Text and photos ©Nancy J. Ondra
For years now, I’ve noticed these white critters on my shrubby dogwoods (Cornus) in August, and I always assumed that they were some sort of caterpillar. I never worried all that much about them, except for noticing that there seem to be more and more each year. Even though they quickly demolish the leaves they’re feeding on, the plants don’t seem much weakened by the damage.
The other day, I overheard a customer asking our retail manager about the same insects on her dogwoods, and I decided I should do a little research to find out what they are for sure, in case I got asked the same question.
Turns out that they’re not caterpillars at all: they’re the larvae of the dogwood sawfly (Macremphytus tarsatus). And that means that Bt sprays wouldn’t work on them, if one were inclined to take control measures. So, I poked around a little more and found this Dogwood Sawfly Fact Sheet from Penn State, which suggests handpicking or else spraying with insecticidal soap or horticultural oil. They advise spraying in July, saying that the larvae stop feeding once they’re over 1 inch long. I think I’d better let those on my plants know, because it looks to me like they’re still very active and feeding quite heartily.
Another reference mentioned that migrating birds will feed on them, so I’ll think I’ll take a hands-off approach for yet another year, even though the damage is getting pretty bad. Here’s a patch of ‘Bud’s Yellow’ red-twig dogwood. You can see that the sawfly larvae have already chowed down on the left side.
There’s not a whole lot left when they’re done.
The first instar stage emerges in June and is a translucent yellow, easily blending into the leaves. I think the one in the center of the photo below might be one getting ready to molt. Those with the powdery white coating are the second instar.
When they molt, they leave their old clothes strewn about.
The yellowish ones with black spots are the third instar.
Not sure which stage this all-black one is at, but it too is a dogwood sawfly larva.
Okay, birdies: come and get ’em. Please!