Hyphantria cunea Drury (Lepidoptera: Erebidae: Arctiinae)
Historically, the fall webworm (Hyphantria cunea) has been a minor pest in hybrid poplar plantations in the Upper Columbia River basin. However, in 2009, H. cunea populations reached unprecedented levels, causing complete defoliation in parts of several planting blocks. Here we alert integrated pest management (IPM) professionals to the potential for H. cunea to become problematic in their plantations and recommend means to monitor and control pest populations.
There are three races of fall webworms in the US, characterized by head capsule color: black, red, or mosaic. Black and red head capsule populations overlap in the eastern US (Oliver 1964), but populations remain reproductively isolated because their mating period is separated in time (Takeda 2005). The mosaic (black-red head capsule) race exists only in the Pacific Northwest (Tufail et al. 2014). Fall webworm colonies produce large and visible silk tents, but IPM managers must realize that other poplar pests may produce silk refuge for protection against vertebrate and invertebrate predators.
Other Lepidopteran species that produce silk webbing include the eastern tent caterpillar, Malacosoma americanum (Fabricius), the forest tent caterpillar, M. disstria Huber, and the western tent caterpillar M. californicum. All belong to the same genus, but they are in the family Lasiocampidae rather than Erebidae. However, M. californicum has multiple subspecies. Ciesla and Ragenovich (2008) identified six subspecies, three of which can be found in local populations within California: M. californicum californicum (Dyar), M. californicum ambisimile (Dyar), and M. californicum recenseo (Dyar). 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. All these species attack poplar and they all spin silk.
Fall webworms develop on 90 species of deciduous shrubs and trees, including poplar, willow, alder, cherry, mulberry, birch, walnut, hickory, pecan, elm, maple, sweetgum, oak, etc.
Webworms are a native species and are common throughout the US and southern Canada. The northern limit of their range is latitude 50–55°N (Morris 1963). Sourakov and Paris (2014) provide excellent information on fall webworms in their southernmost range in the US. They have been accidently introduced to both Europe and Asia. In Japan multiple generations can occur each season (Gomi and Takeda 1996; Gomi et al. 2007).
Fall webworms overwinter (Gomi 1997) as pupae in cocoons in ground litter or lose soil. Depending on an individual’s overwintering site, adult eclosion of cohorts may vary by 30 days the following season. Adults eclose in mid-June through mid-July, mate, and oviposit a flat egg mass consisting of hundreds of eggs (Figure 1A) on the underside of leaves (Hoover 2001). Larvae hatch (Figure 1B) and immediately en masse start to spin silk webbing around the foliage (Figure 1C) where they are feeding. Depending on the quality of food, fall webworms may go through as many as 10 larval instars and several color phases (Figure 1D) before pupating. As larvae develop they expand the webbing, and mature larvae venture away from the web at night to feed on adjacent leaves. Larval development is complete within six weeks and mature larvae descend from the web to the ground where they pupate (Figure 1E). Adult webworms in the Pacific Northwest are white; occasionally there are orange setae on legs or the ventral part of the body, while adults in the southeastern US have dark spots in their white wings (Sourakov and Paris 2014).