Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract

Abstract. 1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development. 2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary. 3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies. 4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups. 5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species. 6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.

Aggregation, defence and warning signals: the evolutionary relationship

“…Alternatively, or additionally, aggregation can yield important non-predatory benefits: including modification of local microclimate for thermoregulatory or hydroregulatory reasons or as a means of overcoming host plant defences (e.g. Clark &Faeth 1997 andReader &Hochuli 2003 and references therein).…”

“…This appears generally true, but is probably a little too sweeping. For example, Reader & Hochuli (2003) state that there are many instances in which early instars are aggregated and later instars are solitary.…”

Benefits of gregarious feeding by aposematic caterpillars depend on group age structure

“…Finally, groups of unrelated larvae may merge on the same host plant (Costa and Ross 2003). Group feeding can have strong fitness benefits by reducing the per capita effects of host defences (or foraging costs in general) (Denno and Benrey 1997;Reader and Hochuli 2003), predation risk (Hunter 2000;Stamp and Bowers 1988;Tullberg et al 2000) and/or microclimate (Battisti et al 2005;Casey et al 1988;Ronnas et al 2010). These benefits can facilitate host plant adaptation (Fordyce and Nice 2004) and may provide a selective pathway for the evolution of highly social life histories from simpler social behaviours (Costa and Pierce 1997).…”

Florivory as an Opportunity Benefit of Aposematism

“…Even where herbivores' colors match flowers', they are likely to be more conspicuous than herbivores feeding on interior leaves. Individual plants are therefore a heterogeneous environment with respect to both mortality risk and energy gain, and so herbivores must decide in which microhabitat to forage (Heinrich 1979;Damman 1987;Stamp and Bowers 1988;Reader and Hochuli 2003;Takeuchi et al 2009) to optimize the trade-off between growth and mortality (Werner and Gilliam 1984;Higginson et al 2012).…”