Dogwood defoliating caterpillars
Thanks for the question.
I believe that these are larvae of the Dogwood Sawfly. Technically they are not termed caterpillars but maggots. See:
Take a look at the following to see what the adult looks like:
They will not harm your dogwood. They are not pollinators. While they might be eliminated with various pesticides, these pesticides would also kill many beneficial pollinators. Other than to improve the overall appearance of your dogwood, there really is no reason to kill them. These larvae will soon drop to the ground, make a cocoon, and over-winter in that form. Adults will emerge next spring. If you wanted to be proactive for next season, I would suggest you do the following:
1). As much as possible, physically remove these larvae from your dogwood and put into a bucket of soapy water. This will kill them and prevent them from overwintering.
2), As part of your fall cleanup, thoroughly remove all plant debris from under your dogwood. Put it in a trash bag and dispose of accordingly. This should remove any larvae that may be existing as cocoons in this debris.
The above two actions should greatly diminish next season’s impact on your dogwood. However since adult sawflies can travel over significant distances, there is no guarantee that you won’t see these larvae again next season. Should that be the case, physical removal would be your best action if you are concerned about not harming any beneficial pollinators.