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Wednesday, August 7, 2013

What's Eating My Dogwoods!?

Dogwood Sawfly

Always trying to give bugs the benefit of the doubt before hurting any good guys, I did my research as to what is decimating my dogwood shrubs. Turns out the Dogwood Sawfly, Macremphytus tarsatus, is a significant pest to the dogwood (Cornus) species.

Variegated Dogwood

Defoliated Varigated Dogwood

The different forms during the larvae stage may make it seem there is more than one type of pest on your plants. The  non-stinging cousin of wasps, adult sawflies lay eggs that hatch on the undersides of  leaves. They are almost a translucent yellow.

The second stage looks like they are covered with a white,  chalky powder, and by the last stage they are about an inch long, yellow with a black head and spots.
The only way to distinguish between caterpillars and sawfly larvae is that caterpillars have five pairs of false legs (prolegs) behind the six true legs while sawflies have seven pairs. 

 Pictured above is a shrublike variegated dogwood, but other susceptible types of dogwood are the gray dogwood, red-twig dogwood, and pagoda dogwood. The common flowering dogwood doesn't seem to be as affected. Being these aren't caterpillars, the natural control that is so safe for such soft bodied pests will do nothing to stop sawfly larvae.

The best way to control them is to understand their life cycle. Adult sawflies emerge from decaying wood in May or June. The females lay eggs inside the leaf tissue and once they hatch, they go through all three phases devouring the leaves, leaving only the midveins behind. Since it is summer's end anyway and the plant is near the end of its growing season, the dogwoods usually survive and leaf out again in the spring. As for the larvae, once they mature, they stop eating, drop to the ground, and burrow into decaying wood matter for the next nine months.

To have control over this natural life cycle, the best you can do is check your dogwood plants at the first larvae sightings and kill the larvae by squeezing them with your fingers or spraying with an insecticidal soap that smothers them. This has to be done before the larvae reach one inch in size, since by then they have begun to prepare for overwintering. Remove any decaying wood lying around the ground and replace rotting wood on out buildings.

As usual, by the time I notice, the damage is done and I am looking at my poor skeleton of a plant. But knowing that it will recover without any help from me, I decided to stay out of nature's business.

The source for this information came from Michael Martin Mills, The Inquirer, Philadelphia, PA

adult dogwood sawfly