Predatory Sawflies

In my last post, I showed a photo of a sawfly visiting extrafloral nectaries, and I alluded to another observation from the same day that made me wonder whether sawflies might benefit plants with extrafloral nectaries in the same way ants do.  Sawflies have strictly herbivorous larvae, and when I have seen adults feeding in the past it has always been on nectar or pollen while visiting flowers.  But when I was reviewing my photos from the Berkshire BioBlitz, I discovered that I had documented some predatory behavior in a sawfly without knowing it.  In fact, I had done so without even realizing I was photographing a sawfly–I thought it was another ichneumon like this one I had photographed an hour earlier, identified by Bob Carlson as a member of the subfamily Diplazontinae (and therefore a parasitoid of aphid-eating syrphid flies):

A diplazontine ichneumon wasp. (~5 mm)

In each of the four photos I got of the sawfly, a tiny wasp can be seen, which I believe to be a platygastrid like the one in this photo–it seemed like every other leaf I looked at that day had one of these wasps, which are parasitoids of gall midges:

Platygaster (Platygastridae; 1.3 mm).

In the first shot, the platygastrid is visible as an out-of-focus blur in the lower right corner, apparently healthy and standing upright:

In the second shot, the sawfly has the platygastrid in its jaws and is turning it on its side:

In the third shot, it looks like the tiny wasp is about to get chomped:

Detail of the third shot.

In the final shot, however, the sawfly is walking away, leaving the platygastrid lying on its back but apparently intact:

I asked sawfly expert Dave Smith for his thoughts on this, and he wrote:

This is a male Tenthredo.  Adults of Tenthredo and some other sawflies with large mandibles (e.g., Taxonus) are known to be predatory.  They are occasionally found catching flies or other insects.  I suppose a platygastrid is as good as anything although they are most often found with flies. I don’t recall records of them taking platygastrids.   It looks like this one attacked it, then left it alone.  Usually they might munch on them for a while.

Given the known diet of predatory sawflies, it seems that the sawfly I saw visiting extrafloral nectaries on choke cherry was unlikely to provide any more benefit to the plant than was the dark-winged fungus gnat that followed it.

While I’m on the subject of sawflies, I might as well take the opportunity to explain what they are–I’ve noticed that whenever I mention them, people either stare blankly or ask, “Did you say ‘soft flies’?”  Sawflies belong to the order Hymenoptera, along with bees, ants, and wasps.  One of the distinguishing features is that the abdomen is broadly attached to the thorax, so that they lack the narrow ‘waist’ of other hymenopterans.  They are called ‘sawflies’ because the females have serrated ovipositors with which they “saw” into plant tissues to lay their eggs.  The larvae, as mentioned above, are herbivorous, and they are easily mistaken for caterpillars.  Dr. Smith told me previously that the larval stages are known for 20% or fewer of the North American sawfly species, so there is much left to learn about these ubiquitous insects.  Below is an example of a sawfly larva, in the same genus as the one shown here attacking the platygastrid–it may or may not actually be the same species.

The larva of Tenthredo grandis, which feeds on turtlehead (Chelone).