In holometabolous insects, the choice of a location in which to undergo pupation is a critical life cycle event, as immobile pupae are especially vulnerable to predation. In most Lepidoptera, the fully grown last-instar larva ceases feeding and empties its gut contents, whereupon, as a pre-pupa, it begins a wandering phase in which it searches out an appropriate place to pupate. Little is known about how a caterpillar navigates from its host plant to a pupation location. We have determined that Epargyreus clarus (silver-spotted skipper) pre-pupae move from their host plant down into the leaf litter, where they construct a refuge out of dried leaves and silk. Here we investigate the sensory cues used by silver-spotted skippers as they navigate towards a pupal refuge, and explore how responses to those cues differ between feeding larvae and non-feeding pre-pupae, both for cohorts and for individuals.
We used Y-tubes to examine larval and pre-pupal responses to light (light/dark), gravity (up/down), and odor (soil + host leaves/soil) cues. Late-instar and pre-pupal caterpillars were placed individually in the loading arm of a Y-tube and observed for an hour. Their behavior was recorded once they had started moving into one of the arms of the tube, or once the hour was over.
Results/Conclusions
Light: Pre-pupae overwhelmingly traveled towards the darkened arm of the Y-tube, while late-instar larvae were significantly more likely to travel towards the light arm, suggesting that pre-pupae use visual cues to navigate towards their preferred pupal substrate of fallen leaves in the litter.
Gravity: Pre-pupae were significantly more likely to travel downward in a vertically oriented Y-tube than were late-instar larvae, suggesting that pre-pupae additionally respond to gravitational visual cues to navigate towards the leaf litter.
Odor: Pre-pupae were significantly more likely to travel towards the soil than were late-instar larvae, though this response was not as strong as the other two. Pre-pupae may be traveling towards soil odor, or away from leaf odor.
Individual larvae tested as late instars and again as pre-pupae showed differing responses to the same stimulus, demonstrating ontogenetic changes in sensitivity and/or reaction to environmental cues. By better understanding the cues that pre-pupae use to locate an appropriate pupation venue, we will be better able to ensure that Lepidoptera, particularly in urban environments, will have access to the habitats they need to complete their life cycles.