WHEAT STEM SAWFLY

A. Stubble Showing cutting by larvae; a. larva in cell within wheat stem; b. pupa in stem. G. Growing wheat plants, x, stem split to show tunneling of larva inside; c. young larva and frays in stem; y, adult sawfly; d. adult female depositing eggs inside stem, with stem cut away to show egg within. (a. b. c. and d, all about three times natural size.)

WHEAT STEM SAWFLY

The wheat stem sawfly is a serious pest of wheat in the northern Great Plains, particularly in Montana and North Dakota. It also attacks rye and to a lesser extent barley, oats, and flax. Several large-stemmed native grasses, such as the wheat, rye, and bromegrasses, are favored host plants.

The adult sawflies emerge from the stubble fields and native grasses in June and July. They fly to the young growing wheat plants, where the females deposit eggs singly in the hollow centers of the stems. The eggs hatch within a week. In the worm stage the insect mines up and down within the growing wheat stem. When full-grown, it cuts a groove around the inside of the stem at about ground level, and makes its overwintering cell in the base of the stem. Winter is spent by the mature larva just beneath the surface of the ground in the cell. Pupation takes place during May and June, resulting in the next generation of adults. There is but one generation of the sawfly annually.

Wheat losses from the sawfly occur in two ways: By shrinkage of the kernels in the heads of tunneled stems, and by total loss of the grain in the heads when infested stems break over in the wind and fall to the ground so that many of the heads cannot be picked up by the harvester.

Control: The sawfly cannot be controlled by the application of an insecticide. Crop losses can be reduced by using certain cultural practices.

Start harvesting as early as possible and before many of the stems have fallen. Where swathers and binders are used, cut the grain before it is quite ripe. In areas where straight combining is practiced, pick-up equipment on combines will salvage many fallen stems.

Cultivate infested wheat stubble in the fall by either deep plowing with a moldboard plow or by extremely shallow tillage. If shallow tillage is practiced, leave as much of the stubble on the surface as possible. Confine spring cultivation to deep plowing. In areas where soil blowing or washing is serious, follow the practice of extremely shallow cultivation.

Do not seed heavily infested fields to sawfly-susceptible wheat. Seed such fields to barley, oats, flax, corn, mustard, or other resistant crops. In Montana the sawfly-resistant wheat variety Rescue should be used if wheat is to be grown on infested fields. Crop rotations, if they provide for the seeding of wheat on sawfly-free fields, will help hold the sawfly in check.