Little known lawn pest shows up in southwest Michigan
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Last week, Kevin Frank forwarded me some photographs of a caterpillar that Chad Follis found in one of his client’s lawns. I emailed Kevin back to say his caterpillar was really a sawfly larva and asked if there were trees around the area where larvae were seen. Sawflies are primitive members of the order Hymenoptera, which includes bees, wasps and ants. We encounter most sawfly pests feeding on the foliage of trees and shrubs. (see photos) Sawfly larvae typically pupate and overwinter in the soil so I first thought Chad’s sawflies had climbed down from a nearby tree or shrub and were wandering around a bit before retiring to the safety of their soil retreats. Wrong!
Chad sent me several of the larvae and they didn’t look like any sawfly I’d seen before. Chad was thoughtful enough to provision the plastic container with a small handful of grass clippings. By the following day, the larvae had converted almost all of the grass into tiny brown pellets of frass. These were definitely grass feeding sawflies. None of my reference books on turf insects listed a sawfly pest. I was finally able to get a beat on the bug by searching the web for grass feeding sawflies. A web site from Oregon State University listed the grass sawfly, Pachynematus setator, as a minor pest of grass seed fields in Oregon. I found a few short reports from a number of other states about sawfly larvae feeding in lawns and cereal grains.
Ross Arnett, author of American Insects An Handbook of the Insects of American North America North of Mexico lists 44 species of Pachynematus (Hymenoptera: Tenthredinidae) However, he lists P. extensicornis as the Grass sawfly, rather than P.setator. It is very likely that most species in this genus are indistinguishable in larval stages.
The Department of Entomology, Oregon State University, provides the best information on these bugs. Here is what they say about P.setator:
Hosts include a number of grasses including wheat and barley, but appear to favor the fine fescues over everything else. There is one generation per year. Adults occur in late spring and lay eggs. Larvae occur as flag leaves begin development and have usually ceased feeding and spun cocoons as the seed crop is set. Mature larvae drop to the soil surface and pupate.
Larvae feed on leaves in late spring to early summer. Larvae are typically leaf feeders that also chew through stems. Damage is usually in the immediate area of taller grass weeds in the field. (see photo) This is because egg laying adults fly to and lay eggs on the tallest grasses in a field. Generally, these sawflies do not cause significant damage to crops or lawns because they are normally heavily parasitized by Icheumonid and Chrysidid wasps.
The larvae resemble caterpillars or foliage feeding cutworms at first glance. Sawfly larvae have 8 pairs of abdominal prolegs as compared to caterpillars, which have no more than 5 pairs.