plants. The wheat stem maggots feed for 1-2 weeks and then pupate within the stems. The adults of the following generation will emerge in late July to August. The eggs laid by the second generation will either overwinter in grasses or emerge and continue their lifecycles in the fall.
The wheat stem maggot infestations that were observed while scouting were relatively minimal. Historically, this is not a yield limiting pest so we do not recommend the use of insecticides to manage it. Even when insecticides are used, no yield differences are observed within the field when compared to areas where insecticides were not applied. This pest can be managed by rotating to a different crop and eliminating volunteer wheat and grass hosts within fields and around the field margins.