Lepidopteran communities in temperate deciduous forests affected by forest decline

We surveyed immature lepidopteran communities in maple forests with varying degrees of forest decline over three consecutive growing seasons (June, July and August; 1987-1989), assuming that leaf-feeding Lepidoptera would be positively influenced by slight or moderate stress to trees. We sampled and identified the species and feeding habits of larvae within the canopy and on sugar maple saplings at 51 stations. Densities of caterpillars in healthy and declining forests did not vary in a consistent pattern among different feeding guilds. Population densities of exposed caterpillars were higher within the canopy of declining forests during 1988 and 1989 but densities on saplings did not vary. Less mobile semi-concealed and leaf-mining larvae in the canopy were significantly less abundant in declining forests in 1988 and 1989 and this may be attributed to the fewer and smaller leaves of declining forests. Nevertheless, these results do not fit Larsson's predictions on the response of folivores with various feeding habits to tree stress, i.e. a stronger positive response of leaf-miners compared to leaf-chewers. The richness of the early season lepidopteran fauna was reduced in the canopy and saplings of declining forests, which suggests that some species respond to stress more than others and that forest decline could lead to an impoverishment of the community. Leaf damage caused by insects was lower in areas of decline in 1987 and 1988 and the differences observed were quite stable throughout the seasons. These results could be explained by lower abundance of semi-concealed larvae and leaf-miners or by a reduced consumption of the leaves by the exposed larvae. Leaves from declining trees could contain higher amounts of phenolic constituents compared to leaves from declining trees because of the higher light penetration in the thinned canopy of declining forests. Variation in population densities of immature Lepidoptera associated with forest decline may be attributable to microenvironmental changes within the canopy such as light conditions, altered leaf chemistry or reduced food availability. If the forest decline phenomenon persists, it could lead to changes in the lepidopteran species assemblage and an impoverishment of species richness for the whole community.