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The Nature Nut

Rosamund Pojar
Elm Sawfly

We had an interesting visitor to our property this week. Jim, my husband, came in the house and handed me a jar containing a huge yellow and black insect that initially looked like a giant hornet or wasp.

I took a photo of it through the glass of the jar as I was not keen on letting it out. It measured 4 cm in length and almost 1 cm wide. It had no obvious stinger and was not pinched between the thorax and abdomen, so it was not a wasp or hornet.

Since I was a bit flummoxed as to what it could be, I called an entomology friend and sent him the photo I took, and he got back to me to tell me it was an Elm Sawfly (Cimbex americana).

Native to North America it has voracious larvae that have been known to defoliate elm trees but will also feed on other trees including willows. They have been reported sporadically throughout BC.

The adult stages are not around for very long (up to 10 days) which could be why we have never seen one before.

The males have a reddish-brown body, whereas the female’s abdomen is striped black and yellow just like wasps and hornets. They buzz their smoky brown wings when disturbed.

In their short adult life, they breed, and then the female lays her eggs in slits or depressions she makes on the underside of leaves. Adults often girdle twigs of host plants, hence the name sawfly.

The huge larvae can reach over 10 cm long, are yellow with a black dorsal stripe and a single eye on either side of the head. The sawfly larvae have more than five pairs of stubby abdominal legs that lack hooks at the tips, whereas true caterpillars have only five pairs of legs each with hooks and are located at the head end of the body. True caterpillars also have more eyes on their head.

The larvae feed on foliage throughout summer until early fall then drop to the ground and overwinter in a cocoon, pupate the following spring, then emerge as robust adults two weeks later.

I am guessing the rather dopey female adult Jim found had just emerged as it was not very mobile. Two days later he saw another one with very little yellow – probably a male.

So now we will be watching for defoliation by large munching sawfly larvae.