Forest tent caterpillar moth vs Elm sawfly
These two are easy to mix up. Here's how to tell a forest tent caterpillar moth from an elm sawfly, and whether either one bites or stings.
Forest tent caterpillar moth
Malacosoma disstria
Elm sawfly
Cimbex americanus
≠The key differences
Forest tent caterpillars are hairy, with a row of pale keyhole-shaped spots down the back, and feed in groups; Cimbex larvae are smooth, hairless and plump.
⚖︎Side by side
| Forest tent caterpillar moth | Elm sawfly | |
|---|---|---|
| To humans | Irritating hairs | Non-toxic |
| Size | 25-45 mm wingspan | 18-25 mm body length (wingspan noticeably larger) |
| Colours | Brown, Pale cream, Black, Blue-grey | Green, Yellow, Black, Brown |
| Family | Lasiocampidae | Cimbicidae |
| Habitat | Deciduous forests, mixed woodlands, urban trees and orchards where host trees occur | Deciduous woodlands, street trees, parks and hedgerows where elm and other host broadleaf trees grow |
| Range | Widespread across much of North America, from Canada through the United States; range varies regionally with host availability | Widespread across much of North America (southern Canada and the United States) |
| Active | Larvae active in spring and early summer (varies by latitude); adults on the wing in late spring to early summer, mostly dusk and night | Larvae active in late spring to summer; adults active briefly in late spring to early summer |
| Pest? | Yes | Yes |
🔍How to spot a forest tent caterpillar moth
- Caterpillars have a row of pale, elongated keyhole- or footprint-shaped dorsal spots on a dark body — distinctive for this species.
- Unlike eastern tent caterpillar (Malacosoma americanum), forest tent caterpillar larvae do not build permanent silk tents in branches; they make ground or trunk mats.
- Adults are small brown moths with faint darker lines and a paler median band on the forewings; identification to species is easiest by larval characters.
🔍How to spot an elm sawfly
- Large, robust adult sawfly with a dark steel-blue to black body, clubbed antennae and smoky wings; females have yellow spots along the abdomen.
- Larvae are plump, pale yellow-green, with a black stripe down the back and black spots along the sides; they often rest curled in a spiral.
- Larvae feed openly on the leaves of elm, willow and other deciduous trees.
- Adults lack the constricted "wasp waist" seen in wasps and bees (sawflies are broad-waisted).
?Frequently asked questions
What is the difference between a forest tent caterpillar moth and an elm sawfly?
Forest tent caterpillars are hairy, with a row of pale keyhole-shaped spots down the back, and feed in groups; Cimbex larvae are smooth, hairless and plump.
Is a forest tent caterpillar moth or an elm sawfly more dangerous?
Forest tent caterpillar moth: Irritating hairs. Elm sawfly: Non-toxic. Adults are harmless. Adults do not sting.
Identification from photos is never certain: don't handle insects you can't identify, and get medical advice for bites, stings or allergic reactions.