Introduced Pine Sawfly Larva

2012 April 14

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[1], 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.

[1] 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.

3 Responses
  1. April 16, 2012

    Wasn’t everything “introduced” in one way or another? If not, how would things like hermit crabs get from one spot on the planet to another?

  2. April 17, 2012

    Yes, and that’s particularly true in places like Michigan, where compliments of the glaciers of the Ice Age there is *nothing* big enough to see that has lived here for more than about 10,000 years[1]. I don’t think that Michigan (and particularly the Upper Peninsula) has had what one would consider a “stable ecosystem” for at least the last 3 million years (unless you consider a sterile glacier to be an “ecosystem”)

    Still, if we take “introduced” to mean “entered the area within the last 100 years or so”, I’ve been noticing that the vast majority of the most-numerous arthropods around here are recent introductions. We are absolutely crawling with woodlice, pillbugs, earwigs, clover weevils, and the like, and have in the past been completely overrun with Asian lady beetles and European paper wasps (and are building up to a big Gypsy Moth infestation). It’s these recent introductions that are prone to massive population explosions.

    [1]Although, in most places, there are a large number of species that evolved where they are, and it was their (sometimes very remote) ancestors that were introduced. Like Central Asia, which is more a source *of* invasive species, than a place for invasive species to *go*.

  3. April 17, 2012

    I’m listening to The Decline and Fall of the Roman Empire right now. The author makes the point that almost every fruit in Europe was brought there by the Romans. When they found they liked something, they planted it everywhere.

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