Experts predict that this year’s tent caterpillar infestation will be more intense than it has been in the last few years although probably not quite as bad as the invasion of 10 years ago. Some of the early victims have been the bushes and trees recently planted at Wilkes. The Bainbridge Island School District reports that it plans to treat the trees and bushes with Bacillus thuringiensis or BT, along with a compost tea and a surfactant called Nu Film P. The spraying is planned for Saturday, May 25, between 9 a.m. and 6 p.m.
The infestation started at Wilkes in early May, stripping bare several of the newly planted Yoshino cherry trees. Caterpillar tents began spreading to birch trees and laurel shrubs. The landscape contractor for Wilkes, Patrick Walker, initially removed the caterpillar tents by pruning the infected branches. But Walker stopped pruning as the infestation spread, out of concern for the trees. The crew tried hand plucking but could not keep up with the spread.
BISD’s Integrated Pest Management Committee, led by Maria Mason and Dale Spoor, guided the process of determining how to proceed. Director of Facilities and Capital Projects Tamela Van Winkle also sought out the input of Maintenance and Grounds staff Bernie Mejia and Chris Rauch. In addition, the District consulted with Bainbridge Gardens and with local gardening expert Ann Lovejoy. The consensus was that the young trees and shrubs at Wilkes were already stressed from recent transplanting, and there was concern that they might not re-foliate and could die.
Further research by the IPM Committee and information from Earthdance Organics turned up the viability of the use of BT in a compost tea. BT is a soil-dwelling bacterium that, during sporulation, produces crystal proteins that have insecticidal action. After consulting with Paul Figueroa, Acting Area Manager for the Washington State Department of Agriculture (WSDA), BISD opted to use the BT mixed into the surfactant Nu Film P and augmented by Earthdance Organics Fertile Tea.
The caterpillars, Malacosoma californicum, are orange and black and build tentlike cocoons. Karen Ripley, DNR Forest Health Program Manager, warns that the caterpillars can defoliate alder, willow, cottonwood, and other broad-leafed trees. However, Ripley points out, despite appearances, healthy trees are not being damaged even if all their leaves are eaten. She says that human intervention is not required for healthy trees. The deciduous trees will replace the eaten leaves within a few weeks, and the trees can even reduce their own digestibility and nutrient content in response. She adds, “The plants growing beneath the caterpillar laden trees are getting extra exposure to sunlight and a shower of nutrients in the feces and leaf fragments that drop from above.”
The Washington State University Extension lists a few additional benefits of outbreaks: Some birds feed on the caterpillars. Small mammals feed on the pupae, and birds and bats eat the moths.
Ripley says these outbreaks are natural periodic events occurring about every nine years with continuing high levels for the ensuing three years.