White or bleached heads are a common sight in wheat fields throughout South Dakota. There are two organisms that could be blamed for this kind of damage, an insect called wheat stem maggot (Meromyza americana) and a disease called Fusarium head blight/Scab caused by Fusarium fungus.
While the damage they cause looks very similar from a distance, there are some easy clues that can differentiate between them. Wheat stem maggot damage results in a completely white/dry head including the stem all the way to the first node where the flag leaf is attached (Figure 1).
Fusarium head blight results in partial or completely dried heads with pinkish kernels and purple discoloration on the stem below the head (Figure 2).
|Fig. 1. Wheat stem maggot damage.
|Fig. 2. Scab infected wheat head.
Wheat Stem Maggot: Pest Profile
The wheat stem maggot is a small yellowish white fly (1/5 inch) with bright green eyes (Figure 3). Adults are nectar feeders and lay eggs on the leaves and stems of wheat plants. The hatched larva burrows into the stem and feeds inside it chewing near the flag leaf (Figure 4).
As a result of this chewing, the nutrient supply to the head is totally cut off leading to the dead and unfilled white head, while the flag leaf and the stem beneath it is still green. When pulled, these white heads come off easily breaking at the chewed point just below the flag leaf.
|Fig. 3. Wheat stem maggot adult.
|Fig. 4. Wheat Stem maggot larva inside the stem.
Phil Sloderbeck, KSU Entomology Department
Wheat stem maggot can survive on several grassy hosts and have two generations in a year. Although the number of white heads in a wheat field might appear concerning, the maggot is considered to be a minor pest. Research at North Dakota State University showed that a foliar application of lambda-cyhalothrin at 4-6 leaf stage or at the flag leaf stage significantly reduced the percentage of damaged stems. However, the study also states that a chemical treatment is not justifiable because it is not economical to treat such a minor insect population.