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COLUMN: Sawflies: a species-specific pest

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By Jeneen Wiche

One of the very first insects that I identified as a young gardener was the pine sawfly. We had planted over a hundred white pine seedlings over 30 years ago, and after a decade or so, we started to lose a couple each year to one problem or another.
I was charged with inspection duty. Looking for and plucking bagworms; collecting beetles in jars for identification at the County Extension Service; or closely noting the color, legs and chewing habits of the various caterpillars I encountered.
The markings of a mature white pine sawfly caterpillar are quite distinctive: the body is pale yellow with four rows of black spots along the back and the head is black. The head looks like a little helmet, almost.
A couple of years ago, two new sawflies came to my attention. When I noticed munching on some trees and shrubs, I took a closer look and discovered caterpillars that looked remarkably like the pine sawfly, but something was different.
First, one was voraciously defoliating a river birch; the other left little of our new Cornus sericea foliage on the plant. They had different markings on their bodies, too. The almost translucent pale yellow color was there, but the dots and stripes were different. In fact, there were no extra markings at all.
In the last decade, the normally northeastern dogwood sawfly has established itself in our area. It is not a pest to fear, like the gypsy moth or emerald ash borer, which can devastate host plant populations. The dogwood sawfly is an ornamental nuisance, not a plant killer. The birch sawfly is the same – a late summer pest that eats as much as it can before it drops to the ground to continue its life cycle as a chunky caterpillar.
All of the sawflies are an ornamental nuisance because they are such voracious eaters. The dogwood sawfly is attracted to two particular dogwood species: the gray dogwood (Cornus racemosa) and the redosier dogwood (Cornus sericea).
We have both here at the farm and, yes, both are being inspected daily for sawfly squishing. The gray dogwood in the pine grove, however, is now nearly defoliated (we were out of town for the weekend); and feeding has just begun on the C. sericea “Midwinter Fire.”
If you have these dogwood species, inspect the underside of the leaves. If you see the pale yellow caterpillars and their dusty black heads, you have the dogwood sawfly. If they are curled, they are at rest; if stretched out, they are feeding.
You may also see different stages of the caterpillar at the same time. And, true to the changing face of a caterpillar, I have seen fully developed caterpillars next to very small ones. With some age, they have a little mottling on their backs and after their second molting, they have a powdery appearance.
By late summer, the sawfly caterpillar reaches maturity and drops to the ground where it will seek a safe place to continue its life cycle. In the case of the dogwood sawfly, the larvae seek out wood for over-wintering; adults emerge in late spring and the female, in her wasp-like form, uses a “saw-like” apparatus to deposit her eggs in leaf tissue of the host plant where the cycle begins again.
As I mentioned, the dogwood sawfly is more of a nuisance because it defoliates deciduous plants that can recover (unlike the sawfly species that prefer evergreens) by putting out new growth mid-season. Plus, many deciduous plants have stored sufficient energy by August for later use the following spring.
Control of the dogwood sawfly on the gray and redosier dogwoods is as simple as doing nothing, hand-picking the offenders, using horticultural oil to smother them or, for the chemically inclined, a systemic insecticide.