A fly that is really a bee

If you’re always looking up to find the birds in the trees, you might miss something interesting on the ground by your feet.  And indeed, I almost stepped on this huge 2- inch long Elm Sawfly that really isn’t a fly at all.

You can see two pairs of wings on this sawfly, which means it is a member of the bee and wasp order of insects, not a fly.  The long orange, club-like antennae are also characteristic of bees, not flies.

One might wonder why a bee would be called a fly — and there isn’t any logical explanation except that the female uses her ovipositor (at the end of the abdomen) like a saw to open cavities in an elm leaf in which she lays about a dozen eggs.  Then she moves onto other leaves and does the same thing, until she has deposited about 150 eggs.  Whitish larvae hatch out and like all sawflies, immediately begin consuming the vegetation, growing about two inches in a month of eating, until they return to the soil to pupate over the winter.  An infestation of sawflies on a young tree may defoliate it so much that the tree becomes stunted, or even dies.

With those wicked-looking, piercing mandibles below its eyes, you might think this insect is carnivorous, but it actually feeds on nectar and pollen and occasionally the tender bark of young twigs.  But females don’t live long enough to do much damage to the plants; once they have deposited their eggs, they die.

This was a very sluggish individual that might have just emerged, since they usually appear at the end of May and early June.

These insects are probably not very numerous in this part of Minnesota since Dutch Elm disease has wiped out so many of their potential host plants.  In addition, both the eggs and the larvae may be parasitized by small wasps and never make it to the pupal stage.  If they reach the pupal stage in the soil, they may be preyed upon by shrews and deer mice, so seeing one just sitting on the trail might be a rare event.