Tent Caterpillars and Fall Webworm

This past summer was a terrible year for tent caterpillars and when we were finally rid of them, out came the Fall Webworm. The insects are easily mistaken as the same caterpillar as both will defoliate trees and form large webbed communal masses on their host tree in their caterpillar stage. They do, however, have some significant differences in their life cycles, nesting patterns and timing of hatch.

The tent caterpillar is active only in the spring and will make their webbed nests on native deciduous trees, most commonly Aspens. These destructive caterpillars hatch from eggs that have been attached in a band to small twigs on their tree host the previous summer. The caterpillars leave their nests at night to feed on the leaves of their host tree causing destruction until the caterpillars are fully mature (about mid-June) at which time they attach their cocoon to the tree. From the cocoon, emerges moths which in turn lay eggs for the following season.

Unlike the tent caterpillars which only cycle through one generation a year, Fall Webworms cycle through multiple generations in a season, with the largest population in the Fall. The Fall Webworm moths lay their eggs on the underside of their hosts leaves, unlike the Tent caterpillars which lay their eggs on twigs of their host. The Fall Webworm is far less discriminating in it’s taste in tree species than the tent caterpillar as it will lay it’s eggs in over 100 species of deciduous tree including cottonwood, elm, and assorted fruit trees.

Tent caterpillars are thought to be the more destructive of the two caterpillars as they cause defoliation in the spring which depletes energy resources of the tree which causes the tree to re-foliate, as the tree needs to work towards storing energy before the dormancy of the winter months. In contrast, the Fall webworm defoliates the tree at a time of year when the trees reserves have been restocked over the summer and the loss of leaves in just weeks away. Trees are most severely affected when they are attacked several years in a row.

The simplest treatment is the physical removal of the masses of caterpillars, assuming of course, that the nest can be reached from the ground or with a tall ladder. The caterpillars are most vulnerable to natural predators in their immature stages, though the caterpillar stage may become decimated by a naturally occurring virus or in some years a fungus. Some companies in Santa Fe will use an insecticide called Bt or Bacillus thruingiensis, but EcoScapes Landscaping has decided that it will not, as it is highly toxic to our bee population and causes more ecological harm than good in the long run.

If you want to read more about Tent caterpillars or Fall Webworms, New Mexico State University’s Carol Sutherland has written some very informative publications. Click link to NMSU


western tent caterpillars 5509530-PPT2253052-PPT


Photos by Jerald Dewey, USDA Forest Service, Bugwood.org

Peggy Wright (Masters in Landscape Architecture)