Sawfly Surprise

As I’ve mentioned a few times before, I’m starting to work on a hostplant-based guide to sawfly larvae that will be arranged similarly to my leafminer guide. In addition to reviewing the existing sawfly literature, I’ve been sporadically collecting larvae to rear, and I plan to do a lot more of that this year. Since the hosts are known for only about a quarter of the North American sawfly species, raising them to adults will often be necessary to figure out what species I’ve found. For the past few years, I’ve been keeping an eye out for sawfly larvae to collect while surveying for galls and leaf mines on Nantucket. Here’s a striking one I collected there on September 5, munching on the edge of a leaf of beaked hazelnut (Betulaceae: Corylus cornuta):

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Five days later, it had molted to its final instar:

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And by September 13, it had spun this cocoon among the crumpled paper at the bottom of the rearing container:

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On April 1, something emerged from the cocoon—but instead of an adult sawfly, it was 44 beautiful little green wasps:

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Although I’m used to them being more nondescript, I eventually recognized these as tetrastichine eulophids. Evidently the larvae are internal parasitoids, since there was no external evidence that anything was amiss with the sawfly larva. Lacking a reared adult sawfly, I have to rely on existing sawfly hostplant records to determine the identity of the larva, but it seems like there’s a pretty good chance it was Arge pectoralis (Argidae), a species more commonly found on birch but also recorded from hazelnut and alder (all plants in the birch family). Tetrastichus trisulcatus is the only tetrastichine eulophid that has been reported as parasitizing this or any other Arge species, and it is apparently a sawfly specialist, but sawfly parasitoids are of course more poorly known than sawflies themselves, so the wasps that came swarming out of that cocoon may or may not have been T. trisulcatus.

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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4 Responses to Sawfly Surprise

  1. Deborah Zinn says:

    Have to say the sawfly larvae on my plumicot (plum/apricot hybrid) here in Melbourne, Australia are solid black, slimy — yuk! They eat a lot & I go out when they are there & squish with my fingers (sorry!). (I’m sure a lot survive unseen.) I read up on them & evidently they are unappealing to birds so that doesn’t keep them in check…. I’ve never noticed the cocoons or the flies. I am an insect lover, but fair’s fair — they tree would be decimated & I don’t use any insecticides (organic gardener). There evidently are different sawflies here in Australia that feast on Eucalyptus trees, but I don’t think I’ve ever seen those.

  2. Elva says:

    Forty Four!! What a surprise. How fascinating.

  3. susantcloutier says:

    Thank you again for giving us insight into a world less traveled!

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