Big maggots dig deeper: size-dependent larval dispersal in flies.

The ability of individual animals to select habitats optimal for development and survival can be constrained by the costs of moving through the environment. Animals that seek overwintering sites underground, for example, may be constrained by the energy required to burrow into the soil. We conducted field and laboratory studies to determine the relationship between individual size and overwintering site selection in the tephritid flies, Rhagoletis juglandis and Rhagoletis suavis. We also explored the effect of site selection on pupal mortality, parasitism, and the ability to emerge from overwintering sites after eclosion. In both species, and in both lab and field tests, larger pupae were found at deeper soil depths. In addition, marginally non-significant trends indicated pupae in deeper sites were 48% more likely to survive the overwintering period. Finally, larger individuals were more likely to eclose and emerge from the soil at a given depth, but flies in deep overwintering sites were less likely to emerge from those sites than flies in shallow sites. Our data indicate that overwintering site selection represents a trade-off between avoiding predators and parasites that occur at shallow sites, and the energetic and mortality costs of burrowing to, overwintering in, and emerging from, deeper sites. The size-dependent overwintering site selection demonstrated here has implications for population dynamics and pest control strategies. Some fly control measures, such as the introduction of parasites or predators, will be mitigated when the deepest and least accessible overwintering pupae represent a disproportionately large amount of the population's reproductive capacity.