Hi Mark,

I believe you've observed the larvae of the dogwood sawfly (Macremphytus tarsatus). Despite resembling caterpillars, they are actually the larvae of a type of wasp. 

The best way to manage these wasp larvae is by spraying them with insecticidal soaps/horticultural oils (which work better than dish soap). And it is imperative to make those applications when the larvae are less than an inch (25 mm) long. Once larvae pass that size threshold, they become more resistant to insecticides. This spraying can be supplemented by shaking them off plant material and crushing them underfoot. 

After they've finished feeding, the larvae seek out decaying or soft wood to burrow into for pupation. If the wood is pressure-treated and properly preserved, these insects will have a very difficult time boring into it. Identify, remove and replace any rotting/soft wood. Wood-boring insects typically cannot infest pressure-treated wood. But if they infest the wood prior to treatment, they may emerge from the wood following pupation. 

Let me know if you have further questions. 

Wow, that's impressive. I will run your question by my entomologists colleagues to see if they have a different ID/control recommendations. I will let you know if they say something different than what I said. 

Hi Mark,

I heard back from my colleagues. They agreed with my ID and the management, but had two things to add.

1) these sawflies can bore into wood that is structurally sound. So the issue is not that the wood has rotted, as you pointed out.

2) the wood can be treated with a residual insecticide to prevent wood-boring. However, I would only apply this when the larvae are mature and are looking for somewhere to bore into (late spring/early summer). The best estimation of when to apply would be when you see larvae over an inch long, or around late July/early August. 

By residual insecticide, I mean one that sticks around for a while. Cyfluthrin is one option. There are other insecticides in the Permethrin class that may work. You'll know it's a permethrin because the active ingredient ends in -"thrin". 

When applying these insecticides, make sure to follow the instructions on the label exactly. Also, try to avoid applying these insecticides on hot or windy days, and not when rain is expected in the following days. 

I hope this helps!