Last year saw the largest defoliation of trees by gypsy moth caterpillars since the early 1980’s. More than 350,000 acres, more than 10 percent of all woodlands in Massachusetts were defoliated in a swath from Cape Cod to eastern Hampden and Hampshire counties. Drought conditions created a perfect environment for an explosion of the gypsy moth population. With very high egg mass numbers on trees this year, concerns were raised that even more acreage of severe defoliation would occur, particularly in areas where winter moth populations were high.
The very wet spring many regions have experienced in 2017 gave hope that the fungus, Entomophaga maimaiga, which attacks gypsy moth caterpillars and has generally kept them in check for 30 years, might be better able to spread widely and result in high caterpillar mortality.
At first, the omens this year were mixed. Some areas again had heavy defoliation by gypsy moths, not only in oaks but also maples and even understory white pines. In other areas, however, despite the high volumes of egg masses, defoliation was less severe and significant numbers of dead caterpillars have been observed.
In mid to late June, the hoped-for crash happened. Reports of thousands of dead caterpillars came from around the state. The wet conditions resulted in huge numbers of caterpillars affected by the E. maimaiga fungus and dying on the trees. The hope now is that the gypsy moth population in 2018 will be considerably reduced, with substantially lower amounts of defoliation.
Repeated defoliations, particularly under drought conditions, can lead to increased mortality of trees. Landowners in the moth infested areas should monitor their woods and see if many trees, particularly oaks, have failed to put out new leaves after defoliation. If they have, landowners may need to consider having a timber sale to salvage the stricken trees before they become unmerchantable. White oak is particularly susceptible to mortality.