Growth and survival of larvae of Thaumetopoea pinivora inside and outside a local outbreak area

1 Outbreaks of herbivorous insects tend to be spatially restricted, possibly because of demographic differences between inside and outside the outbreak area. In some cases, the margin of the outbreak area is distinct, allowing comparisons of adjacent areas that may identify factors leading to such differences in abundance. The northern pine processionary moth Thaumetopoea pinivora presently occurs at outbreak densities within a well-defined area of approximately 3000 ha on the island of Gotland, south Sweden. We investigated whether cohorts of young larvae (first and second instar) had higher growth rate and survival inside or outside the outbreak area. 2 Group-feeding appears to promote outbreaks in certain insect groups. Because T. pinivora larvae are highly social, we also compared larval performance between groups of different sizes inside and outside of the outbreak area: 'small' (33 eggs/group) and 'normal' (100 eggs/group). 3 Averaged over group size, whole colony mortality through the first two instars was two-fold higher in the non-outbreak area compared with the outbreak area. Mortality of individual larvae in the surviving colonies, however, did not differ between the two areas. There were only small differences in food quality (toughness, nitrogen content) between the areas, with no detectable effects on larval performance. 4 Larval relative growth rate did not differ between reduced and normal-sized groups, which is surprising given that growth rate is known to increase with group size in other group-feeding lepidopterans. 5 Reduced group size negatively affected larval survival, particularly in the outbreak area; by contrast, normal-sized groups survived equally well in the two areas. Wood ants (Formica spp.) were more common outside the outbreak area, and appeared to be the main cause of colony mortality at low larval density. A different result was observed with regard to per-capita mortality, which was higher in the outbreak area. We speculate that this could have been due to solitary predators being locally specialized on T. pinivora in the high-density area.