Q. I have a white oak tree in my front yard. Over the Fourth of July, my grandson was leaning against it, and his shirt got all dirty. On closer inspection, I found a number of dead grayish-brown caterpillars. They looked very fearsome. We had the tree sprayed earlier as a precaution against gypsy moths, since we were advised that they might be a problem again in 2006. Were these gypsy moths? Why were they all over the trunk of the tree?
A. If the caterpillars have double rows of five blue dots followed by double rows of six red dots, they are indeed gypsy moth (Lymantria dispar) larvae. Those distinguishing dots may be hard to see if the insects have been killed by disease, because they shrivel up badly.
5 rows of blue dots
6 rows of red dots
When gypsy moth larvae are killed by pathogens, they often remain attached to the trunk and/or branches of their host trees. This is probably related to the behavior of mature larvae in that they feed at night and hide in bark crevices during the day. They probably succumbed to the pathogen while they were down on the tree trunk.
Gypsy moths have been a problem in the northeastern United States since their accidental introduction here in the mid-1800s. Their common name comes from the way they get around. Newly hatched larvae spin a long thread of silk and are picked up and blown around by the wind.
Gypsy moth larvae do not build nests, like eastern tent caterpillars, in the crotch of trees or, as the fall webworms do, out on the ends of the branches of host trees. They were introduced into Medford, Mass., and spread south and west ever since. They are as far west as Wisconsin and extend south along the Appalachian Mountains into Virginia.
When they first arrive in an area, their populations skyrocket because their natural enemies are not present or do not know yet that they like this new food offering. They defoliate many species of trees, including oak, apple, hawthorn, basswood, maple, elm and many others, including conifers. Few species are safe when gypsy moth larvae populations are high.
Deciduous trees that are defoliated in three consecutive years often die because they have spent all of their energy reserves by leafing out twice in a single growing season. Conifers die after a single defoliation because most do not have latent buds necessary to leaf out a second time.
Once native, naturally occurring enemies kick in -- including other insects, birds, small mammals and pathogens -- a balance is achieved, and they no longer seem like a biblical plague. Still, insect populations are cyclical, depending upon favorable weather conditions and abundant food sources.
Pathogens such as bacteria, fungi and viruses have been critical to getting gypsy moth populations under control in our area. Drought years can decimate pathogen populations and allow gypsy moths to gain the upper hand.
Pennsylvania's Department of Conservation and Natural Resources had not conducted aerial spraying since 2002. However, forest entomologists predicted gypsy moth outbreaks in 2006 (based on egg mass counts, populations and damage in 2005), and conducted aerial spraying in eight counties, mostly in the northeast and north and south central regions of the state.
Without laboratory analysis, it is impossible to say which pathogen killed the caterpillars on your white oak, or if the spray application took care of them. You are wise to protect a mature white oak, because few of us could lose such a tree and live long enough to see its replacement grow to similar stature.
If they aren't too bothersome to look at, leave the dead larvae for a few more weeks. They may continue to spread the pathogen to any surviving gypsy moth larvae. By late July, you can safely spray them off with a spray from the hose (NEVER use a pressure washer).