What is forming these webs in some of my trees? ASK THE HORT AGENT
Question What is forming these webs in some of my trees?
Answer Although they are often called bagworms, these worms are actually eastern tent caterpillars. Both of these pests are caterpillars. They have a few similarities, but a lot of differences.
Eastern tent caterpillars emerge in March or April. These are some of the most social caterpillars. They congregate in the fork of a branch and construct a silk tent. The caterpillars will live in this tent for the next 6 weeks. Each day they will expand the tent to accommodate their growing bodies.
Like a house full of fraternity brothers, they lay around most of time. Three times a day they add a little silk to the tent then crawl out to eat a few leaves. Unlike fraternity brothers, the caterpillars have their first meal just before dawn. They eat again at midday, and their final meal is just after sunset.
Bagworms are more like IRS agents. Bagworms emerge from late May through early June. They are solitary caterpillars. While the population of bagworms may be high on any particular tree or shrub, each caterpillar lives in its own silken cocoon. As these caterpillars feed, they enlarge their diamond shaped cocoon to cover their growing body. A few bagworms will not kill a host plant, but high populations can kill the host tree or shrub.
Both tent caterpillars and bagworms eat leaves, but they target different trees. Tent caterpillars prefer wild cherry, apple or crabapple. However, they will also feed on sourwood, sweetgum, willow, maple, oak and poplar. Bagworms also have a large host range, but they strongly prefer juniper, arborvitae and cypress.
Large numbers of eastern tent caterpillars occur at intervals of approximately 10 years. Even then, their host trees usually bounce back from the leaf defoliation which their feeding causes. On the other hand, bagworms may take a couple years to build up a high population on a tree.
An insecticide that contains the bacteria, Bacillus thuringiensis (like DiPel), can be sprayed on the foliage of the affected trees when the caterpillars are small. This will work on tent caterpillars and bagworms. There are several other insecticides that can be used to directly spray the caterpillars. The problem is reaching them. Bag worms have to be sprayed before they develop their cocoons. The cocoons also double as raincoats which makes insecticide treatments useless.
Most folks don’t worry about the tent caterpillars. The leaves will grow back and the worms will be gone in a few weeks.
For more info on eastern tent caterpillars, visit http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note61/note61.html If you have further questions, call me at 910-893-7533 or email me at gpierce@harnett.org
Some folks try to burn their little silk houses with fire (on a long stick). I don’t know what is worse – insecticide or smoldering worms dripping on my head.
Gary L. Pierce
Horticulture Extension Agent
Harnett County | 
