Hazelnut Sawflies

It’s normally a little disappointing to find an ichneumon wasp in one of my rearing containers, since that generally means I failed to rear whatever insect I was trying to rear. So when this one appeared yesterday…

…I braced myself for the bad news as I checked the lid of the jar to see what sawfly larva I had collected sometime last year, kept provided with fresh leaves of its host plant, given a jar of soil to burrow into, and refrigerated over the winter, only to have it succumb to a parasitoid, leaving its identity as mysterious to me as if I had just taken a photo of it on the plant and left it to its fate. But I found that what I had scribbled on the lid was “10/1/22 hazelnut, parasitized, gone 10/2.” Which meant that I already knew it was parasitized when I collected it on October 1, and it burrowed into the soil by the next day—so not a big loss, and a minimal investment of effort on my part. As it happens, I had just made it up to October 1 in sorting through the photos I took last year, so I was able to quickly refresh my memory about who this sawfly larva was. But before I get to that, a little background on the hazelnut sawflies in my yard.

Although there are native beaked hazelnuts (Betulaceae: Corylus cornuta) growing wild around the edges of the yard, it is the hazelnuts we planted—Corylus ‘Medium Long’, which is thought to be a hybrid of American hazelnut (C. americana) and the European hazelnut (C. avellana)—that always seem to attract the sawfly larvae. Just about every year, these larvae show up and get to work defoliating them:

Both of the above photos were taken on September 2, 2016, and at the time I thought I was seeing different-aged larvae of the same species—in fact, they were filed together until today, when I realized the smaller larvae in the top photo belong to a different family (Tenthredinidae; tribe Nematini) from the spotted larva, which is in the family Argidae. It was the spotted larvae that I succeeded in rearing; adults emerged the following June, and Dave Smith identified them as Arge willi.

When I did a thorough inventory of the sawfly larvae in my yard in 2020, larvae of the first type showed up on June 21. I collected them but again failed to rear them; four parasitoids in the genus Ichneutes (Braconidae) emerged the following March.

And I found larvae of Arge willi starting on September 1.

Then, on October 10, I found this beauty (which is something in the subfamily Tenthredininae)…

…but I failed to rear it as well. I looked in vain for more larvae like this in October 2021, and then again this past October. Although no yellow-spotted larvae materialized, I did find yet another species that I hadn’t seen in my yard before.

I think these may be Hemichroa crocea, which normally feeds on alder (Alnus) but is occasionally found on birch (Betula) and hazelnut (Corylus). I was just about finished writing my key to birch-feeding sawfly larvae this winter when I turned my attention to a taxonomic paper on tischeriid moths, but hazelnut will be the next key I do when I get back to that project (well, after musclewood (Carpinus), as I work my way alphabetically through the birch family). Anyway, on the same bush as the larvae above, I found this one, which (based on my knowledge of birch-feeding sawflies) I believe is a Nematus species.

I saw the objects attached to its head and thorax and knew it was unlikely to survive, but I decided to collect it anyway. Ichneumon wasps in the subfamily Tryphoninae attach stalked eggs to their hosts, and I think that’s what the brown object on the head is. As for the white object on the thorax, I believe it is a larva rather than an egg, but whether it’s a tryphonine or something else, I’m not sure.

In this view, the stalk on the egg is more apparent:

I think the ichneumonid that emerged yesterday is in fact a tryphonine, but someone please correct me if I’m wrong. And its sawfly host may have been Nematus corylus; for comparison, here are some larvae of that species that Julia and I collected on Nantucket in September 2020…

…and an adult that emerged the following April:

Looking back through my photos, I think N. corylus was probably the host of the braconids, and it may have been the species that I initially mixed up with Arge willi, but I don’t seem to have photos of those larvae in later instars. Once I put together my key to larvae on hazelnut I’ll have a better sense of how many (known) options there are for larvae with that general appearance. In any case, it’s nice to have our hazelnut bushes providing food for at least four different sawfly species, in addition to occasionally giving us some nuts to eat.