Today on a short walk in the woods behind my house, I found some leaf mines like these on bristly/swamp dewberry (Rubus hispidus):
A closer look at the one in the upper left corner of the above photo:
I first noticed mines like this several Novembers ago, and although there were larvae inside they seemed to have frozen to death, and I assumed they were the result of females of Metallus rohweri, the common sawfly miner of blackberry and dewberry, laying eggs a little too late in the season for their offspring to complete development. However, in early April of 2014, the author of this blog (Dana) wrote to me about larvae he had just found feeding in similar mines in Pennsylvania, while there was still ice on the nearby vernal pool, and wondered if he might be “on to something a bit unusual”. I mentioned my previous observation, and said “I don’t know if they can actually overwinter in the mines. Probably more likely these are offspring of adults that emerged from pupal cells in the ground a week or two ago.” As far as was known, all leaf-mining sawflies overwinter as prepupae in the ground.
He replied, “Interestingly, I had concluded these larvae overwintered in the mines.” He showed me a photo of some distinctly two-toned mines, and said, “My thoughts were that the whitish areas around the leaf edges are mines from last fall, and this spring those mines were expanded into the the rest of the leaf. This photo was taken 4/10/13… isn’t that rather fast leaf-mining for early April? I think those mines were occupied. If so, the sawflies were ahead of the Wood Frogs…”
I agreed that he seemed to be onto something. The following January, when the temperature was well below freezing, he wrote back with photos of occupied mines he had just dug out of the snow, and said that the larvae were feeding within a few hours of having been brought indoors. In May, he wrote again to report that he had succeeded in rearing an adult. When he showed it to me, and later when I examined the specimen, I couldn’t reconcile it with either Metallus rohweri or the raspberry leafminer, M. capitalis. It was oddly pale, like this one…
…in contrast with M. rohweri…
…and M. capitalis, both of which have dark abdomens.
I passed the specimen along to Dave Smith, and after some deliberation, he reported that it was Metallus ochreus, a species he had described in 1988. It was known only from adults caught in Virginia and Maryland, and its host plant was unknown, although Dave had noted that Rubus spp. were present near each collection site. He also noted in the original description that while most sawflies are spring fliers, all specimens of this species had been collected in late September and October. It was a little puzzling, therefore, why Dana’s specimen had emerged in early May, especially since he said it was from a leaf mine he had collected in late March or early April. I wonder now if there was a mix-up and it was really from one of the larvae he had found in January, although even that would be a faster turnaround than I later experienced…
…Because to make sure the biology of Metallus ochreus was properly documented, I collected a large number of larvae mining Rubus hispidus leaves here in Massachusetts in February 2016. Here’s what the mines looked like then; you can sort of see how the early, narrow part of the mine is paler than the rest, although it was much more striking in the examples Dana found:
It was hard to tell if the mines were occupied without picking the leaves and backlighting them:
All of the larvae finished feeding and burrowed into jars of soil within two weeks of my bringing them inside. In August, about six months after I’d collected the larvae, 23 adults emerged, and they were all M. ochreus as expected. They presumably would have emerged even later if I hadn’t brought them indoors in February, and this is consistent with Dave Smith’s impression that the species has a single generation with adults only appearing in the fall. This past spring I found some larvae near my house and checked on them every day; they dropped out of the leaves beginning in late April, and the last one hung around until May 3. Add six months to that, and it makes sense that early November is when the leaf mines start to be noticeable.
All this, along with new host and distribution data for the other North American Metallus species, was revealed last week when my latest paper was published:
Eiseman, Charles S. and David R. Smith. 2017. Nearctic Species of Metallus Forbes (Hymenoptera: Tenthredinidae): Biology and Distribution. Proceedings of the Entomological Society of Washington 119(4):551-564.