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How to identify it
- Infested cones are stuck together with frass bound by webbing, and the webbed castings on the cone surface are free of pitch. The caterpillar leaves a round, clear hole in the cone.[1][2]
- Chewed needles and mined terminal shoots are other signs. On young pines, a pitch mass near the entry point on the trunk gives the insect away.[1]
- The moth is dark gray with white zigzag lines bordered by black across the forewings, and has a wingspread of about 20 to 30 mm.[3]
- The full-grown caterpillar is about 20 mm long and usually reddish-purple with a greenish tinge underneath.[3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Spruce coneworm | Natural Resources Canada calls the fir coneworm similar in many respects to the spruce coneworm (Dioryctria reniculelloides), another cone-feeding Dioryctria of spruce and fir.[1] |
| Zimmerman pine moth | Zimmerman pine moth (Dioryctria zimmermani) is a related gray moth, but in the North its caterpillars feed under the bark of pine limbs and trunks rather than in cones.[3] |
Where it lives
The fir coneworm is transcontinental. It occurs across Canada, in British Columbia and the western states, and in the East apparently throughout the range of its pine hosts, south to the slash and longleaf pine country of the Southern States.[1][2][3]
Biology and life cycle
Hosts include pines, Douglas-fir, true firs and spruces. In the West it is particularly destructive in fir and Douglas-fir cones; in eastern Canada its listed hosts are balsam fir, black spruce, Norway spruce, eastern white pine, jack pine and red pine.[2][1]
The caterpillars prefer cones but also feed on terminal shoots and the bark of the trunk and large limbs, and occasionally mine into branches or the trunk. In California they have damaged grafts of Douglas-fir and ponderosa pine. Pine cones infected with the rust Cronartium strobilinum are especially attractive to them.[1][2][3]
In eastern Canada there is one generation a year, with caterpillars in the cones from mid-June until fall. Near the end of its growth the caterpillar drops to the ground and spins a silk cocoon, overwinters as a last-stage larva or prepupa, and pupates the next year.[1]
Farther south the insect speeds up. In the West, fir cones support one generation and a partial second each year while pine supports only one; in Florida there may be five or six generations, with rust-infected pine cones attacked as early as April.[2][3]
Damage and signs
- Cones are the main loss. The caterpillars can destroy an entire seed and cone crop in a severe infestation, and the insect is among the most destructive pests of slash and longleaf pine cones in the Southern States. Red pine cones in the Lake States and southern Canada are occasionally seriously damaged.[1][3]
- The tree itself is not seriously harmed. In Canada, outbreaks have been limited to occasional local infestations in Ontario and Quebec, mostly in plantations and seed orchards.[1]
Health risk
The forest sources treat the fir coneworm only as a pest of cones and shoots and mention no risk to people.[1][2][3]
Control
Prevention
Because the tree is not seriously damaged, control matters mainly where the seed crop has value, such as seed orchards.[1]
Sanitation
On ornamental trees, Natural Resources Canada advises collecting and burning the cones while the caterpillars are still inside, before mid-August.[1]
Common questions
- Why are the cones on my fir or spruce webbed together and full of sawdust-like frass?
- Webbing mixed with frass holding cones together, with a round hole in the cone, is the sign of a coneworm caterpillar such as the fir coneworm feeding inside.[1][2]
- Will the fir coneworm kill my tree?
- No. It does not cause serious damage to the tree. The loss is the cone and seed crop, and sometimes mined shoots or a pitch mass on the trunk of a young pine.[1]
- Do I need to spray?
- The forest sources suggest no spray for home trees. Removing and destroying infested cones before mid-August, while the caterpillars are still inside, is the recommended step on ornamentals.[1]
Sources
- Natural Resources Canada, Canadian Forest Service, “Fir coneworm, Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/11433 (accessed 2026-11-10)
- USDA Forest Service, “Western Forest Insects, Miscellaneous Publication 1339 (R. L. Furniss and V. M. Carolin, November 1977; scanned copy).” archive.org/details/CAT78702369 (accessed 2026-11-10)
- USDA Forest Service, “Eastern Forest Insects, Miscellaneous Publication 1175 (W. L. Baker, 1972; scanned copy).” archive.org/details/easternforestins1175bake (accessed 2026-11-10)
- iNaturalist, “Dioryctria abietivorella taxon page (order and family).” www.inaturalist.org/taxa/218479 (accessed 2026-11-10)