Q. White "worms" are eating my redtwig dogwoods. Can you tell me what they are and how to get rid of them?
A: The writer attached a photograph showing dogwood sawfly larvae in their white phase, feeding on a redosier dogwood (Cornus sericea). These plants are grown primarily for their red (or in some cultivars, yellow) stems in the winter landscape. They are typically adaptable, fairly problem-free plants. The dogwood sawfly (Macremphytus tarsatus) is one of the few pests that bother them. Gray dogwood (Cornus racemosa) and redosier dogwoods are preferred hosts for this pest.
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.
Very 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.
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).