Insect Pest Topics
Wheat jointworm and wheat strawworm are minor pests of wheat. There are no recent
reports of widespread damage, though there are sporadic, localized infestations that
occur in Montana. Crop rotation prevents these insects from causing damage.
larvae are white in color and form cells in the wall of the stem where they feed on plant sap. The small yellowish grub-like larvae are about ¼ to 3/8 in. (3.6 - 4.2 mm) in length when full grown. Each larva develops and overwinters in a separate cell within the stem gall. Pupation occurs within the cells during the winter.
stem galls usually above the second or third joint. Height of the galls depends on the wheat growth stage at the time when insect eggs were laid in the stem. By harvest, the galls are hard and woody. Yield is affected by abnormal grain development and loss of grain through stem breakage and lodging prior to harvest. Broken or lodged stems may be confused with Hessian fly or wheat stem sawfly damage. Related species include the wheat-sheath gall jointworm and wheat-sheath gall jointworm both reported from northern Utah.
Second generation or summer form (grandis) adults are larger than first generation
adults, measuring 3/8 inch (4.2mm) in length, distinguished by their smooth polished,
black mesothorax. Females only comprise this generation, reproducing without mating
(parthenogenetic) and are capable of dispersing widely. Single eggs are laid in the
most vigorous plants, slightly above the upper joints about the time wheat is in the
boot stage. Small yellow-green larvae reach 6 mm in length and feed inside the stems
near joints, showing little external sign of their presence. Larvae form cells at
the joint, pupating in the fall and continuing through winter.
Timing of egg lay and subsequent larval development influences impact of the summer form on wheat yield. Earlier infestations are more damaging than later which result in only slight injury. Estimates suggest that 22% grain loss can be caused by second-generation strawworms (Phillips & Poos 1953).
Infestations are typically adjacent to infested wheat stubble from the previous year.
Though eggs are laid in other grasses such as barley, oats, or rye, the larvae cannot
complete their development in any plant but wheat.
Doane, R. W. 1926. The reappearance of Harmolita grandis and Harmolita vaginicola in Utah. J Econ. Entomol. 19: 730-732.
Knowlton, G. F. and F. V. Lieberman. 1954. Controlling the wheat strawworm. UT State Agric. College Ext. Circ. 194.
Phillips, W. J. and F. W. Poos. 1953. The wheat strawworm and its control. USDA Farmer s Bulletin No. 1323.
Chamberlin, T. R. 1941. The wheat jointworm in Oregon, with special reference to
its dispersion, injury, and parasititzation. USDA Technical Bulletin No. 784. pp47.