The birch sawfly

by Richard Jones

[The children] were amazed when I told them that instead of a moth, the larva would turn into a sawfly the size of a hornet...

Sawfly larva on a leafWhen the children first found the caterpillar, they were fascinated and asked me what sort of moth it might turn into. They were amazed when I told them that instead of a moth, it would turn into a sawfly the size of a hornet. Some of the parents looked slightly disturbed at the prospect.

This all took place in the nursery section of our local school. The larva was wriggling on the ground beneath one of the medium-sized birch trees, which give such lovely dappled shade to the children playing in the garden there.

It is the larva of the birch sawfly, Cimbex femoratus. At over 35mm long and a good 6mm in diameter, it rivals many a plump and handsome moth caterpillar in its size. Unlike lepidopteron larvae, though, the Cimbex grub has only the six 'true' thoracic legs near the head, and lacks the fat gripping prolegs and claspers towards the tail end, which moth and butterfly caterpillars use to grip the leaf.

Despite textbook assurances that the insect is quite common and widespread, this is only the first time that I have ever found the larva. I'm guessing this is because it spends all of its life up in the birch canopy, even spinning its chrysalis up there. And the only time I have ever seen the truly impressive adult was when a teacher from the same nursery presented me with a dead specimen found flying around indoors last year.

Unlike the berberis sawfly, which has caused quite a running commentary on this blog, Cimbex is never a garden pest, since it never reaches pest proportions. Instead, it remains an elusive but awe-inspiring insect. Unfortunately, this one had been slightly squished, and died later. It now resides in a tube of alcohol. But I'm sure we have a colony up there.

Discuss this blog post

Talkback: The birch sawfly
Your comment will appear after a quick registration step

Gardeners' World Web User 02/07/2009 at 14:34

sorry they are ladybird larvae

Gardeners' World Web User 03/07/2009 at 01:22

is there a bird that eats these massive critter's. make a really good meal probably full of protine.

Gardeners' World Web User 03/07/2009 at 21:59

how do I stat a new blog.You gave us some super information on bumble bees with illustations. Would it be possibl fo you to do the sme on moths?ruth

Gardeners' World Web User 28/11/2011 at 18:39

My wife caually announced there are some strange larvae on your cob nut tree. They have their feet all at one end! In horror for some reason I thought of colorado beetle from her spotty description. Anyway relieved to know now they are a sawfly lavae Thanks to your site