The western hemlock looper creates snags and down wood by severely defoliating and causing the death of all sizes of western hemlock and associated trees in western hemlock stands that are older than 80 years. It also interacts with other agents, such as bark beetles, to cause tree mortality of defoliated trees. By causing topkill in all sizes of trees, L. fiscellaria lugubrosa contributes to the formation of unique limb structures and facilitates the colonization of living tree boles by stem decay. During outbreaks, high numbers of larvae pupae, and moths provide abundant forage for many species of birds and invertebrates. Some of the mortality associated with western hemlock looper defoliation may contribute to the formation of canopy gaps, increasing structural diversity.
Found throughout both states. (See also Important Habitats and Outbreak Dynamics).
Primarily western hemlock, but many other associated trees and understory shrubs may also be defoliated during outbreaks, especially western redcedar, true firs, Douglas-fir, western white pine, and spruces.
Unless defoliation has occurred in prior years, leaving thin, bare crowns, tree symptoms usually are not evident until mid-July, when older larvae begin to feed voraciously upon both new and old foliage. Defoliation begins in the upper crown and may be most concentrated there, although in heavily defoliated trees most of the crown may be affected. Defoliated hemlock trees turn yellowish red, then brown as if they’ve been scorched by fire. Look on affected branches for partially chewed, discolored needles, and on the forest floor beneath affected trees for abundant green needle portions that have been severed at their bases. During outbreaks, most species of trees and shrubs growing intermixed with western hemlock host trees will also be defoliated. Loose threads of silken webbing may be present on trees and understory vegetation when older larvae are abundant.
Larvae (present in June and July) crawl with a distinctive looping motion. Young larvae are banded black and light gray with a black head. Mature larvae are about 30 mm (1 3/16 in) long, with variegated greenish or brownish bodies having 4 dark dots on top of each abdominal segment and complex linear and diagonal markings that also form two gray to cream-colored longitudinal lines. Pupae are mottled greenish brown with black blotches and about 12 mm (1/2 in) long. They are slender and mummy-like, tapering from rounded heads to end in a point, and may be found in August and September in trunk bark crevices, under mosses and lichens, or in debris on the forest floor. Usually present in September and October, Adults are weak flying, fawn colored moths with wingspans of about 35 mm (1 3/8 in). Two dark wavy lines divide the forewings into thirds, and one dark wavy line is present on the hindwings. The wings have somewhat angular posterior margins. During outbreaks, abundant moths flying in infested areas may give the impression of a snowstorm, and dead moths sometimes accumulate in nearby pools and creeks. Single or small groups of eggs may be found beginning in September and throughout the winter on mosses and lichens that are attached to host trees, and in tree bole bark crevices. Eggs are barrel shaped, translucent gray to bronzy brown, and about the size of a pinhead.
Western hemlock looper has one generation per year. There are four life stages: egg, larva, pupa, and adult. Larvae pass through six developmental stages, called “instars”, and adults are small moths. The adult females produce pheromones that attract males for mating. They lay their eggs in September and early October, singly or in groups of 2 to 10, on the trunk and lower branches of trees in bark crevices, mosses, and lichens, and sometimes even on forest floor litter. Winter is spent in the egg stage. After hatching the following May and June, young larvae, which initially crawl about on understory vegetation, are attracted by light and warmth to the upper tree crowns, where they feed on buds and new foliage with little noticeable effect. Older larvae descend to lower portions of the crown by mid-July where they feed heavily on both old and new foliage. They tend to be wasteful feeders, consuming chunks or portions of needles, and clipping many so that portions fall to the ground uneaten. Mature larvae “spin down” on silken threads from feeding sites to lower branches and the ground during August and early September to pupate. During this time they may be found crawling over tree trunks and understory vegetation, and loose webbing may be abundant. Pupation occurs in trunk bark crevices, lichens, mosses, or ground debris, usually lasting 10 to 14 days. Adults generally fly in late August through early October, dying soon after mating and laying eggs.
Mature and old-growth stands predominated by western hemlock that grow in the Cascades and farther west provide important habitat for western hemlock looper. Although present also in the interior portions of Washington and Oregon, habitat in those areas does not appear to be as favorable as that of old-growth forests in the western coastal regions, where large outbreaks causing extensive mortality were recorded from the late 1800’s to the early 1960’s, prior to intensive logging of those areas. Since the mid-1960’s, most recorded defoliation has occurred in the north Cascade Mountains of Washington.
Outbreaks tend to occur in valley bottom stands of mature and old-growth western hemlock. They sometimes extend for long distances up and down the length of affected drainages as high as midslope, but also may occur as more limited areas of defoliation that develop in widely scattered locations at the same time. Outbreaks also sometimes occur in vigorously growing 80 to 100 year-old hemlock stands, and have occurred in stands as young as 60 years of age. Western hemlock looper outbreaks seem to occur repeatedly in the same geographic locations, i.e. they tend to occur in the “footprint” of previous outbreaks.
Larvae disperse vertically within the tree crown and among canopy layers by crawling and by dropping on silk threads produced from their mouths. Small larvae may be blown by wind among trees. Moths provide the primary mode of horizontal dispersal, but are weak flyers.
Outbreaks appear to be sporadic, and usually last about 3 years. The phantom hemlock looper, Nepytia phantasmaria, is sometimes unusually abundant in western hemlock looper outbreaks, and outbreaks of the phantom hemlock looper or the western black-headed budworm, Acleris gloverana, sometimes coincidentally occur at other locations. During the first year of increasing populations, defoliation often goes unnoticed, although moth flight in the fall frequently draws attention. The second year is commonly the peak year, in which larval populations and defoliation are obvious, and extensive moth flights occur in the fall. During the third year, some feeding and spread of affected area occurs, but few larvae reach maturity to lay eggs. Outbreak collapse is usually brought about by the combined effects of diseases, parasites, predators, and sometimes, adverse weather conditions or larval starvation. Two diseases, a nuclear polyhedrosis virus and a fungal pathogen have been identified as significant natural control agents.
During outbreaks, western hemlock looper can cause extensive defoliation of western hemlock and intermixed tree species. Hemlocks are generally intolerant of defoliation, and frequently die when more than 80 percent of their crown is defoliated, especially if defoliation occurs coincidentally with droughty conditions. Trees with 20 percent or less of their crowns defoliated usually completely recover, while intermediate levels of defoliation may result in varying levels of topkill, scattered dead branches, and mortality. Hemlock looper outbreaks create canopy gaps and dead wood structure, increasing structural diversity, but are not likely to change the long-term trajectory of stand development in terms of species composition, because canopy gaps and understories eventually recover to a condition similar to that occurring prior to defoliation.
There is little opportunity for manipulation of western hemlock looper to increase wildlife habitat.
Western hemlock looper outbreaks cause growth loss, topkill and tree mortality, and may reduce tree density and cover to undesirable levels. Losses of existing mature forest cover may degrade for many decades the quality of habitat available for some wildlife species such as the northern spotted owl. Topkill and mortality caused by western hemlock looper may cause detrimental effects to aesthetic values and public safety in recreational areas.
Stands predominated by tree species other than western hemlock or a mix of western hemlock and western redcedar appear resistant to western hemlock looper outbreaks. Although outbreaks occasionally may occur in hemlock stands less than 80 years of age, mortality is uncommon, although some topkill and growth loss may occur. Maintaining a variety of stand age classes and species mixtures across the landscape will help to minimize the risk of adverse effects to western hemlock looper. Single-canopy, well-spaced stands having good vigor should be less susceptible to outbreaks and more resilient to the effects of defoliation. Species compositions with less than 50 percent hemlock or western hemlock and western redcedar also are likely to be less susceptible. Tree species mixtures that are less susceptible to western hemlock looper outbreaks can be encouraged during regeneration, thinning, and selective harvest activities.
Bacillus thuringiensis var. kurstaki (Btk) is a microbial insecticide specific to lepidopterous larvae that is registered for use against the western hemlock looper. It may be used to suppress hemlock looper populations and minimize foliage loss on the areas where it is applied. Btk is most appropriately applied to well defined, high-value areas, giving careful consideration to minimize non-target effects.
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