The more the merrier: Conspecific density improves performance of gregarious larvae and reduces susceptibility to a pupal parasitoid

Abstract

Aggregation can confer advantages in animal foraging, defense, and thermoregulation. There is a tight connection between the evolution of insect sociality and a highly effective immune system, presumably to inhibit rapid disease spread in a crowded environment. This connection is less evident for animals that spend only part of their life cycle in a social environment, such as noneusocial gregarious insects. Our aim was to elucidate the effects of group living by the gregarious larvae of the Glanville fritillary butterfly with respect to individual performance, immunity, and susceptibility to a parasitoid. We were also interested in the role of family relative to common postdiapause environment in shaping life‐history traits. Larvae were reared at high or low density and then exposed to the pupal parasitoid wasp Pteromalus apum, either in presence or absence of a previous immune challenge that was used to measure the encapsulation immune response. Surviving adult butterflies were further tested for immunity. The wasp offspring from successfully parasitized butterfly pupae were counted and their brood sex ratios assessed. Larvae reared at high density grew larger and faster than those at low density. Despite high mortality due to parasitism, survival was greater among individuals with high pupal immunity in both density treatments. Moreover, butterfly pupae reared at high density were able to kill a larger fraction of individuals in the parasitoid broods, although this did not increase survival of the host. Finally, a larger proportion of variation observed in most of the traits was explained by butterfly family than by common postdiapause rearing environment, except for adult survival and immunity, for which this pattern was reversed. This gregarious butterfly clearly benefits from high conspecific density in terms of developmental performance and its ability to fight a parasitoid. These positive effects may be driven by cooperative interactions during feeding.

The Effects of Population Density on the Incidence of Developmental Deformities in Chemosensory Organs of Tobacco Hornworm Larvae (Lepidoptera: Sphingidae)

“…These effects are similar to those of certain other lepidopterans, such as the speckled wood butterfly Parage aegeria ( Gibbs et al 2004 ) and the silkworm Philo samia ( Dutta et al 2013 ). Interestingly, the opposite effects were seen in larvae of the Glanville fritillary butterfly Melitaea cinxia which grew faster and larger when reared in high-density conditions ( Rosa et al 2017 ), as did larvae of the gypsy moth Lymantria dispar ( Pavlushin et al 2019 ) and the moth rice leaf roller Cnaphalocrocis medinalis ( Yang et al 2015 ). Thus, crowding effects on life history traits appear to be common phenomena, but we are unaware of any density-related occurrences of missing or deformed chemosensilla in other species.…”

Viral exposure effects on life-history, flight-related traits, and wing melanisation in the Glanville fritillary butterfly

“…The larvae used in the present experiment, were reared ad libitum on P. lanceolata in family boxes for ten days until they all started to feed and grow. The presence of conspecifics is thought to act as feeding simulant in these gregarious larvae and they generally benefit from the presence of group members ( Rosa et al, 2017 ). At this stage (i.e.…”

Environmental and genetic control of cold tolerance in the Glanville fritillary butterfly

“…We restricted our experimental treatments to the final instar and pupal stage of development, as wild Melitaea cinxia caterpillars live gregariously in all but the final instar. Rearing them individually at earlier stages would increase mortality, thereby reducing the sample size and potentially affecting the measured traits (Rosa et al, 2017).…”