HERBIVOROUS INSECTS AND GLOBAL CHANGE - POTENTIAL CHANGES IN THE SPATIAL-DISTRIBUTION OF FOREST DEFOLIATOR OUTBREAKS

The geographical ranges and the spatial extent of outbreaks of herbivo rous species are likely to shift with climatic change. We investigated potential changes in spatial distribution of outbreaks of the western spruce budworm, Choristoneura occidentalis Freeman, in Oregon, U.S.A. and the gypsy moth, Lymantria dispnr (L.), in Pennsylvania, U.S.A. us ing maps of historical defoliation, climate and forest composition in a geographic information system. Maps of defoliation frequency were as sembled using historical aerial reconnaissance data. Maps of monthly m eans of daily temperature maxima and minima and of monthly precipitati on averaged over 30 years were developed using an interpolation techni que. All maps were at a spatial resolution of 2 x 2 km. Relationships between defoliation status and the environmental variables were modell ed using a linear discriminant function. Five climatic change scenario s were investigated: an increase of 2 degrees C, a 2 degrees increase with an increase of 0.5 mm per day in precipitation, a 2 degrees C inc rease with an equivalent decrease in precipitation, and equilibrium pr ojections of temperature and precipitation by two general circulation models (GCMs) at doubled CO2. With an increase in temperature alone, t he projected defoliated area decreased relative to ambient conditions for the budworm and increased slightly for the gypsy moth. With an inc rease in temperature and precipitation the defoliated area increased f or both species. Conversely, the defoliated area decreased for both wh en temperature increased and precipitation decreased. Results for the GCM scenarios contrasted sharply. For the Geophysical Fluids Dynamics Laboratory model, defoliation by budworm was projected to cover Oregon completely, whereas no defoliation was projected by gypsy moth in Pen nsylvania. For the Goddard Institute for Space Studies model, defoliat ion disappeared completely for the budworm and slightly exceeded that under ambient conditions for the gypsy moth. The results are discussed :in terms of potential changes in forest species composition.