Columns, In the Garden

Dusky Birch Sawflies Defoliate Trees

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photo by Ann Hazelrigg
Dusky birch sawflies raise their bodies in the air as they feed in large groups on the edges of birch leaves.

BURLINGTON – The defoliation in river birch trees may be a sign that dusky birch sawflies (Croesus latitarsus) are feeding on the foliage. Examine the leaf edges for these voracious feeders, which resemble caterpillars (immature moths and butterflies in the order Lepidoptera) but are actually sawflies, in the bee, ant and wasp insect order, Hymenoptera.

Both sawfly larvae and caterpillars have three pairs of thoracic legs, but caterpillars have two to five pairs of prolegs (fleshy, leg-like projections) on their abdomens. Sawflies have six or more.

Sawflies also lack crochets, or hooks, that caterpillars have on their prolegs to help them cling to plants. Without these hooks, sawflies do not cling to plants as tightly and can rear up in the air when disturbed.  

Dusky birch sawfly larvae are yellow-green with black heads and black blotches on their sides and can grow up to an inch long. They feed in large groups on leaf edges and can strip a lot of leaves, often leaving just the midribs, in a very short amount of time! 

There are many sawfly pests that feed on ornamental plants in Vermont, including columbine sawfly, currant sawfly, azalea sawfly and European pine sawfly, to name a few. Pear and rose sawfly slugs are particularly attracted to fruit trees and roses.

The adult sawflies emerge in the spring and tend to be inconspicuous, only feeding on pollen and nectar. The adults usually live only up to a week and their main job is to lay eggs. Once the eggs hatch in May, the larvae will feed until July.

Dusky birch sawflies may have up to two generations each summer and will overwinter in the soil. Their feeding is not harmful to a healthy tree and no control is necessary. However, handpicking and destroying the sawflies can decrease damage to the tree or plant.     

Dr. Ann Hazelrigg is the University of Vermont Extension plant pathologist and director of the UVM Plant Diagnostic Clinic.

Ann Hazelrigg

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