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Hemlock looper

Written by A.C. Skinner Revision 2025

Mature larva on fir twig (length: 32 mm)
Mature larva on fir twig (length: 32 mm) Thérèse Arcand

General information and importance

Hemlock looper is native to North America and is an important forest defoliator. It is common in both forest and urban settings and can be responsible for substantial timber losses and costly defoliation during outbreaks. Larvae are wasteful, voracious consumers and their feeding can lead to tree mortality within a single season. When populations of this insect reach epidemic levels, they can move from their preferred host and defoliate other nearby tree species. They can also cause severe damage in nurseries, plantations, Christmas tree plantations, and stands used for wreath production. Hemlock looper outbreaks often start suddenly and end quickly, typically lasting three years. Outbreaks occur periodically, usually every 11 to 20 years.

Distribution and hosts

Hemlock looper occurs throughout North America. There are three recognized subspecies of Lambdina fiscellaria (Guenée) based on geographic distribution and hosts:

In Canada, although this insect can be found throughout the country, no large outbreaks have been documented in central continental regions. In the United States, significant outbreaks have occurred primarily in states bordering eastern and western Canadian provinces. Outbreaks, however, have been observed as far south as Georgia and as far north as Alaska.

Tree parts affected

Coniferous needles/deciduous leaves

Symptoms and signs

When young larvae emerge in spring after overwintering, they spend their first two instars feeding on needles (foliage) of current-year shoots. As they mature, they move to older needles to feed. Larvae are wasteful feeders and leave partially consumed needles as they feed. Although much of the defoliation by mature larvae occurs on older foliage, they will return to new foliage and eat it as well, once older foliage has been consumed. On Garry oak, western oak looper is a somewhat wasteful feeder because it only partially consumes oak leaves, with leaf veins and parts of leaves unconsumed. During outbreaks, should food supply of the hemlock looper become limited, they will resort to cannibalism.

Conifer trees that are defoliated during a hemlock looper outbreak turn a reddish-brown colour due to the insect’s wasteful feeding habits. Partially eaten, damaged needles die and change colour before falling to the forest floor. The defoliation and colour change are evident in both new and old foliage. Other defoliating insects tend to consume either old or new foliage but rarely both as does hemlock looper.

Hemlock looper larvae have a cylindrical smooth shape, no bristles or hairs, and two pairs of prolegs at the posterior end of the abdomen. Larvae grow from 3 millimetres up to 33 millimetres during development. Larvae range in colour from light green to yellow to black. Larvae produce silken strands to descend to the ground and migrate to new feeding sites or to search out pupation sites in late summer. These silk strands are abundant and cover trees during outbreaks. Pupae are spindle-shaped, 20 millimetres long, and their colour can vary from greenish-brown to beige with darker spots scattered across the hard pupal casing. Adult moths have scalloped wing borders, a wingspan of 35 millimetres, and a body length of 12 millimetres. Colour intensity may vary widely within the species, although most moths are a tan yellow with a dusting of tan or brown to brownish-purple scales. Moths have two darker lines across the forewings and one across the hindwings. A darker spot is usually visible between the two darker lines on each of the forewings. While at rest, moths lay flat in a broad wedge shape. Male moths have wider feather-like antennae than females; these are used to detect pheromones released by female moths for mating. Eggs of hemlock looper are ovoid, about 1 × 0.75 millimetres. Eggs are light green when first deposited and change to a copper-like brown after a few days. Sterile eggs will remain a greenish colour.

Life cycle

All subspecies of hemlock looper have one generation per year. Adult female moths deposit eggs singly or in small clusters of up to five eggs in mid-October on host trees and on nearby forest debris. Eggs hatch in late spring, around May or June, as new tree growth begins. Larvae spend their time feeding on host foliage and rarely leave this location unless their food supply runs out. Western and eastern hemlock loopers pass through five larval instars prior to pupation, except in Newfoundland and Labrador where the insect only passes four instars. Larvae of the Garry oak looper also have only four instars. In mid-August, after two to three months of feeding, larvae search out protected locations to pupate. Pupation lasts about two to three weeks in late August and early September. Adult moths begin to appear in early September and continue into October. The lifespan of adults is about one month, during which time mating and egg deposition occur.

A major limiting factor in dispersal of hemlock looper is their poor ability to fly. This prevents them from migrating far for either mating or egg laying. Outbreaks rarely last more than two or three years as their lack of mobility prevents them from accessing new foraging areas. The combined effects of larval starvation, parasitoids, predators, and diseases are responsible for hemlock looper population declines.

Extreme weather can hasten outbreak collapse and assist in keeping populations in check. Eggs are vulnerable to intense cold spells of -37ºC lasting a few hours per day. Moths struggle during unusually cold and wet weather, and heavy rain during the flight period interferes with mating. Periods of heavy rain or high winds can also displace larvae to the ground leaving them susceptible to either predation or starvation.

Natural enemies play an important role in hemlock looper population dynamics. There are several species within the Hymenoptera and Diptera that are parasitoids on hemlock looper. They attack different life stages (eggs, larvae, pupae) and play a role in controlling populations. Another natural control agent is a nuclear polyhedrosis virus (NPV) disease that is found in looper larvae and is an added driver in outbreak collapse. Several bird species prey on larvae and pupae but their impact on populations is uncertain.

Damage

Stands that are severely defoliated by hemlock looper for one or two seasons often experience significant mortality: up to 92% when defoliation exceeds 90%. Trees that survive defoliation subsequently exhibit decreased radial growth for several years and are vulnerable to secondary pests because of their weakened state following an outbreak. Coniferous trees store fuel (starch) in their needles, and repeated defoliation can significantly influence their ability to recover. Deciduous hosts are generally much less affected and often recover quickly in comparison with conifers. Deciduous stands damaged by hemlock looper rely on starch stored in the stem, trunk, and roots to fuel the production of new leaf growth in spring.

Hemlock looper outbreaks are responsible for millions of cubic metres of timber losses during and shortly after outbreaks. Between 1910 and 1975, hemlock looper outbreaks caused timber losses estimated at 12 million cubic meters in Newfoundland and Labrador and 24 million cubic meters in the province of Quebec. Recovery of individual trees and forested areas is often slow. Large tracts of dead and damaged trees can reduce forest aesthetics for recreational use and tourism. During large outbreaks, larvae and frass (excrement) falling from the tree canopy can be a nuisance in campgrounds and other recreation areas. Risks from forest fires following an outbreak is a major consideration because tree mortality can add dry fuels that are extremely combustible and can act as a “ladder” for the fire to move from the forest floor to crowns. Crown fires are far more damaging than surface fires.

During large outbreaks, a range of ecological effects might occur including changes in stand structure and species diversity, as well as reduced water quality. These effects are often transient and are similar to other defoliating insects whose outbreaks cover extensive areas.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken.

Monitoring hemlock looper populations can be a helpful tool for predicting and treating outbreaks. Aerial surveys complimented with light, pheromone, and oviposition trap catches, assist with making population predictions.

In forest management contexts, outbreaks can also be prevented silviculturally by pre-emptively harvesting host stands of over-mature balsam fir, hemlock, and spruce. Other silvicultural techniques, such as modifying species composition of forest stands to reduce their vulnerability to damage or salvage harvesting of looper-affected forests, are also useful. In areas susceptible to outbreaks and where forest managers have flexibility, planting stands with less than 50% host species is recommended to help avoid or limit the potential for future outbreaks.

During outbreaks, the most common option for pest management is a biological insecticide based on the bacterium Bacillus thuringiensis var. kurstaki (Btk). Btk is a soil bacterium that, although lethal to moth and butterfly larvae that eat treated foliage, is not considered toxic for humans, other mammals, birds, fish, amphibians, or other types of insects. Btk can be applied to both large-scale outbreaks via aerial spraying or individual trees and shrubs. Both Btk and pheromones are defined as pest control products and are regulated in Canada. Products registered for use against hemlock looper under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Photos

Larva

Larva

Thérèse Arcand

Dorsal and ventral views of pupa (length: 20 mm)

Dorsal and ventral views of pupa (length: 20 mm)

Thérèse Arcand

Larva

Larva

Thérèse Arcand

Mature larva on fir twig (length: 32 mm)

Mature larva on fir twig (length: 32 mm)

Thérèse Arcand

Larva

Larva

Carole Germain

Eggs laid on the stem of a fir twig (length: about 0.9 mm)

Eggs laid on the stem of a fir twig (length: about 0.9 mm)

Thérèse Arcand

Adult at rest (wingspan: 32 mm)

Adult at rest (wingspan: 32 mm)

Thérèse Arcand

Severely defoliated balsam fir forest in July

Severely defoliated balsam fir forest in July

Luc Jobin

Severely defoliated balsam firs in July

Severely defoliated balsam firs in July

Luc Jobin

Male parasitoid emerged from hemlock looper prepupa

Male parasitoid emerged from hemlock looper prepupa

Carole Germain

Female parasitoid emerged from hemlock looper prepupa

Female parasitoid emerged from hemlock looper prepupa

Carole Germain

Severely defoliated balsam firs in July

Severely defoliated balsam firs in July

Luc Jobin

Severely defoliated balsam fir forest in July

Severely defoliated balsam fir forest in July

Luc Jobin

Severely defoliated forest

Severely defoliated forest

Christian Hébert

Adult

Adult

Christian Hébert

Selected references

Harris, J.W.E.; Dawson, A.F.; Brown, R.G. 1982. The western hemlock looper in British Columbia, 1911–1980. Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. Information Report BC–X–234. 18 p.

Hébert, C.; Berthiaume, R.; Dupont, A.; Auger, M. 2001. Population collapses in a forecasted outbreak of Lambdina fiscellaria (Lepidoptera: Geometridae) caused by spring egg parasitism by Telenomus spp. (Hymenoptera: Scelionidae). Environmental Entomology 30(1): 37–43. https://doi.org/10.1603/0046-225X-30.1.37

Hébert, C.; Jobin, L.J. 2001. The hemlock looper. Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre. Sainte-Foy, Quebec. Information Leaflet LFC 4. Rev. 17 p.

Hudak, J. 1997. The eastern hemlock looper, Lambdina fiscellaria fiscellaria (Guen.) (Lepidoptera: Geometridae) in Newfoundland, 1983–1995. Natural Resources Canada, Canadian Forest Service, Newfoundland and Labrador Region. St. John’s, Newfoundland and Labrador. Information Report N–X–302. 84 p.

Humphreys, N. 1995. Western oak looper on Saltspring Island. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. FIDS Pest Report 95–16. 2 p.

Iqbal, J.; MacLean, D.A.; Kershaw, J.A., Jr. 2011. Impacts of hemlock looper defoliation on growth and survival of balsam fir, black spruce and white birch in Newfoundland, Canada. Forest Ecology and Management 261(6): 1106–1114. https://doi.org/10.1016/j.foreco.2010.12.037

Legault, S.; Hébert, C.; Blais, J.; Berthiaume, R.; Bauce, E.; Brodeur, J. 2012. Seasonal ecology and thermal constraints of Telenomus spp. (Hymenoptera: Scelionidae), egg parasitoids of the hemlock looper (Lepidoptera: Geometridae). Environmental Entomology 41(6): 1290–1301. https://doi.org/10.1603/EN12129

Levin, D.B.; Laitinen, A.M.; Clarke, T.; Lucarotti, C.J.; Morin, B.; Otvos, I.S. 1997. Characterization of nuclear polyhedrosis viruses from the three subspecies of Lambdina fiscellaria. Journal of Invertebrate Pathology 69(2): 125–134. https://doi.org/10.1006/jipa.1996.4633

MacLean, D.A.; Ebert, P. 1999. The impact of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) on balsam fir and spruce in New Brunswick, Canada. Forest Ecology and Management 120(1–3): 77–87. https://doi.org/10.1016/S0378-1127(98)00527-1

Otvos, I.S. 1977. Mortality of overwintering eggs of the eastern hemlock looper in Newfoundland. Fisheries and Environment Canada, Forestry Service. Ottawa, Ontario. Bi-monthly Research Notes 33(1): 3–5.

Otvos, I.S. 1977. Weather and outbreaks of the eastern hemlock looper in Newfoundland. Fisheries and Environment Canada, Forestry Service. Ottawa, Ontario. Bi-monthly Research Notes 33(1): 3.

Raske, A.G.; West, R.J.; Retnakaran, A. 1995. Hemlock looper (Lambdina fiscellaria). Pages 141–147 in J.A. Armstrong, W.G.H. Ives, editors. Forest insect pests in Canada. Natural Resources Canada, Canadian Forest Service, Science and Sustainable Development Directorate. Ottawa, Ontario.

Sabbahi, R.; Royer. L.; O’Hara, J.; Bennett, A. 2018. A review of known parasitoids of hemlock looper (Lepidoptera: Geometridae) in Canada and first records of egg and larval parasitoids in Labrador forests. The Canadian Entomologist 150(4): 499–510. https://doi.org/10.4039/tce.2018.27

Turnquist, R. 1991. Western hemlock looper in British Columbia. Forestry Canada, Pacific Forestry Centre. Victoria, British Columbia. FIDS Report 91–08. 42 p.

Van Frankenhuyzen, K.; West, R.J.; Kenis, M. 2001. Lambdina fiscellaria fiscellaria (Guenée), hemlock looper (Lepidoptera: Geometridae). Pages 141–144 in P.G. Mason; J.T. Huber, editors. Biological control programmes in Canada, 1981–2000. CABI Publishing, Wallingford, United Kingdom.

Cite this fact sheet

Skinner, A.C. 2025. Hemlock looper. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.