Sawflies are best-kept secret in insect studues
August 19, 2001

An unusual, bumblebee sized insect was brought to a local naturalist for identification. It proved to be an elm sawfly (Cimbex americana).

It is said that sawflies are one of the best kept secrets in the world of insect study. ONe might read many books on insects and still know nothing about the numerous species of common sawflies.

Sawflies are in the same order as bees, wasps and ants (Hymenoptera). Though adult sawflies may resemble bees and wasps they are identifiable by the broad attatchment joining the thorax and abdomen whereas in bees, wasps and ants the attatchment is narrowed to a threadlike waist.

The elm sawfly belongs to the largest kind of sawflies, the size of bumblebees but are not hairy.

They have seven segmented antennae which are slightly clubbed. They do not sting but can inflict a strong pinch with their mandibles.

The larvae resemble the caterpillars of moths and butterflies, but there is a difference.

The easiest way to identify them is to look at their legs. Although both caterpillars and sawfly larvae have three pairs of hardened legs near the head, after which there are pairs of softer, leg-like projections call prolegs, sawfly larvae have six or more pair of these while caterpillars have five or less.

The larvae of sawflies are distinctive as they curl their abdomens around the object on which they are feeding and they curl their heads up and back when disturbed.

The specimen at hand is a male elm sawfly, with a dark blue body marked with yellow spots on the abdomen. The female has a large whit spot on the prothorax. The wings of both are smoky colored.

The sawfly larvae, when mature, spin silken cocoons. They may overwinter as larva in these, or pupate, and emerge in the spring.

In July, 1992, Woodland Dunes was infested with alder sawflies. The leaves of the alder trees along the Cattail Trail were all chewed up until only the central ribs remained.

AT that time a Woodland Dunes column dated July 30, 1992, discussed the life history of sawflies, which might bear repeating.

In early spring adult sawflies emerge from their parchment-like cocoons. They mate, and eggs are laid on the chosen foliage.

The name sawfly is indicative of the egg-laying organ (ovipositor) which has saw-like edges.

The eggs hatch in about a week and the larvae feed voracioulsy on the chosen plant.

The cocoons that are formed usually fall to the ground.

In the case of the 1992 invasion they probably fell into water as the level was high.

A question at the end of the 1992 column: what is the future of the alders and will there be another infestation?

The alders recovered and there has never been even a slight return of alder sawflies ever since.

In the absence of elms, the elm sawfly larvae also feeds on willows. To date there is no known report of larval activity in our area.