Wait... That's Not A Caterpillar

One of the more common trees in the hill country of Texas is Cedar Elm (Ulmus crassifolia) and my backyard is no exception.  There is a large Cedar Elm that arcs over much of my backyard, providing shade to the back deck, the kitchen and living room downstairs, even shading the upstairs windows of the master bedroom.  A couple of weeks ago, there was a large amount of frass, or insect excrement, near the grill on the back deck.  Not long after the frass began appearing, I saw what looked like a caterpillar on one of the Cedar Elm leaves and, as I was in a hurry to head to the airport, assumed it was the larval stage of a butterfly or moth.  When I arrived back in Texas from visiting a client, I got a closer look and saw exactly what it was.  And, for our conversation, what it wasn't!

Working its way down the trunk of the tree and onto my backyard fence was the same larva I'd seen eating away at the Cedar Elm leaves.  Much to my surprise; however, in spite of the way it looked, this larva was no caterpillar!  Instead of a Lepidoptera, I would be spending a few minutes admiring a Hymenoptera.

Elm Sawfly (Cimbex americana) larva 

Hymenoptera?! Lepidoptera?! Wait a second... For those of you that have never been on a nature walk with me, I don't just throw out big words for the sake of proving I can speak multisyllabically.  I always break these things down so we can all understand what we're talking about.  So, before you close your internet browser or click over to another site, give me a second.  

Butterflies and moths are insects belonging to the order Lepidoptera, which can be literally translated to "scale wing".  This refers to the scales that all butterflies and moths have on their wings.  Ever pick up a dead moth and get that fine dust on your fingers?  That isn't dust, but wing scales rubbing off onto your fingers.  

Wasps, bees, ants, and sawflies are insects belonging to the order Hymenoptera, which can literally be translated to "membrane wing".  This refers to the delicate membranous wings, from the root "hymen".  It would be this order of insect that my visitor belongs.  

Sawflies, including the Elm Sawfly (Cimbex americana) are Hymenoptera that look more like bees, but are more closely related to wasps.  They lack that tight, petite waist that transitions from thorax to abdomen in wasps, which often confuses people into thinking they are bees.  True to their name, Elm Sawflies  use a saw-like ovipositor to cut into the twigs and small branches of members of the Elm (Ulmus) family of trees to lay eggs.  They aren't; however, strictly confined to Elms, as they will often lay eggs on the twigs of members of Maple (Acer) and Willow (Salix).  Another defining characteristic of sawflies are larvae that strongly resemble Lepidoptera caterpillars.  I think we can all agree the photos above and below look extraordinarily like caterpillars!

Count the prolegs of the Elm Sawfly (Cimbex americana) larva in this photo and you'll quickly see how to distinguish it from the caterpillars of butterflies and moths.

Now, you're likely wondering how to tell the larvae of sawflies from the caterpillars of butterflies and moths.  Well, it's just as easy as it can be.  Simply count the prolegs, or the short, unjointed appendages that look like stubby legs on the larva!  Lepidoptera larva have five pairs of these appendages.  Sawfly larva, like the above pictured Elm Sawfly, have anywhere from six to eight.  Most often, as seen above, there are seven pairs of prolegs.  Being unjointed means these aren't true insect legs, giving them them the name "prolegs".  Prolegs also have really cool hooks on their tips that look like crochet needles which they use for gripping.  They're even called crochets.  For a quick comparison, just flip to my last entry about Lepidoptera.  You'll quickly notice the Wilson's Wood-nymph caterpillar in the photos has five pairs of prolegs.  Remember, count the short, stubby and unjointed appendages; not the jointed true legs at the very front of the larvae!

Elm Sawflies are pretty widespread and you are likely to see them over much of the United States.  They don't tend to do enough damage to trees to be considered an economic problem and they have their fair share of predators.  Often, they are attacked by other members of the order Hymenoptera, like Ichneumon Wasps  They may strip your favorite shade tree of some of its leaves or rain frass down upon your patio, but their populations vary quite a bit from year-to-year.  If you see a lot of them this year, you may very well not see many at all next year.  The larvae will burrow into the ground and pupate in a rather papery cocoon, where they won't emerge as adults until next year.  Occasionally, when you're digging near trees that sawflies have used as a food source, you'll find last year's cocoons scattered quite shallow below the surface of the ground.

So, next time you see what you think is another caterpillar, take a closer look.  You just might have a sawfly larva of your very own!  As always, let me know if you see anything cool out there in the field and in the woods!