The spongy moth (Lymantria dispar) was brought to Boston in 1869 during an attempt to initiate a profitable silk industry in the United States. It soon escaped confinement and has been expanding its range ever since. It is now one of the most challenging insect pest species in North America. With hundreds of known plant hosts, many of which are completely defoliated during outbreak years, the spongy moth has no shortage of food. Defoliation lasting two or more years can cause significant tree mortality, which may increase the possibility of wildfires. Spongy moth larvae may also outcompete the larvae of native butterflies and moths.
Similar to spiders, the smallest spongy moth larvae can create a silk thread that is caught by the wind, carrying them to new locations by a process called "ballooning." Typical movements of this sort are short (less than a kilometer), yet during strong, sustained winds this is a likely means of population establishment at Isle Royale. However, egg masses or small larvae may also be accidentally transported by people. Prior to loading gear onto boats or the float plane, please inspect any gear you are brining to the island for both small caterpillars and for quarter-sized fuzzy, brown egg masses that are often tucked into crevices.
Pheromone traps that attract adult male moths have been placed near docks around Isle Royale since 1998. The first moth was trapped in 2000, but there have been dramatic increases in the number of male moths captured since 2007. The females cannot fly, which helps slow the spread of spongy moth populations, and the male moths were probably blown across the lake on strong winds. So far there are no known reproducing populations on the island, and the island needs your help in keeping these moths out.