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How to identify it
- The giveaway is the web: larvae spin loose silk at the base of the needles, pull cut needles into it and leave their frass inside, so a feeding site becomes a large web packed with droppings. The older needles are eaten and the new growth is left sticking out beyond the stripped older shoots.[2][1]
- Mature larvae are up to 27 mm (about an inch) long with a brown head marked with dark patches. Younger larvae are mottled yellowish brown with dark brown lines down the back, the belly and along the spiracles, black thoracic legs, and two prominent black spots above each thoracic leg and on top of the last abdominal segment.[1][2]
- The final instar turns green with a faint dark green stripe down the back, but keeps the black spots above the legs and on the last segment.[1]
- Adults are about 0.6 inch long, shiny blue-black with a reddish head, pronotum and legs, and emerge in spring or early summer.[2][1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Pine false webworm | Another needle-webbing sawfly on pines. The western webspinning sawfly is the species of British Columbia, Alberta and south to California whose mature larva has a brown, dark-marked head and paired black spots above each thoracic leg and on top of the last abdominal segment.[1] |
| Pine sawflies | Check for the web: western webspinning sawfly larvae feed from inside a loose silk web at the base of the needles that fills with cut needles and frass.[2][1] |
| Fall webworm | The western webspinning sawfly's webs are on pine (or spruce), hold cut needles and frass, and are active in June and July; the larvae are up to about an inch long with black spots above the legs.[1][2] |
Where it lives
Western North America. It occurs on the south coast and in the southern interior of British Columbia, in mountainous western Alberta, and south to California; the PNW handbook treats it as a landscape pest of the Pacific Northwest.[1][2]
Biology and life cycle
Natural Resources Canada describes it as restricted to pines, including ponderosa, lodgepole, shore and western white pine, while the Pacific Northwest handbook lists it on spruce as well as some pines.[1][2]
There is one generation a year. It overwinters as a prepupal larva (the handbook says larva or pupa) in an earthen cell in the soil, and adults emerge in mid-spring to early summer.[1][2]
Females lay eggs on the old needles. The larvae feed on older foliage for up to about three weeks, active mainly in June and July, pulling cut needles into their web. When mature, usually in June in the Pacific Northwest or from late July to August in British Columbia, they drop to the ground and form earthen cells about 3 inches below the surface.[2][1]
Damage and signs
- Larvae consume the older needles, leaving stripped older shoots with the current growth extending beyond them, and cover the feeding site with a large web full of frass.[2][1]
- It occasionally causes significant damage to pines planted in urban areas and to lodgepole pine in seed orchards, and heavily infested trees may be covered with large unsightly webs.[1]
Health risk
Neither source mentions any effect on people or pets; the insect is described only as a defoliator of pines and spruce.[1][2]
Control
Biological control
In Austria, nematodes applied to the soil gave good control of this sawfly when the soil pH was kept between 5.0 and 7.0 with lime.[2]
Pesticides
The PNW handbook refers readers to its landscape pest chemical control table rather than naming products for this sawfly.[2]
Common questions
- What is the dirty web full of droppings on my pine?
- Probably a western webspinning sawfly colony. The larvae spin loose silk at the needle bases, drag cut needles into it and fill it with frass while they eat the older foliage.[2][1]
- Will it kill the tree?
- The sources describe stripped older needles and unsightly webs, with the new growth left intact, and note occasional significant damage on urban pines and seed-orchard lodgepole pine; neither reports tree death.[1][2]
- Is it on spruce or pine?
- Natural Resources Canada lists only pines (lodgepole, shore, western white, ponderosa), while the PNW handbook lists it under spruce and some pines.[1][2]
Sources
- Natural Resources Canada, Canadian Forest Service, “Western webspinning sawfly (Cephalcia californica), Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/7446 (accessed 2026-11-10)
- Pacific Northwest Pest Management Handbooks (Oregon State University, Washington State University, University of Idaho), “Spruce (Picea)-Western webspinning sawfly (Pacific Northwest Insect Management Handbook).” pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/spruce-picea-western-webspinning-sawfly (accessed 2026-11-10)
- iNaturalist, “Cephalcia californica taxon page (order and family).” www.inaturalist.org/taxa/738455 (accessed 2026-11-10)