True Armyworm vs. Grass Sawfly

Published: June 6, 2011

The grass sawfly (Pachynematus spp.) can be confused with small true armyworm larvae in both grass pastures and wheat. Grass sawfly larvae usually occur 1-2 weeks earlier than true armyworm larvae in both crops, but often are present at the same time through May and early June. As previously described, true armyworm larvae are pale green in color when small. They have four sets of abdominal prologs and one anal pair of prologs. Larger larvae are not green and tend to spend more time on the top of the grass or wheat plant. The grass sawfly is pale green through all larval stages and may appear to possess transparent skin over its body. Thus, it looks like you can see all the good stuff inside the larvae. It is commonly observed in wheat just before and after head formation. Although only a rare economic pest of grass pastures in Missouri, it is present in high numbers in many fescue fields this year. This insect is known to cause problems in turf throughout the US and in wheat and barley in many eastern states. In Missouri, larvae of the grass sawfly are generally heavily parasitized and cause only limited damage to wheat. Populations occasionally build in turf, but problems in grass pastures are rare unless heavy populations build and severely defoliate grass vegetation.