Gypsy moths (Lymantria dispar) in the Niagara Region : population density variation, introduction of an entomopathogenic fungus (Entomophaga maimaiga) and occurrence of nuclear polyhedrosis virus

摘要

The gypsy moth, Lymantria dispar, a major defoliator ofbroad leaf trees, was accidentally introduced into North America in1869. Much interest has been generated regarding the potential ofusing natural pathogens for biological control of this insect. One ofthese pathogens, a highly specific fungus, Entomophaga maimaiga, wasaccredited with causing major epizootics in populations of gypsy mothacross the north-eastern United States in 1989 and 1990 and isthought to be spreading northwards into Canada. This study examinedgypsy moth population densities in the Niagara Region. The fungus, .E..maimaiga, was artificially introduced into one site and the resultingmortality in host populations was noted over two years. Therelationship between fungal mortality, host population density andoccurrence of another pathogen, the nuclear polyhedrosis virus (NPV),was assessed.Gypsy moth population density was assessed by counting eggmasses in 0.01 hectare (ha) study plots in six areas, namely Louth,Queenston, Niagara-on-the-Lake, Shorthills Provincial Park, ChippawaCreek and Willoughby Marsh. High variability in density was seenamong sites. Willoughby Marsh and Chippawa Creek, the sites with thegreatest variability, were selected for more intensive study.The pathogenicity of E. maimaiga was established in laboratorytrials. Fungal-infected gypsy moth larvae were then released intoexperimental plots of varying host density in Willoughby Marsh in1992. These larvae served as the inoculum to infect field larvae.Other larvae were injected with culture medium only and released intocontrol plots also of varying host density. Later, field larvae werecollected and assessed for the presence of .E.. maimaiga and NPV. Agreater proportion of larvae were infected from experimental plotsthan from control plots indicating that the experimental augmentationhad been successful. There was no relationship between host densityand the proportion of infected larvae in either experimental or controlplots. In 1992, 86% of larvae were positive for NPV. Presence andintensity of NPV infection was independent of fungal presence, plottype or interaction of these two factors.Sampling was carried out in the summer of 1993, the year afterthe introduction, to evaluate the persistence of the pathogen in theenvironment. Almost 50% of all larvae were infected with the fungus.There was no difference between control and experimental plots. Datacollected from Willoughby Marsh indicated that there was nocorrelation between the proportion of larvae infected with the fungusand host population density in either experimental or control plots.About 10% of larvae collected from a nearby site, Chippawa Creek,were also positive for .E.. maimaiga suggesting that low levels of .E..maimaiga probably occurred naturally in the area. In 1993, 9.6% oflarvae were positive for NPV. Again, presence or absence of NPVinfection was independent of fungal presence plot type or interactionof these two factors.In conclusion, gypsy moth population densities were highlyvariable between and within sites in the Niagara Region. Theintroduction of the pathogenic fungus, .E.. maimaiga, into WilloughbyMarsh in 1992 was successful and the fungus was again evident in1993. There was no evidence for existence of a relationship betweenfungal mortality and gypsy moth density or occurrence of NPV. Theresults from this study are discussed with respect to the use of .E..maimaiga in gypsy moth management programs.