Introduced Pine Sawfly Larva
Sandy caught this on September 18, 2011 while sweep-netting in the grass on our septic drain field. It was about an inch long, not an unusual size for a caterpillar.
One thing that we noted right away, though, is that it wasn’t actually a caterpillar. If we count the prolegs on the abdomen, we see that there are eight pairs. If this were a butterfly or moth caterpillar, it would only have no more than five pairs of prolegs.
This narrowed things down considerably. Larvae that look like caterpillars, but have too many prolegs, are generally some species of Sawfly, which are related to ants, bees, and wasps.
The coloration pattern of this one is pretty distinctive, with those yellow spots and stripes on a black background. A quick BugGuide search on “sawfly larva” quickly turned up an excellent candidate: the Introduced Pine Sawfly, Diprion similis.
“Introduced”. It figures.
These were accidentally introduced in Connecticut sometime before 1914, and have since spread west to most of Canada and every state east of the Mississippi, with some also present in Washington State and probably a number of other western states. They eat the needles from a variety of different pine trees, but they prefer the 5-needled pines like White Pine. Some of which we have growing in our back yard.
The eggs are laid inside of pine needles in the spring, with about 10 eggs per pine needle. When they hatch out, they are gregarious, with masses of the larvae stripping all the needles off of a given branch. They don’t usually kill the trees, but they can stunt them, and chewed-on branches don’t look very nice. Which is a problem when they get into christmas-tree plantations.
When the larvae mature, they disperse (which is why we found this one in the grass instead of in a pine tree). They make a hard, brown cocoon that they overwinter in, and emerge as adults in the spring. Although, evidently, they don’t always emerge the next spring, sometimes they’ll stay cocooned for as long as 3 years before emerging. This is probably how they got introduced, by the way – some of the long-duration cocoons were probably stuck to some imported object, and they were off and running.
 The prolegs on sawfly larvae are evidently a case of convergent evolution with caterpillars. Genetic studies of their proleg development have shown that a different set of genes is used to grow them than are used by butterfly/moth caterpillars. It’s just that something like prolegs are very useful to a leaf-feeding grub, because without them one tends to fall off the leaf a lot. So sawflies evolved them independently, even though they are practically unrelated to caterpillars.