The spruce budworm has historically been one of the most important conifer defoliating insects in North America. A single generation occurs annually. Adult female moths are strong at flying and can disperse great distances (50 miles) if riding a wind current. Females lay eggs in clusters of 10-150 on host plant needles by approximately July and through August. Eggs take 1-2 weeks to hatch. First instar caterpillars are tiny and may complete mild feeding on their host plants, and in some cases are capable of short distance dispersal. Caterpillars quickly (while still in the first instar) spin a "hibernaculum" (an overwintering shelter similar to a cocoon) within which they molt into the second instar and remain dormant through the winter until the following spring. In the spring when temperatures warm (roughly by May), the second instar caterpillars again become active and may do a bit of feeding on male host plant flowers or begin to mine previous year's needles. Eventually, the larvae enter expanding vegetative buds and feed on them, destroying several buds before shoot elongation occurs. Caterpillars continue to grow and feed, and by the fourth or fifth instar, they tie the tips of foliage together on new shoots, where they create a nest-like structure. At this stage, they may feed by severing needles which remain attached to the shoot by silk where they turn reddish brown in color. From a distance, the trees that are most heavily infested may look like they have scorched foliage or branch tips that are without needles. At maturity, spruce budworm caterpillars are approximately an inch in length. Defoliation from spruce budworm may not become visually noticeable until approximately late June. Pupation occurs with adults emerging again in July and August, laying eggs at the end of their life cycle.
First instar larvae appear in the midsummer of their first season, but feed very little. The second instar larvae overwinter and resume major feeding in the following spring. Host plant needles are fed upon, beginning by approximately May. Usually only of concern in northern New England. The spruce budworm is rarely concerning on ornamental host plants in managed landscapes, unless perhaps a large regional outbreak is occurring. For example, outbreaks in Alaska in 1978 were experienced in residential and park areas as well as forested locations. If four or five years of consecutive, severe defoliation occurs, tree mortality is common. Outbreaks of spruce budworm historically occur on a cyclical basis, every 30-40 or 60 years. These outbreaks can last several years when they occur. The last major spruce budworm outbreak to impact northern New England (ex. Maine) was in the 1970's and 1980's. Damage to host plants includes: destruction of buds, abnormally shaped new twigs, and defoliation of current year's shoots. Heavily infested branches, if shaken, may reveal numerous caterpillars dangling from silk. Defoliation may begin at the top of the tree, with current year's needles being partially or completely fed upon. In the case of heavy population outbreaks, previous year's needles may also be fed upon. Spruce budworm caterpillars are also known to feed on male flowers and cones. Defoliated trees turn a rust color due to killed and dried needles. By the fall, trees may appear gray in color after the killed needles have been blown away by the wind. A single year of defoliation may have little impact on the host plant, perhaps weakening the tree and making it more susceptible to secondary pests or pathogens. Multiple years of defoliation can severely impact tree or forest stand health.
Maine is anticipating another outbreak to occur soon. Migrating moths from Canada were well documented in 2019 (Maine Forest Service). An interactive outbreak map is available here: https://www.sprucebudwormmaine.org/map/ . Monitoring for spruce budworm can be conducted using pheromone traps, light traps, overwintered larval sampling, aerial survey, and ground survey.
Spruce budworm trapping involves use of a trap, lure, and an insecticide to kill the moths that are attracted to the trap by the lure formulation. Multi-pher I © traps, synthetic pheromone lures (95:5 E:Z - 11 tetradecenal), and insecticides within the trap (to immobilize the insects) are combined during spruce budworm monitoring programs. Traps are deployed in mature forest stands with at least 50% white spruce or balsam fir. Traps are hung at eye level (this has changed over the years; in some research traps have been hung in the upper canopy, more recently they are 6.5 feet above the ground) at least 130 feet apart. Historically, traps are collected at the end of the moth flight period (Sanders, 1996; Rhainds et al., 2015).
Historically, high population cycles of spruce budworms were linked to overmaturity in stands of spruce and balsam fir. In the management of forest stands, which is not thoroughly discussed in this guide, preventing overmaturity or reducing dependence on fir (a favored host plant) helps reduce the risk of spruce budworm outbreaks.
Mating disruption studies have been conducted with Hercon flake formulations of spruce budworm pheromones. They have been shown to interfere with the ability of male spruce budworms to locate pheromone traps in treated forest plots. This is especially true at low spruce budworm densities. When spruce budworm densities are high, the disorientation of the male moths by the pheromone mating disruption technique is lower (Dimond et al., 1984).
Natural enemies such as parasitoid wasps (Meteorus trachynotus and Phaeogenes hariolus) and flies (Lypha setifacies) are important factors that help reduce or regulate spruce budworm populations. Adverse climatic conditions can also reduce spruce budworm outbreaks (Johnson and Lyon, 1991). Dipteran parasitoids (Nemorilla floralis and Phyrxe vulgaris) and a hymenopteran parasitoid (Itoplectis conquisitor) were reported as natural enemies of the spruce budworm collected in Massachusetts and Rhode Island by Schaffner (1959). Vertebrate predators, including birds, also feed on spruce budworms. Warblers, kinglets, sparrows, vireos, juncos, nuthatches, thrushes, and purple finches historically all respond to increasing spruce budworm densities (Crawford and Jennings, 1989). Recent studies have also shown that certain species of web building spiders are also important predators of spruce budworms. These include but are not limited to: Grammonota angusta, Pityohyphantes subarcticus, Dictyna brevitarsa and Estrandia grandaeva. Additionally, another species of spider described as a wandering hunter (Philodromus rufus) is also an important predator of spruce budworm (Bowden et al., 2022). qPCR-based molecular sorting is also being used to identify spruce budworm natural enemies, including dipteran and hymenopteran parasitoids as well as microsporidian infections (Nisole et al., 2020).
Abamectin (NL)
Acephate (NL)
Acetamiprid (L)
Azadirachtin (NL)
Bacillus thuringiensis subsp kurstaki (NL)
Beauveria bassiana (NL)
Bifenthrin (NL)
Carbaryl (L)
Chlorpyrifos (N)
Chlorantraniliprole (NL)
Chlorpyrifos (larvae) (N)
Chromobacterium subtsugae (NL)
Emamectin benzoate (L)
Flonicamid+cyclaniliprole (N)
Horticultural oil (L)
Indoxacarb (L)
Insecticidal soap (NL)
Lambda-cyhalothrin (L)
Malathion (L)
Neem oil (NL)
Pyrethrin+sulfur (NL)
Spinetoram+sulfoxaflor (N)
Spinosad (NL)
Tebufenozide (NL)
Active ingredients that may be applied systemically include: Abamectin (injection), acephate (injection), acetamiprid (injection), chlorantraniliprole (soil drench), emamectin benzoate (injection), and neem oil (soil drench).
Emamectin benzoate is labeled for use against western spruce budworm, specifically.
When used in a nursery setting, chlorpyrifos is for quarantine use only.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
