Fine-scale abundance and distribution of wild silk moth pupae

Although several factors influence herbivore insect distributions at any particular scale, the most important determinants are likely to differ between species with different life histories. Identifying what these factors are and how they relate to life history forms an important component of understanding the population dynamics of species, and the habitat requirements necessary for their conservation. The pupal stage of two wild silk moth species, Gonometa postica Walker and G. rufobrunnea Aurivillius (Lepidoptera: Lasiocampidae), is the target of harvesting practices that are totally dependent on the availability of pupae from natural populations. Consequently, and partly due to poor knowledge of the species' biology, there is substantial interest in the distribution of pupae among and within trees for both these species. It was investigated whether between- and within-tree pupal distributions in these two species are non-random, and if so, whether there are relationships between pupation site use and tree characteristics such as tree size, available pupation space and branch position. Between-tree patterns in pupal abundance were random in terms of absolute spatial position, but markedly non-random with respect to tree characteristics. The apparent G. postica pupae were aggregated on large larval host plants, whereas the cryptic G. rufobrunnea pupae were aggregated on non-host plants. These patterns reflect the life history differences of the two species. In contrast, at the within-tree scale, branch position, aspect and tree shape influenced pupation site choice similarly for both species. These patterns might be related to microclimate. Documenting between-tree and within-tree patterns in Gonometa pupal distributions is the first step towards explaining pupation site selection, as well as identifying possible evolutionarily selective factors in the species, and generating testable hypotheses from these.