Columns, In the Garden

Dogwood Sawfly are Lookalike Caterpillars

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photo by David Cappaert
Although the just-hatched larvae of the dogwood sawfly are tiny and translucent, as they grow and molt, they become covered with a white, waxy coating.

by Ann Hazelrigg

BURLINGTON – 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. 

photo by Bruce Watt 
Mature dogwood sawfly caterpillars, which are about an inch long and yellow with black markings, can cause damage by boring into decaying or soft wood.

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.   

photo by Whitney Cranshaw
Immature dogwood sawflies are caterpillars that change colors, textures and appearances several times as they grow and will be white or cream-colored with small, black spots as they near maturity.

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.

[Ann Hazelrigg is an extension plant pathologist at the University of Vermont.]

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