Mountain Ash Sawfly
Hosts: American and European mountain ashes
Evidence: Look for defoliation of all but the midribs of leaves (a). Larvae, which are yellowish-green with prominent black spots (a,b), feed gregariously and tend to defoliate an entire branch before moving on to another.
Life Cycle: There are two generations per year, except in the northernmost part of the sawfly's range. Winter is spent in the ground as a cocoon, and pupation occurs in the spring. First generation adults emerge and lay eggs throughout June and into July. Eggs are laid in slits along the toothed margin of leaflets, producing a blistered appearance. Larvae of the first generation mature and drop to the soil to pupate by August. A portion of these reach adulthood and lay eggs, producing a second generation which feeds from late August through September.
Management: Defoliation fluctuates with weather conditions and the abundance of natural control agents such as predators and parasites. Branches that contain leaflets with eggs or larvae can be pruned out and destroyed. Larvae can be treated with residual insecticides. Bacillus thuringiensis is not effective against sawfly larvae.
Figure a: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT
Figure b: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT
Drooz, A.T. 1985. Insects of Eastern Forests. USDA Forest Service Miscellaneous Publication 1426. p 404-405;
Johnson, W.T. and Lyon, H.H. 1991. Insects That Feed on Trees and Shrubs . 2nd edition. Cornell University Press. p 128-129;
Rose, A.H. and Lindquist, O.H. 1982. Insects of Eastern Hardwood Trees. Canadian Forest Service Publication, Forestry Technical Report 29. p 278-279.