First sawflies of the season!

I took all my overwintering larvae and pupae out of the fridge on March 1, and so far two agromyzid flies, two scathophagid flies, two braconid wasps, two eulophid wasps, and seven sawflies have emerged as adults. Apart from the braconid and eulophid parasitoids, these are all species that have a single generation per year, with larvae that are only active for a few weeks in spring or early summer, so every individual that emerges nearly a year later feels like a major triumph.

The first sawfly appeared on March 12, from a jar of soil into which at least seven larvae had burrowed between June 18 and July 11 of last year. I had collected three larvae on June 13 from some field horsetails (Equisetaceae: Equisetum arvense) in the ditch across the road from my house, and several additional larvae appeared in the jar over the next couple of weeks—they had been either eggs or tiny larvae that I didn’t notice when I collected the three larger larvae. Here’s what one of them looked like on June 15:

And one on June 18:

This winter, between putting together my “Bugs in Winter” online course, revising portions of Leafminers of North America, peer-reviewing hundreds of pages of manuscripts written by other leafminer researchers, and getting a few papers of my own ready to submit, I managed to make reasonable progress on my guide to sawfly larvae, including writing the section on horsetails. What I learned as I put together the horsetail section is that Leblanc & Goulet (1992) published a key to larvae they had raised from eggs laid by six species of Dolerus (Tenthredinidae) they had collected in eastern Canada and put in cages with field horsetail plants. Twenty other Dolerus species and subspecies in North America are likely to feed on horsetails because they belong to subgenera that have been exclusively associated with horsetails in North America and Europe (mostly as caught adults), but the larvae are completely unknown for all of these. So as far as I know, this adult that emerged four days ago is the first North American horsetail-feeding sawfly that anyone has actually reared from larva to adult:

I haven’t made any attempt to identify it yet, but the larvae don’t match any of the six species described by Leblanc & Goulet, so it seems like some progress has been made here. I think they are the same as sawfly #19 from last year’s yard list, which was likewise found on field horsetail, and I think they’re also the same as the larvae I found on wood horsetail (E. sylvaticum) by the beaver pond down the hill from my house—an adult emerged from that jar today, but I haven’t had a chance to photograph it yet.

I fully expected the first sawfly adults to emerge to be Dolerus, since I’ve seen adults of this genus as early as March under natural conditions. I also expected that other early emergences would include some of the first larvae I found last year. On May 31, I collected five larvae like this from a little aspen sapling in my front yard (sawfly #3 from the yard list):

IMG_2208

At the time the sapling was unambiguously a bigtooth aspen (Salicaceae: Populus grandidentata), but over the course of the summer it transformed into an unambiguous quaking aspen (P. tremuloides). I haven’t yet attempted to make any sense of the 20+ North American sawfly species that are known to feed on Populus, but as I mentioned when I first found these larvae, I suspect the genus is either Euura or Nematus. Whatever species this may be, I now know what the adults look like, because all five of them emerged on March 14 and 15.

Sawflies overwinter as larvae in their cocoons, waiting until spring to pupate. So apparently these two species can be convinced that spring has arrived after being exposed to warmer temperatures, then pupate, transform to adults, chew their way out of their cocoons, and claw their way to the soil surface, all in the space of 11 to 14 days.

With any luck, there will be more updates soon!

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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5 Responses to First sawflies of the season!

  1. Lynn says:

    I love the unambiguous aspens. No wonder I’m confused. And I shall have to kneel before Equisetums this summer, in hopes of finding yet more confusing arthropods.

  2. lifelessons says:

    The sawfly larva is gorgeous.

  3. DonRecklies says:

    Ditto (sort of) Lynn’s comment. I ruefully recall the annoying variety of leaves dropped by the (%#!&) aspens in a study plot for a class project the names of which changed 3 times before I was done – and I’m still unsure what they really were!

  4. Marko Prous says:

    The nemtine from Populus is almost certainly Euura (non-Amauronematus Euura I think). The larva resembles E. caeruleocarpus, but the adult does not fit this species at all.

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