Pest description and damage Caterpillars are the larval form of butterflies and moths. Caterpillars tend to have long cylindrical bodies with a visible head capsule, six true legs and a series of five pair or fewer prolegs. Most caterpillars have chewing mouthparts (mandibles) that they use to bite, chew and feed on leaf tissues. These larvae are variable in size, shape, color and hairiness. Some caterpillars are so small that they fit inside the confines of seeds and some may mine the tissues between the upper and lower leaf surfaces (leafminers). Some caterpillars may burrow into wood, roots, or buds. Other caterpillars can be as large as one to three inches long and resemble twigs or branches. Caterpillars may be hairless and others may have hairs that are thick, bristle-like, or soft and silky.
Biology and life cycle Moths and butterflies have complete metamorphosis: egg, caterpillar, pupa (chrysalis, cocoon or lined cell in the soil or wood) and adult. Moths and butterflies may be strong or weak fliers, and some are flightless. Knowing this provides clues on whether these caterpillars will build up quickly in an area.
Pest monitoring Look for caterpillars or their fecal pellets below host plants (use sticky plates hung horizontally below suspected hosts or check leaf surfaces). Some caterpillars make visible webbing trails, tie leaves together or create tents. Pheromones are available to attract male moths to sticky traps.
Control weeds, grasses, and debris in the vegetable garden that provide cover. Encourage natural enemies of caterpillars like birds and spiders.
Many caterpillars are eaten by other insects or are heavily parasitized by wasps and parasitoid flies. Certain flowering species can be planted to provide nectar and pollen to attract these parasitic wasps and flies into the home landscape. Tiny Trichogramma wasps parasitize the eggs. Birds, bats and many small mammals feed on moths, larvae and pupae and some eggs. These parasitoids and predators that feed on the caterpillars can be purchased and released in the landscape, or one can manage the landscape in a way that keeps the parasitoid and predators healthy. This may mean tolerating a low number of these pests in your landscape plants.
For more information
LaGasa, E., T.Murray, and C. Looney. 2011. PNW Defoliators (http://invasives.wsu.edu/defoliators/species_faqs.html)
Miller, J. C. and P. Hammond. 2003. Lepidoptera of the Pacific Northwest: Caterpillars and Adults. HJ Andrews Publication Number 3739 (http://andrewsforest.oregonstate.edu/pubs/webdocs/reports/pub3739.cfm?to...)
Rosetta, R. 2009. Web of destruction: caterpillars of concern in the Pacific Northwest (http://oregonstate.edu/dept/nurspest/caterpillar%20pests.htm)