Forest and Western Tent Caterpillars

Forest and Western Tent Caterpillars

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R. Rodstrom, GreenWood Resources, Portland, Oregon, John Brown, Department of Entomology, Washington State University
Grown as a monoculture for biofuel, pulp, and non-structural saw timber, hybrid poplars suffer serious insect pest infiltration. Depending on location, hybrid poplar crops can be attacked by 40 different identified species of insect pests. These pests consume foliage, burrow into stems and bole, and destroy roots. Each publication in this series focuses on an individual identified insect pest of hybrid poplars. The publications are designed to aid poplar growers and insect pest managers by characterizing an insect pest, discussing the damage it causes, and suggesting strategies for managing it.
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Malacosoma disstria Huber and M. californicum pluviale (Dyar)

(Lepidoptera: Lasiocampidae)


Several web-spinning caterpillar species attack hybrid poplars in eastern Oregon and Washington. In addition to forest and western tent caterpillars, the fall webworm [Hyphantria cunea Drury (Lepidoptera: Erebidae)] is another serious defoliator. Our objective is to alert professional pest managers to potential threats these larvae pose to poplar biomass production. Management decisions should be based upon an accurate identification of the pest, knowledge of the pest’s life history, monitoring their population, and having a management strategy to control these pests.


Several tent caterpillar species (Lepidoptera: Lasiocampidae) are found in the western US. In addition to the forest tent caterpillar, Malacosoma disstria Huber, found throughout the hardwood forests of the entire US and southern Canada, several others have more limited west coast ranges. Generically these can be grouped as the western tent caterpillar, M. californicum, which includes multiple subspecies. Ciesla and Ragenovich (2008) identified six subspecies. Malacosoma californicum californicum (Dyar), M. californicum ambisimile (Dyar), and M. californicum recenseo (Dyar) can be found in local populations within California. M. californicum pluviale (Dyar), known as the northern tent caterpillar, is found in the Pacific Northwest and southern Canada. M. californicum lutescens is a Great Plains species that extends west to Montana, and M. californicum fragile is found in the southwest US. This information sheet will address just two species: the forest tent caterpillar, M. disstria, and the western tent caterpillar, M. californicum pluviale.


Forest tent caterpillars attack most deciduous trees. In their northern range, forest tent caterpillars attack poplar, aspen, willow, birch, cherry, ash, basswood, oak, red alder, elm, hawthorn, and sugar maple. Additional hosts in southern states include water tupelo, sweetgum, and swamp blackgum. Species not attacked include conifers, sycamore, and red maple (Batzer and Morris 1981). Western tent caterpillars attack poplars, willow, red alder, crabapple, ash, birch, hazel, hawthorne, choke cherry, multiple woody shrub hosts, and California oaks and madrone (Ciesla and Ragenovich 2008).


Forest tent caterpillar are found throughout the US and Canada where deciduous hardwood trees grow (Furniss and Carolin 1977). Western tent caterpillars have been collected in Oregon, Washington, and southern Idaho (Ciesla and Ragenovich 2008).

Life History

Forest tent caterpillars and western tent caterpillars are both univoltine, having just one generation per year throughout their range. Egg masses containing overwintering larvae are found on smaller branches, completely surrounding the diameter of the twig. Each larva hatches within its eggshell, but overwinters as a pharate larva, meaning a fully formed larva still within the eggshell (Ciesla and Ragenovich 2008). Larvae then emerge from these egg masses as the twig swells the following spring. In the Pacific Northwest, larvae hatch in late April or early May. Larvae feed in aggregations but do not form extremely large webbed tents like other members of this family (other tent-making caterpillars). Both M. disstria and M. californicum pluviale do spin copious amounts of silk, marking their trails to food sources and creating silken-mats on large branches or the trunk where they congregate to molt or rest (Meeker 2013). M. californicum pluviale does spin small web tents that become larger as larvae grow (Ciesla and Ragenovich 2008). Both M. disstria and M. californicum pluviale have five larval instars. The ultimate instar (Figure 1) starts a wandering stage in search of a suitable pupation site (Figure 2). Larval development through all five instars is rapid (30–42 days) and pupation may require an additional 12–18 days. Adult moths emerge in late July and early August and mate, and the females lay egg masses on small branches or twigs.


Copyright 2017 Washington State University

Published June, 2017

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