Egg spreading in the ant-parasitic butterfly, Maculinea alcon: from individual behaviour to egg distribution pattern

Oviposition site selection can be viewed as an act of parental care; it is a life history trait with significant consequences for offspring development and success. Herbivorous insects may adopt different strategies for spreading their eggs (e.g. single oviposition events versus egg clustering). Host plant use results from the interplay of available cues and the responsiveness of the ovipositing female. We studied how egg-laying behaviour of individual females resulted in the egg distribution patterns observed at the population level in the ant-parasitic butterfly Maculinea alcon. Eggs are laid on marsh gentians, Gentiana pneumonanthe, but survival also depends on the presence of Myrmica host ants as young caterpillars leave the host plant and survive only after being adopted by ants. Several studies have focused on egg distribution patterns, but less so on the behavioural mechanisms behind the oviposition patterns. Our observational data show evidence for some level of egg clustering, as predicted by the obligate myrmecophilous lifestyle. There was no evidence of egg-laying avoidance when gentians already carried several eggs. Phenological time windows for oviposition on individual flower buds and gentians were much wider than reported before. Detailed tracking of egg-laying females in the field showed high rates of oviposition and suggest relatively high average levels of fecundity compared to fully herbivorous lycaenid butterflies. Results are discussed in relation to adaptive oviposition strategies of this highly specialist ant parasite of conservation concern.