Sawflies

Wheat sawflies were found attacking wheat flag leaves in localized areas of Umatilla County's wheat growing region (2003). Wheat sawflies, Pachynematus spp., are rarely a problem in Oregon's wheat fields. The last time they were an economic problem was in 1971 in the Willamette Valley. They are regularly present but not at significant numbers. Parasitic wasps are typically able to keep populations in check under most conditions. The level of infestation varies widely and the potential for impacting yields needs to be accessed on a field-by-field basis.

Photo: SawflySawfly adults are wasps (adults have "broad waists" - unlike bees and wasps with "thread-like" waists). They do not sting. The larvae (caterpillars feeding on the wheat leaf) can be distinguished from cutworms, armyworms and loopers in that they have fleshy abdominal prolegs on abdominal segments 2-7 or 8 and 10. Armyworms cutworms have prolegs on segments 3,4,5,6 & 10, loopers on 5,6 & 10.

Leaf feeding sawflies usually occur on grasses. The last large outbreak of leaf feeding sawflies in OR was in fine fescue grown for seed in western OR in 1971 (300 larva/25 sweeps). The pest was Pachynematus setator and does not have a common name. In California wheat had been grown for 75 years without a sawfly noticed until 1953 when 200 acres were destroyed. The following year 12,000 acres were sprayed. It has not been a problem since.

The sawfly infesting wheat in Umatilla County probably is similar to the one in the Willamette Valley with the following life cycle: Wasp emerges from the soil in spring (April and May). Up to 80 eggs/female are laid singularly in leaves.

Within 10 days eggs hatch. A larva feeds for 3 to 4 weeks (longer in colder weather), drops to the soil and forms a cell where it will stay until the next spring and emerge as the wasp. In the Willamette Valley most larvae have completed development and stop feeding by early June.

Why the Outbreak?

Probably a combination of events-mild winters, upset in the natural biological control of the sawfly larvae (they are heavily parasitized by other wasps), no-til practices, and perhaps a build-up on grasses in the area.

What to Expect

It is believed this species has only 1 generation a year. Expect most of the damage to occur in MAY and JUNE this year and possibly next year. Usually an outbreak year is followed by a population collapse with very few pests seen in the next or following years. This is assuming biological and natural control (weather) take their toll on the over-wintering larvae in the soil. However, there is a two-year outbreak of sawflies on hybrid poplar in Clatskanie OR at this time.

Control

Chemical: Most all insecticides labeled for "worm control on wheat" will control sawflies. They are very susceptible to insecticides with the exception of those usually used only for aphid control. Do not expect dimethoate and DiSyston to control sawflies well.
Cultural: cultivation crushes and/or buries the overwintering larvae. For years plowing has been used to control many sawfly pests in cereals.
Natural/biological: Parasitic wasps usually help keep sawflies at very low levels.

When to Control

The simple answer is when the cost of a spray will prevent yield loss in excess of treatment cost. BUT there is no simple relationship here between numbers per sweep and damage to expect. Nor are there established thresholds-such as numbers per sweep or numbers per plant. However, consider the following factors:

Are the larvae defoliating the flag and/or second leaf--these two leaves can account for up to 100 percent of the variation in wheat yield! A mature larva will be about 3/4 inch long. How big are the larvae in relation to the growth stage of the wheat and how much longer will the bulk of the feeding last? Do many of the larvae show signs of being parasitized (black marks and discolorations noted on the skin). Are the larvae distributed throughout a field or only occur in certain areas like in association with field margins, ridges, adjacent grass land or no-til fields?

Source: Glenn Fisher, OSU Extension Entomologist, May 19, 2003

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