Unlike most other forest insects, spongy moth (Lymantria dispar) [formerly known as the “gypsy moth”] populations go through dramatic changes in abundance. Most of the time, their population is relatively low and you rarely see a caterpillar unless you look for one. Surprisingly, you can have up to 25,000 caterpillars per acre and not notice any effect on the trees! Periodically, however, the population can explode as a result of a combination of favorable conditions and will increase at a very rapid rate. Typically, the population will continue to grow until there are so many caterpillars that they consume all the available food and begin to starve. It’s at this “outbreak” stage in their population cycle that we have problems with spongy moth. It usually takes only one to three years before starvation, disease, and natural enemies cause the population to return to low levels, but in the meantime damage is done to trees.