Wheat jointworm and Wheat strawworm

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.
 
 

Wheat jointworm

The jointworm is native to North America and occurs throughout the U.S. Jointworm was first reported in the Pacific Northwest in 1926 and has been observed in parts of Montana. Wheat is the primary host of the jointworm whose infestation is characterized by the occurrence of woody galls that weaken the stem causing stem breakage above the gall.
 
 

Description and Life Cycle

The shiny black wasp-like adults, about 1/8 in. (2.5 - 3.2 mm) in length, have yellow markings on leg joints and thorax and resemble a small, winged ant. Adults emerge from small circular holes chewed in the stem galls that remain from the previous year plant residue. Adults can be found for a four-week period when wheat is jointing. Adults are not strong fliers but may be carried greater distances with the aid of wind. Females lay several eggs in stems just above joints, that hatch in about 12 days. Newly hatched 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.
 
 

Injury

Larva form cells within the stem wall and feed on plant sap causing 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.
 
 

Management

Because the jointworm is a minor pests, there little concern for management. Reduced tillage has probably enhanced the survival of wheat jointworm. Cultural controls such as plowing stubble and late seeding suppress jointworm populations, however these techniques are not compatible with production practices used in Montana. Crop rotation to a non-host grain crop like barley or broadleaf crops that are not subject to attack by the jointworm can reduce jointworm populations. Several small wasp parasitoids are important in biological control of this pest. Unfavorable environmental conditions, unusually wet weather during cold winter months increases the overwintering mortality of this insect. Insecticides are not effective in control of this insect.
 
 

Wheat strawworm

Description

The strawworm has two generations per year, the first of which is the more damaging. The overwintering or spring form (minuta) adults emerge from stubble in late winter. The adult is a wingless, ant-like insect approximately 1/8 in (2.8 mm) in length with yellow-banded legs. Eggs are laid by first generation females in young tillering-stage winter wheat plants. Straw-colored larvae emerge in 10 days and develop within tillering stage wheat, feeding on the embryonic head. The first generation completes development in about 30 days (May to early June).

 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.
 
 

Damage

Larvae of the first generation are the most damaging. Infested tillers result in stunted stem that remains vegetative with little or no grain produced. Damage is often caused to the main tillers. First generation larval feeding may kill plants, and grain yield is usually severely restricted or eliminated. Infested tillers resemble Hessian fly infested wheat, with the main stem undeveloped and leaf tissue a darkened green color.

 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.
 
 

Management

Because first generation strawworm is wingless and its dispersal is limited, crop rotation or isolating a wheat crop from a previous year's infestation is recommended. Plowing and cover cropping prior to the next wheat crop has been recommended, but is not compatible with conservation or no-till practices. Volunteer wheat is an important source of strawworm and its timely elimination can reduce this pest. Six wasp species parasitize strawworm (Phillips & Poos 1953) although they are not thought to entirely control strawworm populations.

References

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.