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Insect Pests of Winter Wheat
November 26, 2024
Occurrence of Common Insect Pests Associated with Winter Wheat
Table 1. Common Insect Pests of Winter Wheat*
*Occurrence may differ depending on geography. Occurrence may be earlier in the Southern Plains and later in the Northern Plains.
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Aster Leafhopper
Adult aster leafhoppers are wedge-shaped, small, about 1/8 inch long, and are light green to yellow. The adult aster leafhoppers have clear wings that cover their abdomens. Although present on nymphs and adults, the two distinct spots present between their eyes are more easily observed on adults. In addition, there are six distinct black markings on the front of the head. Aster leafhoppers feed by piercing leaves, which creates white or yellow spots on the leaves called stippling.
Army Cutworm
The army cutworm is a late fall to early spring pest that occurs sporadically in the Western Plains of the United States. Adult moths lay their eggs in soil in the fall and the larvae feed during the fall and early winter. Early injury on wheat leaves resembles windowpane holes. Army cutworm larvae are greenish-brown to greenish-grey caterpillars, approximately ½ to two inches in length. Larvae hide in loose soil at the base of plants, emerging to feed in the evening. Army cutworms are climbing cutworms and only feed on above ground plant parts. In fields in which the plants are well established, an insecticide application is rarely justified in the late fall or early winter. However, some fields will not “green up” in the spring because of constant cutworm feeding. Scouting fields frequently during warm periods in the early to late spring is recommended if the preceding fall was dry. Moisture availability, crop condition, and regrowth potential are factors that can impact yield loss to army cutworms. Fields that are planted late under dry conditions with poor tillering may have economic injury with less than two larvae per square foot. In most fields, treatment will not be necessary until there is an average of four to five larvae per square foot. Vigorous, well-tillered fields under optimal growing conditions can tolerate even higher populations—as many as nine or 10 larvae per square foot—without measurable yield loss. Infestations in later stages of crop development are less damaging than early ones.
Bird Cherry-Oat Aphid
The bird cherry-oat aphid is a very dark olive green, although its color varies depending on the temperature and growth stage. Nymphs are usually pale yellowish green, darkening as they mature to a deep olive green in the adult stage. Normally, a dark brown spot is visible spanning both cornicles (tail pipes) which are always black. High populations that persist past the boot stage can cause the flag leaf to twist into a corkscrew, resulting in “fish-hooked” heads. Feeding causes no chlorosis or other visible damage to wheat plants, but heavy infestations can reduce grain quality, protein content, test weight, and even impede protein assimilation by grazing cattle. The major economic impact of the bird cherry-oat aphid comes from its role as a key vector of plant viruses, especially Barley Yellow Dwarf Virus (BYDV). Virus infections are more damaging when they occur in early growth stages of the wheat plant. The economic threshold for treating these aphids is quite high, and treatment of wheat in vegetative stages is not recommended. To determine bird cherry-oat aphid levels from boot stage until heading, count the number of aphids present on each of a series of 25 to 50 randomly selected tillers across a zigzag transect of the field. If an average of 50 or more aphids per tiller are present, treatment with an insecticide broadly labeled for aphid control on wheat may be considered.
Brown Wheat Mite
The brown wheat mite is approximately the size of a pencil tip. It has a rounded body, is metallic dark brown or black, with short hairs on its back. The front legs are about twice as long as the other three pairs of legs. The brown wheat mite is most prevalent in dry weather, and the population increases on wheat suffering from moisture deficiency. It occurs across the High Plains of the US. Usually, insecticides are not recommended for managing this pest because infestations are associated with drought, so the infested crop is likely under drought stress.
Chinch Bug
The chinch bug is a pest of small grains in the eastern High Plains. The ideal host plants for this pest are wheat or barley, followed by corn or sorghum. The early immature stages have a yellowish-brown head and thorax, which darken in the later developmental stages. The abdomen is yellowish or reddish orange in young nymphs, which darkens to almost black in the final stage. All nymphs have a characteristic white band across the middle of the body. Adults are pale right after the final molt, but later develop a distinctive black-on-white ‘X’ pattern on the wings.
English Grain Aphid
English grain aphids range from pale green, to yellowish green or yellow, to even orange or reddish brown. The English grain aphid is a large aphid with long black cornicles, long legs, and antennae slightly longer than half the body length. English grain aphids feed on wheat heads right up until maturity and may cause some kernels to shrivel. Although they are very common, they do not normally cause economic damage.
Fall Armyworm
Fall armyworm larvae vary from light tan to nearly black, with three thin light-yellow lines down the back. There is a wide dark stripe and a wavy yellow-red splotched stripe on the side of the larvae. Fall armyworms can be confused with both true armyworms and corn earworms; however, fall armyworms have a prominent inverted "Y" mark on the front of the head. Older fall armyworm larvae also have four raised dark bumps in a square shape near the end of the abdomen. They occur late in the season and can be a pest on winter wheat seedlings. They do not overwinter in most major wheat areas but migrate to those areas late in the summer.
False Wireworm
Adult false wireworms are also called darkling beetles. These large black or reddish-brown beetles cannot fly and can be recognized by the odd angle at which the body is held when they run. The larvae appear similar to wireworm larvae, except that false wireworm larvae have longer legs and antennae. Larvae typically damage wheat in the fall by feeding on seeds, seedlings, and young plants, resulting in lost stand.
Grasshopper
There are many species of grasshopper that can damage small grains. Not all grasshoppers are crop pests, although most can cause injury in large numbers. The major pest species include the differential grasshopper, red-legged grasshopper, and the two-striped grasshopper. Damage can occur in both the fall and spring. Typically, grasshoppers invade from field margins as fall planted or spring planted wheat emerges. Counts of seven to 12 grasshoppers per square yard signal potential problems. Once wheat has been planted, three or more hoppers per square yard within the field can destroy seedling wheat.
Greenbug
Greenbugs are likely the smallest aphid found on wheat. They are lime green or yellow. They have a dark green line down the middle of the back and antennae that are as long as or longer than their body. A substance in greenbug saliva causes young leaves to turn yellow and older leaves to develop orange-red spots. Colonies are concentrated on the undersides of leaves. Greenbugs often occur in concentrated patches within a field, damaging small circular patches of plants that radiate from dead spots.
Hessian Fly
The Hessian fly is a serious pest that can damage cereal crops, causing stunted growth, poor-quality crops, and reduced grain yields. The damage is caused by the immature maggot stage of the fly, which feeds on the leaves, stems, and tillers of the plant. Hessian fly injury to wheat is usually more serious during years of abundant rainfall and can occur in both fall and spring. Infestations are more common with wheat planted no-till directly into the previous year’s wheat stubble. Fall-infested plants have a characteristically thickened, dark bluish-green appearance, and will often die off over the winter. These insects overwinter in wheat as pupae commonly called “flax seeds”. Infestations in the fall often give rise to a spring generation that may cause plant lodging, most likely above the node, leading to yield reductions. The larvae or pupae will be visible on the wheat stems when the leaf sheaths are carefully removed to expose the stems.
Pale Western Cutworm
Pale western cutworms cause damage by feeding on the stems and leaves of small grains, corn, and other crops. They will cut the stem at base, preventing it from producing grain. Young larvae are about ½ inch long with smooth, gray-white bodies, while mature larvae are about 1.5 inches long and a gray-green color. The caterpillars of pale western cutworm lack any distinct markings. Larvae are usually more common in loose, sandy, or dusty soil and are found most easily in the driest parts of the field. Outbreaks are associated with dry conditions during the previous spring.
Russian Wheat Aphid
The Russian wheat aphid is a small, lime-green aphid with a football-shaped body and very short cornicles, legs, and antennae. They prefer to feed on the newest growth of the wheat plant, rolling up the leaves to create a protected microhabitat. Resistant wheat varieties are less prone to leaf rolling, leaving the aphids more exposed to predation and parasitism. Infested leaves exhibit purple, yellow, or white longitudinal streaks along the leaves and leaf sheaths. Heavily infested plants may appear flattened, with young tillers lying almost prostrate to the ground.
Soil Insects
This group contains three major insect pests: wireworms, white grubs, and false wireworms. Wireworms are the larval stage of click beetles and there are many species that can attack wheat. Wireworms primarily injure the seed and seedling growth stages of wheat. Wireworms are light to dark brown with a hard body and six legs, and are primarily a pest in the early spring. White grubs are the larval stage of May and June beetles. White grubs are cream colored and shaped like a C with six legs.
Stinkbugs
Stinkbugs can reduce wheat yields, but their impact is usually small and greatly reduced after the milk stage. However, high populations feeding during the dough stage can reduce the test weight. Stinkbug thresholds for wheat vary by region but are typically around one stinkbug per square foot or one stinkbug per five to 10 heads during the milk and soft dough stages. Stinkbug damage is greatly reduced once wheat reaches the hard dough stage. Populations are usually confined to the edges of the field.
True Armyworm
The true armyworm feeds on a variety of plants, preferring grasses. Females lay their eggs in large clusters on vegetation, particularly lodged small grains. The larvae are green to black with stripes of various colors. The head capsule is medium brown with dark markings. Feeding mostly at night, each larva can consume 43 linear inches of wheat leaf to complete its development, or the equivalent of three whole plants. However, 80% of consumption occurs during the last three to five days of feeding. Treatment is usually not necessary when average larvae levels are below four to five larvae per square foot, but an insecticide treatment may be required when there are five to eight larvae per square foot depending on growth stage in relation to crop maturity.
Wheat Curl Mite
The wheat curl mite is microscopic, cigar-shaped, white, and wingless. Because they are too small to see with the unaided eye, a hand lens with at least 10x magnification is needed to observe wheat curl mites on leaves in the field. There are two pairs of legs near the head and a small fleshy pod at the posterior. The wheat curl mite has three life stages: egg, two larval stages, and adult. Females lay eggs in straight lines along leaf veins, and a generation can be completed in as little as 10 days in warm weather. Wheat curl mites feed on new leaf growth, causing leaves to roll up and sometimes trapping the grain head. The major impact of the mite is the transmission of wheat streak mosaic virus which can cause up to 100% yield loss in young wheat plants.
Wheat Head Armyworm
Wheat head armyworm larvae are grayish or greenish-gray caterpillars with white, green, and brown stripes. The moths emerge in the spring and females deposit eggs on wheat or barley. Larvae feed on grain heads at night, primarily in late May and mid-June. It is a minor pest of wheat in most years but can occasionally cause noticeable damage to wheat kernels.
Winter Grain Mite
Winter grain mites have a dark brown to almost black body with conspicuous reddish-orange legs. Their front legs are longer than the others, but not as pronounced as on the brown wheat mite. There are generally two generations per year. The first generation hatches in September or October as weather conditions become favorable for the over-summering eggs. Populations reach a peak in December or January. Fields with loose sandy or loamy soils are more at risk than those with hard, clay soils.
Wheat Stem Maggot
Although very common, the wheat stem maggot is usually considered to be a minor pest problem. The larvae are greenish maggots about 1/4 inch in length. The maggots sever the stem at the flag leaf, resulting in a white grain head that can be easily pulled from the plant.
Wheat Stem Sawfly
The wheat stem sawfly has historically been a pest in spring wheat growing, but it has recently developed into a winter wheat production problem across the Great Plains regions of Wyoming, Nebraska, Montana, and Colorado. An adult sawfly is black with three yellow bands around the abdomen and yellow legs. The larvae are cream colored and assume an S shape when removed from the stem. They have a brown head and a short blunt projection on the posterior. There is a single larva within each wheat stem. When the plant reaches at least 50% moisture, the larva will cut a V-shaped notch just above the soil line that may result in a lodged stem. The remaining wheat “stub” acts as an overwintering chamber.
Management strategies and recommendations for pests will vary by location, so consult your local extension office for current management recommendations.
Additional Resources
Zukoff, A., McCornack, B.P., and Whitworth, R.J. 2024. Wheat insect pest management. Kansas State University Research and Extension. https://bookstore.ksre.ksu.edu/pubs/wheat-insect-pest-management-2024_MF745.pdf
Blodget, S. and Kieckhefer, J. 2020. Chapter 22: Insect pests of wheat. In, Best management practices for wheat production. South Dakota State University. https://extension.sdstate.edu/sites/default/files/2020-03/S-0005-22-Wheat.pdf
Bynum, E., Knutson, A., Reed, B., and Gonzales, J.D. 2018. Managing insect and mite pests of Texas small grains. Texas A&M AgriLife Extension. https://extensionentomology.tamu.edu/resources/management-guides/managing-insect-and-mite-pests-of-texas-small-grains/
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