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How to identify it
- Mature larvae are about 18 mm (0.7 inch) long with an orange to brown head and a pale greenish-yellow body marked with faint gray-green stripes along each side.[1]
- Larvae feed gregariously, so colonies and their damage are concentrated at the tips of branches just below the expanding new shoots in mid to late summer. Damaged trees keep only the new foliage at the shoot tips.[1]
- In midsummer, females insert eggs into the base of new needles, leaving scars that stay visible for several weeks; eggs are laid singly in pockets cut on the outer edges of one or both needles of a pair, and a female tends to lay her whole complement on one shoot.[1]
- Like other sawfly larvae it has a pair of fleshy prolegs on six or more abdominal segments, whereas caterpillars have five or fewer pairs.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Jack pine budworm | The Swaine jack pine sawfly is a sawfly larva (six or more pairs of prolegs) feeding in colonies on old needles from midsummer into fall, with an orange to brown head and faint gray-green side stripes.[1] |
| Red pine sawfly | A related sawfly of jack, red and pitch pine that hatches in May and finishes feeding by early July. The Swaine jack pine sawfly's adults do not even fly until June to late July, and its larvae feed in mid to late summer into September and October.[1][3] |
| Pine sawflies | Of the colony-feeding Neodiprion sawflies, this one is tied almost entirely to jack pine and feeds late in the season.[1] |
Where it lives
East of the Rocky Mountains in Canada from Alberta to Nova Scotia and in the Great Lakes states of the United States. It is most common in the eastern part of jack pine's range, from west of Lake Superior in Ontario and Minnesota through the Great Lakes region to the Lac Saint-Jean region of Quebec. Cool climates at the northern limit of its range do not allow the feeding stages to complete development, and warming trends favor its survival and range expansion.[1]
Biology and life cycle
Jack pine is by far the main host. Other pines near an infestation may be slightly damaged, but the sawfly is less likely to complete its life cycle on them.[1]
Early in the season larvae eat only older foliage because the fresh current-year needles contain a feeding deterrent, identified as 13-keto-8(14)-podocarpen-18-oic acid. Its concentration falls as the needles mature, and by August, at about 60 days old, the new foliage becomes acceptable to the larvae.[2][1]
There is one generation a year. Adults emerge from June to late July and lay eggs in the new needles. The larvae from one egg cluster form a colony and feed until cooler weather in September and October, then drop to the ground, spin a thin cocoon in the upper soil, molt to a prepupa and overwinter in diapause. Some stay in diapause for a second year.[1]
Completing the cycle in one year needs a relatively warm autumn. If temperatures fall too low before the larvae mature, survival drops sharply, which is why only minor infestations occur north of 49 degrees N and why open, sunny stands suit the sawfly.[1]
Damage and signs
- Because the larvae feed late and prefer old needles, little damage is done during the period of fastest shoot growth, and new foliage is mostly spared. Trees are left with only new needles at the shoot tips.[1]
- Even so, during the widespread outbreaks before 1970, tree death was reported in Quebec. The most intense infestations were in mature, open-grown jack pine on poor soils.[1]
Health risk
Both sources treat the insect only as a jack pine defoliator; neither mentions any risk to people or pets.[1][2]
Control
Prevention
Stand risk can be rated from temperature: open stands exposed to warm sun, sites south of 49 degrees N, and sites near large bodies of water with milder autumns are at greater risk of outbreaks.[1]
Biological control
A naturally occurring nuclear polyhedrosis virus found in Ontario and Quebec outbreaks in the 1950s remains a significant cause of death in dense populations. Virulent strains were selected in the laboratory, and experimental aerial sprays in the early 1960s gave excellent control, though the method was not developed further after 1966 as outbreaks became rarer.[1]
Pesticides
Registered products change from year to year; Natural Resources Canada advises checking the current pesticide registration database, treating only when population size warrants it and against the approved life stage, and consulting a local tree-care professional.[1]
Common questions
- Why do the larvae leave the new needles alone?
- Fresh jack pine foliage contains a chemical that deters feeding. As the needles age the deterrent fades, and by August the current year's foliage becomes acceptable to the larvae.[2][1]
- When should I look for it?
- Egg scars appear at the base of new needles in midsummer, and colonies of larvae feed at the branch tips from mid to late summer until cool weather in September or October.[1]
- Is it still a serious forest pest?
- Outbreaks have been localized since the last large one in Quebec in 1994, and a naturally occurring virus keeps dense populations in check.[1]
Sources
- Natural Resources Canada, Canadian Forest Service, “Swaine jack pine sawfly (V.G. Nealis, 2025), Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/6585 (accessed 2026-11-10)
- American Association for the Advancement of Science (Science), “Chemical basis for feeding adaptation of pine sawflies Neodiprion rugifrons and Neodiprion swainei (Ikeda, Matsumura and Benjamin 1977, Science 197:497-499).” doi.org/10.1126/science.197.4302.497 (accessed 2026-11-10)
- Natural Resources Canada, Canadian Forest Service, “Red pine sawfly (V.G. Nealis, 2024), Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/6575 (accessed 2026-11-10)
- iNaturalist, “Neodiprion swainei taxon page (order and family).” www.inaturalist.org/taxa/871577 (accessed 2026-11-10)