The expectation is that the phenology of an herbivore is influenced by abiotic factors and its own population density during development. In this study, we investigated how the pupal density affected the pupal traits and emergence of Thaumetopoea pityocampa moths over a period of 3 years in two clearings in a pine forest. The pupae were larger in years when the pupal density was high and in the clearing exposed to less solar radiation. There was no relationship between the time of pupation and pupal size. Large pupae were positively correlated with an early emergence of adult moths and a longer period of adult emergence. Up to 13.9% of the pupae developed without cocoons, especially in years when they were abundant, but this did not affect the emergence of the moths. Incidence of pupal diapause was density-dependent and only occurred at a low level in dense populations. Overall, our results indicate that gregariousness confers important fitness-related advantages in this species.
Maternal care within the treehopper subfamily Centrotinae has been considered a rare trait. We present observations on maternal care and nymphal behaviour made in 16 rain forest plots in Peninsular Malaysia and Sabah, Borneo, from 1996 to 1998. Females of 11 Centrotinae morphospecies from the tribes Centrocharesini (Centrochares Stål), Gargarini (Ebhul Distant), and Hypsaucheniini (Gigantorhabdus Schmidt, Hybandoides Distant, Pyrgauchenia Breddin, Pyrgonota Stål) produced egg clutches and guarded them. Maternal care was restricted to egg-guarding in all species. Pyrgonota ?bulbosa Buckton eggs hatched successively over a couple of days and females deserted their clutches when most, but not all, 1st instar nymphs had hatched. Nymphs of egg-guarding species lived gregariously and/or by subaggregating, as did some non-guarding species. Leptobelus dama Germar, Gargara semibrunnea Funkhouser, and two Tricentrus Stål spp. were solitary as nymphs and adults.
Many gregarious insects aggregate in naturally occurring refuges on their host plants. However, when refuges are filled, they may be forced to aggregate on exposed areas of the plant. This study examines the effects of refuge saturation on group size and defence against parasitism in larvae of Ammalo helops Cramer (Lepidoptera: Arctiidae) that form day-resting groups on the trunks of weeping laurel, Ficus benjamina L., in El Salvador. Population densities, group sizes and parasitism were recorded on eight trees for each of four generations in 1995 and 1996. When population densities were low, all larvae were located in small groups in naturally occurring structural refuges (rotted out holes, spaces between crossing branches and under aerial roots) on the host plant. In contrast, when population densities were high and structural refuges were full, many larvae formed significantly larger groups (density refuges) on the open trunk. Between 20 and 24% of late-instar larvae were parasitized and this was inversely dependent on the size of within tree populations, in spite of populations being fragmented among structural refuges. Similarly, in a study carried out at a different location on young trees without structural refuges, parasitism of larvae was inversely related to group size. Although parasitism rates decreased with increasing group size, most larvae preferentially selected the small naturally occurring refuges, where groups were restricted to low densities. If this behaviour is an adaptive trait, I speculate that parasitism (or some other unmeasured mortality factor) is lower in naturally occurring refuges than in large open groups.