Alder Sawfly
Eriocampa ovata and related species
A group of sawflies whose larvae defoliate alders, including red alder. The most familiar is the woolly alder sawfly, whose larvae are coated in a white woolly wax; along with the striped and green alder sawflies it can strip alder foliage over large areas, thinning and browning the canopy.
🔎 How to spot it
Sawfly larvae look like caterpillars but are the larvae of a non-stinging wasp. Woolly alder sawfly larvae are covered in a white, waxy, hairlike coating that hides a pale green body; striped alder sawfly larvae are yellowish to greenish-brown with two black stripes down the sides; and green alder sawfly larvae are a vivid bluish-green and over three quarters of an inch long. Early on, look for a shotgun-blast pattern of small holes in the leaves.
🥀 Damage it causes
Young larvae chew small holes that give a characteristic shotgun-blast look, then older larvae eat all the soft tissue between the veins, leaving only the veins and midrib so the foliage turns thin and brown and the thinning is easily seen from the ground. Severe, repeated defoliation weakens alders and, because alders fix nitrogen, heavy outbreaks can sharply cut the nitrogen they return to the soil.
🛡️ Prevent it
Watch alders from early summer on and act while larvae are small and clustered. On smaller trees, hand-pick the larvae or knock them off with a strong jet of water, repeating through the summer as needed. Since the insects drop to pupate in the soil or in woody debris, these simple measures plus tolerating light feeding usually keep healthy alders fine.
🧯 If it is already here
Hand-picking or a strong water spray controls light to moderate infestations on accessible trees. Note that Bt (Bacillus thuringiensis) does not work on sawflies because they are not caterpillars; if an insecticide is needed, use insecticidal soap, horticultural oil, or spinosad on the young larvae. Avoid spraying near streams and riparian areas, since many alders grow along salmon-bearing waters where insecticides harm aquatic life.
💡 Good to know
Woolly and striped alder sawfly adults emerge from about mid to late June, while the green alder sawfly emerges earlier in mid-May; the larvae feed for several weeks, then drop to overwinter as prepupae in the soil a few centimeters down or, for the green alder sawfly, in stumps and woody debris. All species feed on alders, and heavy red alder defoliation has been recorded in southeast Alaska.
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.