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):

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

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

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

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.