Poison Ivy Sawfly

Mary Bennett (MN) sent me this image with psychedelic color right out of the '60s.  After a few frustrating minutes I sent it on to Kevin Firth who identified it as the larva of the Poison Ivy Sawfly, Arge humeralis.  As you probably already guessed, the larva feeds on poison ivy, Toxicodendron (Rhus).  It is not alone, there are over 100 insect species in the US that have been collected and reared on poison ivy!

When is a fly not a fly?  One rule of thumb is when its name is combined in one word as in "sawfly."  True flies of the family Diptera have their common name separated as in "House Fly."  A. humeralis is actually a sting-less wasp!

   Tom Klein CC

 Color variation - Tom Murray CC

This little purple cat will become an adult with a lot of unwasp-like characteristics.  For instance it lacks the typical wasp waist that is fashionable in most of the others creatures in the order  Hymenoptera.  Another is the resemblance of the larva to the caterpillars of Lepidoptera.

It is actually the differences in the larvae that will help me next time, if I can remember them that long.  Sawfly larvae have six or more pairs of prolegs, the leg like structures on the abdomen, while Lepidoptera cats have five and none on the first two abdominal segments.  Many of the sawfly larvae are able to squirt a noxious smelling substance from their last segment when they are harassed.

Seven pair of prolegs and counting, starting at the first abdominal segment.

The "saw" refers to their saw-like ovipositor that they use to cut into plants to lay their eggs.  This of course does no good for the plant and some species cause serious damage to cultivated plants.  On the other hand, A. humeralis has been considered as a possible biological control for poison ivy, a plant that 70% of the population is sensitive to.  Few of us would mourn its passing but that is an unlikely extinction.

Details on the Poison Ivy Sawfly are at this link.