FOREST TENT CATERPILLARS

Question I've had those green army worms all over the trees in my yard in the past, will they come back, and what can I do about them?

Answer

The "green army worms" you describe are most likely Forest Tent Caterpillars (FTC) that are native insects that have population explosions about every ten years. We saw over two million acres of trees defoliated by FTC last year. We expect even more of the insects starting this June in the area from Brainerd and Mille Lacs north to Canada. Some may even be seen as far south as the northern metro area. We can expect them to be a nuisance for about three weeks in June.

You can do several things to reduce the nuisance and protect your buildings and trees. Although they won't damage buildings, their excrement may stain siding. To reduce this, brush or wash them off buildings daily. They come off easier if you do this every day. Bag, bury or compost the caterpillars and cocoons. You can spray your foundations (avoid painted surfaces) with an insecticide containing Malathion to inhibit the insects from climbing on your buildings.

The favorite food of FTC is leaves of deciduous trees, especially aspen. They tend to avoid red maple. In the forest, FTC can consume 60% of the leaves two to three years in a row without seriously affecting the tree. In the urban setting, where trees are under many environmental stresses, FTC can be a serious threat. Most vulnerable are birch and oak defoliated two years in a row and any newly planted trees. The birch and oak can be weakened enough to allow wood boring insects to kill the tree. Defoliation of newly planted trees can kill them outright. Fruit trees will also have reduced crops if FTC has defoliated them.

You can reduce the risk to your trees by hand picking all the egg masses off valuable plants by early-May and dispose of them. Also hand pick caterpillars off plants and dispose of them. If you can determine that there are no egg masses in a tree or if you have sprayed the tree, you can prevent migrating caterpillars from climbing up the trunk by using barriers. Construct a barrier band around the trunk made of duct tape, tin foil or tar paper and coat it generously with grease (Tangle foot or Vaseline). Never apply grease directly to the tree bark. The barrier band should be in the shade or you run the risk of killing the bark and cambium under the band. Check the barrier band daily to see if more grease or Tangle foot is necessary. Remove the band in early July after the caterpillars have formed cocoons.

You can spray an insecticide to kill caterpillars. Each product has restrictions about which plants and sites it can legally be applied. If applying to shade and ornamental trees, the label should say it is for use on shade and ornamental trees. Please read and follow label directions. Biological insecticides containing Bt, (a bacterial product made of Bacillus thuringiensis) are recommended to use for FTC control in the backyard because of their safety and the low toxicity to non-target organisms. Bt products are only toxic to caterpillars; they do not kill bees, flies, mosquitos, etc. However, Bt products are slightly slower to act since caterpillars must eat them before they take effect. Apply Bt to the leaves of host plants not to the bark or other non-edible materials. It is most effective on young (small) caterpillars. Chemical insecticides can also be used but would normally be a second choice after Bt, due to safety considerations. Commonly used chemical insecticides contain Malathion (Malathion), acephate (Orthene), carbaryl (Sevin) or methoxychlor (Methoxychlor). These products can also kill bees and other organisms, so exercise caution when using them. (Brand names are not meant to be an endorsement of a particular commercial product.)

FTC moths are attracted to lights during early July. Turning out your yard and exterior lights may reduce egg-laying on your trees and thus reduce next year's population.

The most important thing you can do for your trees is to keep them well watered. Supply 1 inch per week if you do not receive that much in rainfall from May 1 through September 1. Do not fertilize trees or use a weed and feed product on your lawn during an outbreak. Heavy nitrogen fertilization encourages the tree to produce more leaves which may deplete energy reserves and put additional stress on the tree. Stressed trees are easily attacked by other serious insect or disease pests.  

DNR-FORESTRY