Population ecology at the range edge

Outbreaks of pest species can cause extensive damage to arable crops and to forest production. Some pests also present severe health hazards to humans and animals. A recent outbreak of the Northern pine processionary moth Thaumetopoea pinivora on Gotland, southern Sweden, has caused concern for the welfare of local residents as this species is known to cause itching rashes in most people that come in contact with infested areas. This thesis examines larval traits that are important to the fitness of T. pinivora, and may thus be a factor in explaining why the species sometimes reaches outbreak levels. As there has been much concern that the outbreak will lead to the moth expanding its range, I have also studied the species' dispersal patterns on a local scale on Gotland and its distribution and colonization history on a global scale. Colony size was found to be important for the growth and survival of the gregarious larvae of T. pinivora. The number of individuals in a colony determines the benefits gained from both predator protection and thermoregulation, indicating that high population densities are favourable for larval survival.The larvae of T. pinivora hatch early in the spring. This enables them to evade predators that do not become active until later in the season. Predator activity increases later in the spring, and early hatching larvae have then had time to grow and are therefore better defended from predation than larvae that hatch later.Although several populations were found outside the outbreak area on Gotland, none of these populations were founded recently and the migration level between the populations was low. This indicates that there is little or no expansion of the outbreak range. The distribution of T. pinivora populations is globally fragmented. Genetic studies show that the colonization history is recent, which is surprising considering the limited dispersal ability of the species. The pattern could possibly be due to a loss of habitat patches that have previously been available for colonization in a stepping-stone fashion.