Willow Sawfly
Nematus ventralis
A sawfly whose striking black-and-orange larvae feed in colonies on willow leaves and can quickly defoliate a plant, leaving only the midribs. It is a common willow pest across North America and also feeds on poplar; sudden outbreaks can strip a small willow in days.
🔎 How to spot it
The larvae look like caterpillars but are the larvae of a non-stinging wasp. They are shiny black with a row of slightly raised orange-yellow spots along each side, curl into an S shape when disturbed, and feed in groups of roughly five to ten. A look-alike, Nematus oligospilus, is plainer, with a flesh-colored head and a light green, faintly striped body. The adult is a small dark sawfly.
🥀 Damage it causes
Early-stage larvae chew holes and notches, but later stages eat whole leaves down to the midvein, and a feeding colony can defoliate branches or strip a small willow rapidly. An older, established willow usually survives even dramatic defoliation, but repeated stripping can badly set back a young plant.
🛡️ Prevent it
Scout willows through the summer and act while colonies are small, since the larvae cluster and are easy to spot and remove. Knocking the colonies into a bucket of soapy water, or off onto the ground, controls them without chemicals. Because the larvae overwinter in the soil, cultivating the ground around willows over winter exposes them to cold.
🧯 If it is already here
For light to moderate infestations, hand-removal or hosing the colonies off is enough. Bt (Bacillus thuringiensis) does not work on sawflies because they are not caterpillars; if a spray is needed, use insecticidal soap, horticultural oil, or spinosad on the young larvae. Spot-treat the colonies rather than the whole plant to spare beneficial insects.
💡 Good to know
There are at least two generations a year, with the first larvae hatching in spring and a second, often heavier generation later in the summer, and the larvae overwinter in cocoons in the soil. The bold black body with orange-yellow side spots makes this colony-feeding sawfly easy to recognize on willow.
🌱 Plants it attacks
3 plants in the library can be attacked by this pest
For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.