Entomologist USDA-APHIS Buzzards Bay, Massachusetts
In 1987, the Elkinton lab launched an ambitious study of mortality factors controlling Lymantria dispar populations. We deployed artificial populations of L.dispar, (ranging from 45,000 to 1.1 million eggs per ha.), into a forest with very low background densities of L.dispar. The result was a strong, positive, spatially density-dependent population decline. The key parasitoid driving declines was Compsilura concinnata, an introduced generalist fly. The results suggested that C. concinnata can suppress spatially separated L. dispar caterpillars, dropping densities to levels where mouse predation prevents population increases. This fly requires multiple hosts to complete four generations a year, which raised concerns about non-target impacts to native Lepidoptera. We deployed 500 Hyalophora cecropia caterpillars and tracked their daily survival; however, none of these caterpillars survived beyond 4th instar. To determine the role of parasitism, we had also deployed 100 caterpillars of each instar for 5-7 days and reared these indoors. Cumulative mortality from C. concinnata explained 81% of overall H. cecropia mortality. This fly has been recovered from over 200 species of Lepidoptera in the US. However, C. concinnata may also be controlling another serious pest, the brown-tail moth. We verified this using sentinel populations deployed at inland and coastal sites in Massachusetts. At inland sites the moth was decimated by C. concinnata; however, sites where browntail moth currently occurs were nearly C. concinnata free. Despite the success of this generalist fly in controlling two serious pests, we strongly encourage the use of species-specific parasitoids to avoid irreversible impacts to native fauna.
