Abstract:
The gypsy moth, Lymantria dispar, a major defoliator of
broad leaf trees, was accidentally introduced into North America in
1869. Much interest has been generated regarding the potential of
using natural pathogens for biological control of this insect. One of
these pathogens, a highly specific fungus, Entomophaga maimaiga, was
accredited with causing major epizootics in populations of gypsy moth
across the north-eastern United States in 1989 and 1990 and is
thought to be spreading northwards into Canada. This study examined
gypsy moth population densities in the Niagara Region. The fungus, .E..
maimaiga, was artificially introduced into one site and the resulting
mortality in host populations was noted over two years. The
relationship between fungal mortality, host population density and
occurrence of another pathogen, the nuclear polyhedrosis virus (NPV),
was assessed.
Gypsy moth population density was assessed by counting egg
masses in 0.01 hectare (ha) study plots in six areas, namely Louth,
Queenston, Niagara-on-the-Lake, Shorthills Provincial Park, Chippawa
Creek and Willoughby Marsh. High variability in density was seen
among sites. Willoughby Marsh and Chippawa Creek, the sites with the
greatest variability, were selected for more intensive study.
The pathogenicity of E. maimaiga was established in laboratory
trials. Fungal-infected gypsy moth larvae were then released into
experimental plots of varying host density in Willoughby Marsh in
1992. These larvae served as the inoculum to infect field larvae.
Other larvae were injected with culture medium only and released into
control plots also of varying host density. Later, field larvae were
collected and assessed for the presence of .E.. maimaiga and NPV. A
greater proportion of larvae were infected from experimental plots
than from control plots indicating that the experimental augmentation
had been successful. There was no relationship between host density
and the proportion of infected larvae in either experimental or control
plots. In 1992, 86% of larvae were positive for NPV. Presence and
intensity of NPV infection was independent of fungal presence, plot
type or interaction of these two factors.
Sampling was carried out in the summer of 1993, the year after
the introduction, to evaluate the persistence of the pathogen in the
environment. Almost 50% of all larvae were infected with the fungus.
There was no difference between control and experimental plots. Data
collected from Willoughby Marsh indicated that there was no
correlation between the proportion of larvae infected with the fungus
and 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% of
larvae were positive for NPV. Again, presence or absence of NPV
infection was independent of fungal presence plot type or interaction
of these two factors.
In conclusion, gypsy moth population densities were highly
variable between and within sites in the Niagara Region. The
introduction of the pathogenic fungus, .E.. maimaiga, into Willoughby
Marsh in 1992 was successful and the fungus was again evident in
1993. There was no evidence for existence of a relationship between
fungal mortality and gypsy moth density or occurrence of NPV. The
results from this study are discussed with respect to the use of .E..
maimaiga in gypsy moth management programs.
