By Ann Hazelrigg
Extension Plant Pathologist
University of Vermont   

You may think that caterpillars were devouring your dogwoods this year, but these lookalike larvae are actually sawflies in the order Hymenoptera and are related to bees, wasps and ants. (True caterpillars are the larvae of moths and butterflies and are in the insect order Lepidoptera.)

Sawflies skeletonize leaves and are gregarious, usually feeding in large numbers so can cause a lot of damage to plants very quickly. If the feeding occurs late in the season, the damage does not typically impact the health of the plant, but if early in the season, large outbreaks may need to be controlled. This can be done by hand picking or using insecticidal soaps or horticultural oils when the larvae are small.

Several sawflies attack our trees and shrubs in Vermont, including the pear slug, rose slug, European pine sawfly and birch sawfly, among others. The dogwood sawfly (Macremphytus tarsatus) attacks various species of dogwoods. It is an interesting sawfly because the immature forms have the ability to change colors, textures and appearances several times as they grow and mature.

The adult is a slender, black wasp-like insect that emerges from May to July. The female lays up to 100 eggs on the undersides of leaves using a “saw-like” ovipositor, a tube-like organ for depositing eggs.  When the larvae hatch, they are tiny, yellow and translucent. As they grow and molt, they become covered with a white waxy coating. 

The young caterpillars skeletonize leaves leaving the veins while older larvae may leave only the midrib. The larvae eventually shed their waxy coating and become cream-colored with black spots.

At maturity, the larvae are about an inch long and become yellow and black. The mature caterpillars bore into decaying or soft wood and were recently found boring into a rotted deck in Vermont. The larvae can form cells in logs, landscape timbers and lawn furniture! Fortunately, there is only one generation of this sawfly per year in Vermont.