Larvae overwinter in chambers constructed in rotting wood, pupate in
spring and adults emerge from late May through July. Females lay
eggs on the undersides of leaves, and can lay as many as 100 eggs
per leaf! Larvae hatch and begin feeding en masse on a leaf until it
has been skeletonized, then they move on to another leaf.
Young larvae are a translucent yellow; after the second molt, they
appear covered with a powdery white substance. After the final molt
before winter, larvae appear yellow with shiny black heads and spots
along their backs.
Redtwig Dogwood with variegated foliage
mature larvae do not feed, but spend time looking for a safe place
to spend the winter. Dogwood sawflies overwinter as mature larvae in
rotting wood, and may attack wooden structures or porch furniture.
The subsequent damage caused by woodpeckers looking for a tasty
snack is much worse than what the insects cause. There is one
generation of dogwood sawflies annually.
The white phase of this insect is an example of protective
coloration -- they look like bird droppings, so predators leave them
alone. Mature larvae can create significant damage to host plants.
Fortunately, this occurs late in the growing season; the damage can
be unsightly but is not a life-threatening situation for the shrub.
Controls & Sprays
If you absolutely cannot live with the damage (remember, the plants
can!), horticultural oil, insecticidal soap, Montgomery Garden
Insect Spray or Captain Jack's Deadbug Brew (spinosad), and Sevin (carbaryl)
are labeled to control dogwood sawfly larvae in the home garden.
Handpicking is an option, too. Put on a pair of gloves
(especially if you are squeamish), pick the larvae off the leaves,
and drop them in a container of soapy water (dish soap is fine for
drowning pests, but may not be for spraying plants).