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Hemlock Looper

The hemlock looper is a destructive forest pest that primarily feeds on hemlock and fir trees, causing defoliation and potential tree mortality during outbreaks.

Hemlock Looper

The Hemlock Looper (Lambdina fiscellaria) is a native North American insect belonging to the family of Geometridae. Known for its distinctive ‘looping’ method of movement, it is a defoliator with a wide host range, feeding on over 28 species of trees and shrubs.

Host Plants

Hemlock Looper primarily targets trees such as Eastern Hemlock (Tsuga canadensis), Western Hemlock (Tsuga heterophylla), and balsam fir (Abies balsamea). In addition, it also feeds on a variety of other coniferous trees, including spruces, larches, and pines, as well as some deciduous species like birch and maple.

Regions impacted

The Hemlock Looper can be found across North America, from the Pacific to the Atlantic coast and from Canada south to Pennsylvania, Wisconsin, and California.


The adult Hemlock Looper is a rather inconspicuous moth. It has a wingspan of approximately 1-1.5 inches (25-38 mm) and varies in color from mottled gray to brown, which helps it blend in with tree bark.

The larval stage, or caterpillar, is where the Hemlock Looper gets its name. These caterpillars move in a distinctive “looping” motion, arching their bodies as they crawl along branches and foliage. They are generally greenish, although color can vary depending on their diet and age, and measure up to 1 inch (25 mm) in length.

There are three regional subspecies of hemlock looper: the Eastern Hemlock Looper (Lambdina fiscellaria fiscellaria), the Western Hemlock Looper (Lambdina fiscellaria lugubrosia), and the Western Oak Looper (Lambdina fiscellaria somniaria).

Life Cycle

The Hemlock Looper has a life cycle similar to other moths and undergoes a complete metamorphosis, which includes egg, larva (caterpillar), pupa, and adult stages. Here’s a detailed look at the life cycle of the Hemlock Looper:

Egg: The life cycle begins in late summer or early fall when female moths lay their eggs. These eggs are deposited in large masses on the needles and twigs of their host trees. The eggs overwinter on the trees, protected from the elements and predators by a waxy coating.

Larva (Caterpillar): In the spring, the eggs hatch into small caterpillars. These caterpillars are voracious feeders that grow rapidly and consume the needles of their host trees. They pass through five or six stages, known as instars, over several weeks. They shed their old skin with each instar and emerge larger than before. The caterpillars are usually greenish, which helps them blend in with the foliage of their host trees and move in a distinctive “looping” motion, which gives them their common name.

Pupa: In late summer, fully grown caterpillars transition to the pupal stage, descending on silken threads to lower branches and the ground. During this period, they may crawl on tree trunks and vegetation, and loose webbing may be visible. Pupation occurs in trunk bark crevices, lichens, mosses, or ground debris, typically lasting 10 to 14 days.

Adult: Adults emerge in late summer or early fall. They are short-lived, surviving just long enough to mate and, in the case of the females, lay eggs for the next generation.

After laying eggs, the adult moths die, and the cycle starts anew, with the eggs overwintering until the following spring. The Hemlock Looper typically has one generation per year, but in some locations and under certain conditions, there may be a partial second generation.

Damage and Detection


Hemlock Looper caterpillars feed on the foliage of their host trees, consuming needles and causing defoliation. Light to moderate defoliation can usually be tolerated by healthy, mature trees, but severe or repeated defoliation can weaken trees and eventually lead to tree mortality.

Defoliation can disrupt the tree’s ability to photosynthesize and produce the necessary nutrients for growth and survival. Additionally, trees under stress from significant defoliation become more susceptible to other pests and diseases.

In outbreak conditions, the Hemlock Looper can cause widespread defoliation and significant tree mortality, impacting both the ecosystem and commercial forestry interests.


Detecting an infestation of Hemlock Loopers involves both visual inspections for signs of damage and active monitoring for the presence of the pest. Here’s what to look for:

  • Defoliation: Look for trees that are losing needles or showing signs of thinning foliage. This is usually most noticeable in the upper crown of the tree initially.
  • Caterpillars: In the late spring and early summer, look for small green caterpillars on the needles, twigs, and trunks of trees. As the caterpillars grow, they can often be seen moving in a characteristic “looping” motion.
  • Silk webbing: Mature larvae produce an abundance of silk webbing, which is evident in defoliated stands.
  • Moths: Adult Hemlock Looper moths are active in late summer to early fall. They are nocturnal and can be attracted to lights, so they might be noticed around outdoor lighting at night.
  • Egg Masses: Female moths lay their eggs in masses on the needles and twigs of host trees. These egg masses can often be seen during the fall and winter.

Regular monitoring can help detect an infestation early, allowing for more effective management and control. This can involve visual inspections, as well as trapping adult moths to determine their numbers and distribution.

Prevention and Control

Prevention and control of the Hemlock Looper require a multi-faceted approach that encompasses several strategies.


  • Forest Diversity: Encouraging diversity in tree species and age can help limit the spread and impact of Hemlock Looper infestations. Monocultures or forests dominated by susceptible species are more likely to suffer severe damage.
  • Tree Health: Maintaining the overall health of trees can make them less susceptible to severe damage from Hemlock Loopers. This includes proper watering and fertilizing practices, as well as controlling other pests and diseases.
  • Monitoring: Regular monitoring of susceptible forests can help detect the early stages of an outbreak. This can involve visual inspection of trees for signs of defoliation and the use of pheromone traps to capture adult moths and assess their population levels.


  • Natural Predators: Birds and various insects can prey on Hemlock Looper caterpillars and pupae. In many cases, these natural predators can help limit Hemlock Looper populations. Encouraging these predators can be part of an integrated pest management strategy.
  • Biological Control: Biological control agents such as the bacteria Bacillus thuringiensis (Bt) and certain types of parasitic wasps can be effective against Hemlock Looper caterpillars. Bt is a common ingredient in many biological insecticides and can be sprayed onto trees during the caterpillar stage. It is toxic to the caterpillars but has minimal impact on non-target species.
  • Chemical Control: Insecticides can be used to control severe infestations of Hemlock Loopers. This often involves aerial spraying of affected areas. However, chemical control can have negative impacts on non-target species and should be used judiciously.
  • Silviculture: Forest management practices, such as selective logging or thinning of trees, can help limit the spread of Hemlock Looper infestations and reduce their impact. Removal of severely infested trees can also help reduce the population of the pest.

Remember that Hemlock Looper infestations can be cyclical, with populations building up to outbreak levels over several years, then crashing due to disease, predation, or depletion of food resources. Monitoring and intervention can help manage these cycles and reduce their impact on forests.

While every effort has been made to describe these plants accurately, please keep in mind that height, bloom time, and color may differ in various climates. The description of these plants has been written based on numerous outside resources.

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