Elm sawfly

📖Overview

Cimbex americanus is a member of the sawfly family Cimbicidae, notable for large, robust adults and big, caterpillar-like larvae. It occurs across much of North America. Females use a saw-like ovipositor to insert eggs into host leaves; larvae hatch and feed on leaves before spinning cocoons in leaf litter or soil, where they overwinter. Outbreaks can cause noticeable defoliation on individual trees, but populations are usually low and do not cause long-term tree mortality.

Adults are non-aggressive and do not sting people. The species has one generation per year in most of its range. Larvae are most often seen in late spring and summer when feeding, while adults are present briefly for mating and egg laying.

🔍How to identify an elm sawfly

Adults are large, robust, wasp-like sawflies with clubbed antennae. The body is dark steel-blue to black; females usually have a row of yellow spots along each side of the abdomen, and wings are smoky brown. Adult body length is typically about 18-25 mm. Larvae resemble moth caterpillars: plump and pale yellow-green with a black stripe down the back and black spiracles along the sides. They often rest curled in a spiral and can squirt a harmless defensive fluid, and they reach about 4-5 cm when mature.

1. Large, robust adult sawfly with a dark steel-blue to black body, clubbed antennae and smoky wings; females have yellow spots along the abdomen.
2. 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.
3. Larvae feed openly on the leaves of elm, willow and other deciduous trees.
4. Adults lack the constricted "wasp waist" seen in wasps and bees (sawflies are broad-waisted).

Lookalikes

↻Life cycle

1. 🥚

   Egg

   The female cuts slits in leaf tissue with her saw-like ovipositor and inserts eggs. Eggs hatch in about one to two weeks depending on temperature.

2. 🐛

   Larva (sawfly caterpillar)

   Larvae feed on foliage for several weeks, mostly singly, growing through several instars; they are the stage that causes visible leaf damage.

3. 🫛

   Pupa

   Mature larvae spin a tough cocoon in leaf litter or loose soil, overwinter inside it and pupate in spring.

4. 🐝

   Adult

   Adults emerge in spring to mate and the females lay eggs on host trees, completing the annual cycle.

🍃Diet and predators

🌱Effect on plants and gardens

Larvae feed on the leaves of elm, willow and other deciduous trees such as birch, maple and basswood; adults sometimes chew strips of bark from twigs. Heavy localized feeding can cause defoliation, but healthy mature trees usually recover.

How to get rid of elm sawflies

Usually no treatment is needed. For small numbers, handpick larvae from branches and drop them into soapy water. Encourage natural enemies (birds, predatory insects, parasitoid wasps and flies). For larger infestations on small or valuable trees, use an insecticide labeled for sawfly larvae, such as insecticidal soap or spinosad, following label directions; note that Bacillus thuringiensis kurstaki (Btk) does not work on sawflies because they are not caterpillars. Consult an arborist for large trees.

📍Where and when to find it

Look on the leaves and branch tips of elm and nearby deciduous trees for feeding larvae; adults may be seen on foliage or flying nearby in spring.

✨Fun facts

• Sawflies are in the same order as wasps and bees (Hymenoptera) but lack the narrow waist of most wasps.
• Despite their caterpillar-like appearance, sawfly larvae are not butterflies or moths and have more prolegs than true caterpillars.
• Cimbex adult sawflies are unusually large compared with most other sawfly species.

?Frequently asked questions

📷Photos of the elm sawfly