By Raymond Cloyd, Professor and Extension Specialist in Horticultural Entomology/Plant Protection.
We have received several inquiries regarding caterpillar like insects feeding on green ash (Fraxinus pennsylvanica) trees. The insects in question are larvae of the brownheaded ash sawfly, Tomostethus multicinctus, which are sporadic, early season, defoliating insect pests. The larvae feed extensively causing noticeable leaf damage and producing frass or fecal material.
Brownheaded ash sawfly larvae are approximately 15 to 20 millimeters (1/2 to 3/4 of an inch) long, yellow green, with white and green stripes extending the length of the body (Figure 1). The larvae have a brown head, and there are prolegs (fleshy, leg like appendages) on every segment of the body, which distinguishes sawfly larvae from caterpillars. Brownheaded ash sawfly larvae feed primarily on green and white ash trees. Adults are wasp like in appearance. Brownheaded ash sawfly pupate in the spring, with adults emerging and females laying eggs inside leaves. Larvae emerge (eclose) from eggs and congregate in groups, feeding from May through June.
Larvae create shot holes or pin holes on young leaves (Figure 2), but as they increase in size, the larvae consume entire leaves, especially terminal leaves (except the main veins), resulting in almost complete defoliation. By June, larvae are fully grown and shed a papery like skin that is attached to the leaf (Figure 3). The larvae move toward the base of the tree and construct protective cocoons. High numbers of larvae may be present at the base of trees (Figure 4). Brownheaded ash sawfly overwinter as full grown larvae or pre pupae within silken lined cells located on the top of the soil at the base of previously infested trees. There is one generation per year in Kansas.
Rainfall will quickly remove larvae from trees. In addition, the larvae can be removed by hand and placed into a container of soapy water to kill them. An insecticide application is not warranted unless brownheaded ash sawfly larval populations are causing extensive damage that compromises the aesthetic value of an ash tree. Insecticides with contact activity can be applied, but thorough coverage of the tree canopy, in particularly, the leaf undersides, is important. Do not apply an insecticide containing the active ingredient, Bacillus thuringiensis subsp. kurstaki, because the product will not kill sawfly larvae.
For more information on brownheaded ash sawfly, you can access the following extension publication:
Cloyd, R. A. 2016. Brownheaded ash sawfly. Kansas State University
Agricultural Experiment Station and Cooperative Extension Service. Kansas State University; Manhattan, KS. MF3297. 2 pages.
http://www.bookstore.ksre.ksu.edu/pubs/MF3297.pdf
Related: Brownheaded ash sawfly