Forest tent caterpillar populations are high in many parts of the state. We are predicting defoliation in southern Vermont, and an expansion to northern areas, especially the Champlain Valley. You can expect defoliation of many species of trees, shrubs and even vegetable gardens by this voracious native insect. Trees that are particularly favored include sugar maple, ash and oak, but it feeds on most deciduous species. Southern Vermont has experienced two years of high FTC populations.
The window of concern for trees is May and June when the FTC larvae are actively feeding. Egg hatch has already begun in southern Vermont. Hatch is staggered over several weeks so throughout the season you may observe caterpillars of different sizes. Initially, caterpillars may stay clustered until favorable feeding temperatures are reached. Small larvae are easily dispersed by wind. The larger caterpillars are often observed congregating on tree
boles, on man-made structures (around homes). When tree foliage is depleted, larvae migrate to other vegetation and become a nuisance to homeowners. Once they reach about 2 ½ inches in length, they stop feeding, form a cocoon in leaves, bark, or other dark structures, and pupate. Feeding for the year is then complete. By early July, most caterpillars will have pupated.
What can be done?
Be observant
Landowners are encouraged to observe insects and defoliation on their trees in order to learn if they have trees at risk. Just because FTC populations are high in their region does not mean they will see defoliation on their trees. There are also other insects that may be responsible for tree health issues (see “Natures Complexities” below).
Reduce tree stress
Reduce additional stress on defoliated trees by avoiding disturbance around trees:
On forestland, be flexible when scheduling timber harvests. Postpone harvesting if populations build. Delay timber harvesting at least 3 years after the outbreak. Thinning can allow drying of soil, stressing trees. Also, by reduce the number of trees in the stand, you may get more insects on each remaining tree.
On landscape trees, avoid root damage and pruning, and, when applicable, provide adequate water during dry conditions or on newly planted trees.
Choose biological insecticides for spraying on ornamental trees.
The most common reasons landowners may want to spray FTC are to protect leaves on valuable ornamental trees or to avoid caterpillar nuisance around their homes. Foliage of small trees and shrubs can be successfully treated from the ground, but tall, mature trees require special spray equipment to reach the tops. Some locations are just not accessible using ground equipment.
If a landowner chooses to spray, we recommend using the biological insecticide, Btk (Bacillus thuringiensis kurstaki). This is a naturally occurring bacterium that is effective against FTC and less toxic to the environment than other insecticides. Btk is sold locally at some hardware stores under Brand names such as Dipel, Foray, etc. Some brands of Btk are certified for organic farming. If you want to know what is available in your area, you can make some calls to local businesses.
Btk must be eaten by FTC caterpillars to be effective, so it needs to be sprayed on host plant leaves. Best results can be obtained if this is done when the caterpillars are relatively small (1.5 inches). There will be a range of caterpillar sizes so keen observations are needed to accurately determine the appropriate timing to apply insecticides to successfully control insects. As with any spraying, weather conditions will play an important role in effectiveness. Check the label for appropriate spray conditions.
Choose a Knowledgeable Arborist
While we are not able to recommend any particular arborist or tree service company to use for ground spraying, these questions will help with the choice: are they insured, do they have their International Society of Arboriculture’s arborists certification or have a Vermont Plantsman’s Horticultural Certificate, and do they have a professional pesticide applicator license?
Be patient with trees
Tree recovery from defoliation takes time. First you can expect that defoliated trees will develop a second flush of leaves in July to help produce energy for the tree. Then, if no defoliation occurs the following year, trees will begin producing more abundant leaves. Urban trees are often growing under stressful conditions and may have some dead branches following defoliation. It’s best to wait 1-3 years after defoliation before pruning or tree removal to allow for full recovery.
Nature’s Complexities
FTC population outbreaks generally collapse after several years in a given region due to the activity of natural enemies. An increase in parasitic flies and diseased caterpillars has been observed in many FTC infested areas. In time, we expect the population to crash and remain low for 10 or more years.
Apart from identifying FTC, you should be aware that there are several other insects that may complicate diagnosis and control strategies.
Some areas of Vermont continue to see apple and cherry trees defoliated by Eastern Tent Caterpillars (ETC). This insect is a close relative, hatches a bit earlier, and can be easily misidentified as FTC. If the caterpillar is associated with a “tent” and has a solid white stripe down the back, it’s an ETC. FTC larvae have white, keyhole markings down the back. Another hairy caterpillar, the gyspy moth, was more common in 2005, and may be accompanying FTC in feeding on hardwoods this year. Gypsy moth caterpillars have blue and red dots down their back.
Another tree health concern for sugar maples comes in the form of a small scale insect, European fruit lecanium scale, which has been implicated in tree declines where both FTC and the scale were present. Sorting out which insects are present in your area will require some detective work before you assume FTC is the only culprit. Make personal observations before making your diagnosis or asking for assistance.
![Vermont Division of Forestry [photo]](../img/banner_protection.jpg)
