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How to identify it
- The adult is a yellowish-white fly about 5 mm (1/4 inch) long with three conspicuous black stripes along the thorax and abdomen and bright green eyes. The Pacific Northwest species are light to dark green or green-yellow with a dark stripe down the top of the thorax.[1][2]
- The maggot is white and legless with a pair of hook-like mouthparts; a single maggot is found inside the straw just above the last node of a damaged stem.[1]
- During seed development the head and upper internodes of an infested stem turn white while the lower stem and leaves remain green. The stem pulls out easily, and if no larva is found the stem is usually cut off inside.[1][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Hessian fly | Hessian fly damage is managed with the same delayed planting dates, but wheat stem maggot damage is the white head on an otherwise green stem with a tunnel two or three inches long and a single white maggot above the top node.[1] |
| Wheat stem sawfly | Both insects feed inside wheat stems; a wheat stem maggot stem pulls out easily just above the last node and holds a single white, legless maggot with hook-like mouthparts, in a tunnel two or three inches long.[1] |
Where it lives
The High Plains IPM Guide treats Meromyza americana as a pest of wheat and other small grains and grasses on the High Plains; the Pacific Northwest handbook covers the related Meromyza pratorum and M. saltatrix, which have severely damaged spring barley at Klamath Falls, Oregon.[1][2]
Biology and life cycle
The wheat stem maggot overwinters as a larva in the lower stems of wheat and other hosts, pupates in spring, and the adults emerge in June. Females lay about 30 eggs on leaves and stems; the newly hatched maggots enter the leaf sheaths and tunnel into the tender stem tissue, feeding for about three weeks before pupating.[1]
A second generation of flies emerges in midsummer and lays eggs on volunteer grain and other grasses, and the fall generation emerges in late August to early September and lays eggs in the newly planted winter wheat.[1]
Hosts are wheat, rye, barley and oats plus bluegrass, millet, timothy and a range of other native and introduced grasses. Two parasitic wasps, Bracon meromyzae and Coelinidea meromyzae, are important in keeping populations low.[1][2]
Damage and signs
- Maggots construct tunnels two or three inches long in the stem. The damage shows during seed development as white heads and upper internodes on green plants, and the Pacific Northwest handbook says the pest is recognized most easily by these white heads.[1][2]
- Larvae sometimes attack young tillers, cutting off the central shoot, and occasionally feed in heads, destroying floral parts or developing seed. Meromyza species have severely damaged spring barley at Klamath Falls, Oregon.[2]
Health risk
The sources treat it only as a pest of small grains and grasses and mention no effect on people.[1][2]
Control
Prevention
Plant winter wheat after the dates recommended to escape Hessian fly, which is an effective practice against the wheat stem maggot as well. Resistant wheat varieties are not available.[1]
Rotating non-susceptible crops such as corn, sunflower, flax, soybeans, safflower and legumes into the system reduces numbers of the pest.[1]
Sanitation
Destroy volunteer wheat and other volunteer plants, which the midsummer generation uses to bridge to the new crop.[1]
Biological control
The parasitic wasps Bracon meromyzae and Coelinidea meromyzae attack the maggot and are important in maintaining low populations.[1]
Pesticides
The Pacific Northwest handbook states that insecticides are not currently labeled for control of these pests, and the High Plains guide refers growers to local pest management personnel for current labeled products.[2][1]
Common questions
- Why are some wheat heads white while the plant is still green?
- A white head and upper stem on a plant whose lower stem and leaves are green is the classic sign of a wheat stem maggot tunnel; the stem pulls out easily just above the last node, and a single white maggot is usually inside.[1][2]
- Can I spray for wheat stem maggot?
- Not in the Pacific Northwest, where no insecticides are labeled for it; the High Plains guide relies on delayed planting, volunteer control, rotation and the maggot's parasites, and tells growers to check locally for labeled products.[2][1]
- What else does it attack besides wheat?
- Barley, rye and oats, and grasses including bluegrass, millet and timothy; the midsummer generation lays its eggs on volunteer grain and other grasses.[1][2]
Sources
- High Plains IPM Guide (Colorado State University, University of Nebraska, University of Wyoming and Montana State University), hosted by Bugwood, “Wheat Stem Maggot (F. B. Peairs, G. L. Hein and M. J. Brewer), High Plains Integrated Pest Management Guide, Bugwood wiki.” wiki.bugwood.org/HPIPM:Wheat_Stem_Maggot (accessed 2026-11-10)
- Pacific Northwest Pest Management Handbooks (Oregon State University, Washington State University and University of Idaho), “Small grain-Wheat stem maggot, Pacific Northwest Insect Management Handbook.” pnwhandbooks.org/insect/agronomic/small-grain/small-grain-wheat-stem-maggot (accessed 2026-11-10)
- iNaturalist, “Meromyza americana taxon page (order and family).” www.inaturalist.org/taxa/1292982 (accessed 2026-11-10)