VT a. Larvae of the Moutain ash sawfly consume all but the midveins of leaves. When Mountain ash sawfly larvae are distrubed, they tend to rear up into an S-shaped position. 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. Defoliation fluctuates with weather conditions and the abundance of natural control agents such as predators and parasites.