Sawfly Connections

It’s always satisfying to meet some strange new insect, only to discover that it is another life stage of a very familiar one.  This has happened to me twice now with sawflies.

In early July, I photographed this sawfly in my yard.  It looked much like most other sawflies, except the hind tarsi were distinctively flared out.

I’ve just learned that this is Craesus latitarsus (Tenthredinidae), the “dusky birch sawfly.” Actually, two other eastern Craesus species look essentially identical as adults, but they feed on hazelnut and chestnut.  This sawfly was right next to a birch tree, and there was no hazelnut or chestnut nearby, so I think it’s safe to call it this species, especially since the other two are much less common.  I know the larvae of C. latitarsus well; there were so many of them on the gray birch saplings around my new house this fall that they were almost completely defoliated.  (This, incidentally, causes me no concern whatsoever.  If a native insect wants to defoliate trees in my yard, it has every right to.  Birches are hardly in danger of becoming scarce.)

Two Julys before I photographed the adult C. latitarsus, I met a sawfly that so resembled an ichneumon wasp in its color pattern and frenetic behavior that it had me fooled at first.

I later learned that this is Macremphytus testaceus, a species whose larvae eat dogwood leaves.  This made perfect sense, since the previous summer the dogwoods on the other side of the house had been covered with the larvae of this species.

When feeding, the larvae have a white, waxy covering like the one on the left.  When mature, they molt one last time and look like the larva on the right.  I once tried to raise these larvae, and had them in a Ziploc-type bag by my desk.  One night when sitting at my desk, I noticed some motion across the room out of the corner of my eye.  When I went over to investigate, I discovered that one of the larvae had chewed its way out of the plastic bag and had been trying to bore into the windowsill when it fell into a spider web, from which it was now struggling to free itself.  I later found a small pile of sawdust under a nearby chair, which the larva had evidently tried boring into before heading for the windowsill.  It turns out that Macremphytus species burrow into dead wood to overwinter and pupate.

Sawfly specialist Dave Smith once told me that when attempting to rear unknown sawfly larvae, he puts them in jars with sand/soil, corks, and corrugated cardboard so they will have a variety of options for pupation sites.  I think the story I’ve just related illustrates why jars are preferable to plastic bags.