This buggy ballet was filmed by Holly Welch who watched these dusky birch sawfly larvae feeding on her river birch leaves. They are gregarious, feeding together as a family. They tend to eat from the edge inward until they reach the petiole before moving on to another leaf.
These sawfly larvae have a defensive
startle reflex, designed to prevent predation. You can see it in the
photos above one second apart after Holly tapped the leaf. Her video shows that this occurs in unison, increasing the startle of the predator.
Like lepidoptera larvae, sawflies have prolegs but theirs lack the little hooks called crochets that they use to cling onto the plant. Prolegs don't have joints or flex like insect legs. Instead movement is by extending the body and getting a new grip (think inchworm). She reported that "I was looking at the larvae again this evening. It appears that the
younger/smaller larvae feed from the underside of the leaves whereas the
older/larger larvae feed only from the edges. Either way, not much is
left of the leaves when they are finished."
Most sawfly species larvae are social, feed in large groups and strip a
leaf clean. They seldom harm the tree although they can be an unsightly
pest in an urban setting. Some species are a serious problem in
horticulture. They get their name from a saw-like ovipositor they use
to put their eggs under the bark. Like dragonflies and damselflies,
their common name is one word rather than two (think house fly) letting you know
that they aren't true flies in the Hemiptera family.
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| Dusky Birch Sawfly- Charley Eiseman CC |
Sawflies are actually members of the wasp family. Bug Lady describes the evolutionary path of the wasp family from laying eggs in wood, through species that make galls, paralyze prey with chemicals and finally the ovipositor becoming a stinger in what we commonly think of, and fear. Sawflies lack a stinger and can't harm us. The adults live for around a week and breed quickly without feeding. For that reason they are not commonly seen.