Last updated: July 21, 2016
In the mid-1860's, E. L. Trouvelot imported gypsy moth egg cases from France to his home in Medford, MA and began his amateur endeavor to create a superior silkworm using a non-native species. By 1869, several adults had escaped into the wild, blown off a windowsill on a windy day. Lacking natural predators, they reproduced rapidly. Soon, gypsy moths were defoliating the Medford landscape.
As Trouvelot's interest turned to astronomical drawings and a return to his native France, attempts to physically remove gypsy moths failed. In 1910 and 1911, several fungal pathogens were released. No lasting infections of the gypsy moth population were observed, and hope for a solution faded.
Gyspy moth outbreaks continued to occur over an ever-expanding range, peaking in 1981 with the defoliation of roughly 300,000 acres of forest in Massachusetts alone. With nothing to lose, scientists released the fungal pathogen Entomophaga maimaiga a second time. Once again, no lasting infections were observed. Then, in 1989, a large number of gypsy moth larvae in southwestern Connecticut were found to have died from E. maimaiga infection. The infection spread and outbreaks seemed to be a thing of the past.
Gypsy moth caterpillars are once again defoliating huge tracts of deciduous trees. Fungal growth has been limited by unusually dry and warm spring and summer conditions in 2014 and 2015. As a result, the gypsy moth population is exploding. Ken Gooch, director of the Massachusetts Department of Conservation and Recreation Forest Health Program, has projected upwards of 150,000 acres of defoliated forest from gypsy moth caterpillar herbivory this year. The outlook for 2017 is not good as the dry, warm weather continues in 2016.
Concerns about environmental damage are rising. Mature trees can die following repeated defoliation events. Though many trees will push out a second growth of leaves, lammas growth, the trees must expend energy in the effort. At the same time, defoliation deprives trees of the ability to create and store energy through photosynthesis. Weakened trees will eventually die if they are not given the opportunity to fully recover.
Ecosystems are complex systems, of course, and the impacts of the alien gypsy moth are potentially more wide-ranging than they might appear at first glance. Ecosystems evolve over long time scales, and the individual species woven into ecosystems become uniquely fit to leverage the resources inherent in the evolved system without disrupting its balance. Let's take a closer look at how gypsy moth caterpillars may be disrupting the balance.
Oak trees drop large quantities of acorns on some years and much smaller quantities on others. The bountiful years are known as 'mast' years, and seem to occur every two to five years. Still somewhat of a mystery, mast year acorn production can be looked at as a response to environmental factors such as weather conditions and as a reproductive strategy. Gypsy moths prefer oak tree leaves, and it is reasonable to assume that defoliation due to gypsy moth herbivory disrupts normal mast year production.
A series of investigations led by Joseph Elkinton in the 1980's and 1990's illuminated the relationship between oak mast year acorn productivity and white-footed mouse population. Following mast years, the population of white-footed mouse grows due to increases in winter breeding and survival rate of overwintering individuals. Elkinton was interested in this dynamic because white-footed mouse is a significant predator of gypsy moth caterpillars and pupae. In fact, it may be that white-footed mouse can control low-density gypsy moth populations. Could it be that a disruption of the mast year productivity and white-footed mouse population dynamic released a low-density gypsy moth population from predation and then weather conditions drove an outbreak?
Perhaps we should think about the way defoliated areas appear as a mosaic on the landscape. One can see an area of defoliated trees abutting an area that seems undisturbed. Could this mosaic pattern be related to the mosaic nature of mast years? Possibly related to the mast year and white-footed mouse relationship? Could the pattern be related to elevation of the terrain and drainage characteristics of the underlying soil? How complex might the situation be?