Some lepidopteran species have larvae that live gregariously, especially in early instars. Colony-living species may benefit from improved protection from predators, thermoregulation, and feeding facilitation, for example. While many studies have compared solitary and gregarious life styles, few data exist as to the relationship between size of the larval colony and larval performance in gregarious species. The present study was aimed at understanding the importance of colony size for growth and survival of the northern pine processionary moth (Thaumetopoea pinivora) larvae. Field studies, comparing three different sizes of colonies of T. pinivora larvae, showed that individuals in larger colonies had a higher survival rate compared with those living in smaller colonies and also a faster growth rate. The higher survival rate of large colonies was attributed to improved protection from predacious arthropods. In early spring, the young larvae bask in the sun to increase their body temperature. In field experiments the thermal gain was higher in large colonies, and individuals in such colonies also grew faster. As growth rate was not affected by colony size when the ability to bask was experimentally removed in a laboratory experiment, the higher growth rate of the larger colonies was probably due to improved thermoregulation rather than feeding facilitation. The size of larval colonies of gregarious insects depends on natural mortality events as well as on female oviposition strategy. Our results show that decreasing colony size can lead to a reduction in growth rate and survival. It is therefore important to understand whether or not small colonies will benefit equally from the gregarious behaviour

Summary Accurate estimates of gamete and offspring dispersal range are required for the understanding and prediction of spatial population dynamics and species persistence. Little is known about gamete dispersal in fungi, especially in lichen‐forming ascomycetes. Here, we estimate the dispersal functions of clonal propagules, gametes and ascospores of the epiphytic lichen Lobaria pulmonaria.We use hierarchical Bayesian parentage analysis, which integrates genetic and ecological information from multiannual colonization and dispersal source data collected in a large, old‐growth forest landscape.The effective dispersal range of gametes is several hundred metres to kilometres from potential paternal individuals. By contrast, clonal propagules disperse only tens of metres, and ascospores disperse over several thousand metres.Our study reveals the dispersal distances of individual reproductive units; clonal propagules, gametes and ascospores, which is of great importance for a thorough understanding of the spatial dynamics of ascomycetes. Sexual reproduction occurs between distant individuals. However, whereas gametes and ascospores disperse over long distances, the overall rate of colonization of trees is low. Hence, establishment is the limiting factor for the colonization of new host trees by the lichen in old‐growth landscapes.

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.

It is challenging to unravel the history of organisms with highly scattered populations. Such species may have fragmented distributions because extant populations are remnants of a previously more continuous range, or because the species has narrow habitat requirements in combination with good dispersal capacity (naturally or vector borne). The northern pine processionary moth Thaumetopoea pinivora has a scattered distribution with fragmented populations in two separate regions, northern and south-western Europe. The aims of this study were to explore the glacial and postglacial history of T. pinivora, and add to the understanding of its current distribution and level of contemporary gene flow. We surveyed published records of its occurrence and analysed individuals from a representative subset of populations across the range. A 633 bp long fragment of the mtDNA COI gene was sequenced and nine polymorphic microsatellite loci were genotyped. Only nine nucleotide sites were polymorphic in the COI gene and 90% of the individuals from across its whole range shared the same haplotype. The microsatellite diversity gradually declined towards the north, and unique alleles were found in only three of the northern and three of southern sites. Genetic structuring did not indicate complete isolation among regions, but an increase of genetic isolation by geographic distance. Approximate Bayesian model choice suggested recent divergence during the postglacial period, but glacial refugia remain unidentified. The progressive reduction of suitable habitats is suggested to explain the genetic structure of the populations and we suggest that T. pinivora is a cold-tolerant relict species, with situation-dependent dispersal.