From Extension Educator Tom Worthley:
dead caterpillars on tree

Photo: Tom Worthley

“The attached photo is of a 26-inch diameter oak near my home with lots of caterpillars on it, and all of the caterpillars are dead. They exhibit symptoms of the fungus that attacks gypsy moth caterpillars, particularly when populations are high. So while I cannot say it with absolute certainty, I am of the opinion that the Entomophaga maimaiga fungus is at work (finally). Dry spring seasons the last couple years had a depressing effect on the fungal activity, leading to the caterpillar outbreaks we are seeing this year, but we’ve had a wetter spring and while the defoliation ‘damage has been done’ in many areas (almost total defoliation in my area, Higganum) we are now seeing increased fugal activity killing off the caterpillars.

Most trees will re-foliate. This requires some drawing upon stored reserves of carbohydrates by the individual tree, in order to send out new leaves and the evidence of gypsy moth activity will likely appear as reduced diameter growth. Some trees that have been stressed by repeated defoliations in multiple years and perhaps by drought or other issues, might not survive. We will know in the next few weeks.
We will also know later this summer whether many gypsy moth caterpillars have survived to maturity. Non-flying, mostly white females will take up positions in sheltered spots on the bark of trees, and males (more tan, or buff-colored) will by flying around seemingly at random.”
Extension Educator Donna Ellis adds: “As the caterpillars decompose, the fungus reproduces inside the cadavers and on the ground around the trees.  Entomophaga will further spread in the area and can persist in the soil for many years. We recommend that property owners leave the caterpillars in place on the trees to allow the fungus to continue to develop and spread naturally. It remains to be seen how successful the fungus will be in reducing future gypsy moth populations, but hopefully it will have an impact.”