Entomologists commonly consider the number of larval instars as characteristic of holometabolous insect species. However, it is not uncommon for instar number to vary considerably, presumably in response to environmental stressors. We noted that Epargyreus clarus (Hesperiidae), the silver-spotted skipper, can add an additional larval stadium between the third and fourth instar, and that both host plant quality and ambient temperature seemed to induce this shift in life history. To better understand the interaction amongst and relative importance of these environmental factors, we raised larvae on high-quality (kudzu) or low-quality (wisteria) host plants in a growth chamber under three temperatures (20, 26, and 32 degrees C) and two day-length regimes (constant or decreasing photoperiod). For each treatment, we recorded larval survival, incidence of extra instars, duration of each instar, total days to pupation, and pupal mass. In addition, to determine the generality of the relationship between host plant quality and incidence of extra instars, we reared E. clarus larvae in the laboratory under relatively constant temperature and day length conditions on six different host plants, and quantified incidence of supernumerary stadia. We also monitored larvae in the field on the same six host plants for presence/absence of extra instars.
Results/Conclusions
E. clarus larvae raised on the poor-quality host commonly included an extra instar at all three temperatures and under both photoperiodic regimes (50-100% incidence), whereas larvae reared on the high-quality host were much less likely to do so (0-39% incidence). For larvae on both host plants, and under both light treatments, the high-temperature treatment yielded the highest proportion of extra instars (100% on poor-quality host and constant daylight), with lower levels of extra instar production at the low and intermediate temperatures.
Larvae reared on all six hosts in the laboratory at least occasionally added an extra instar, but incidence seemed to track host plant quality, as measured by % foliar N. In the field, incidence of extra instars increased at the end of the season, as cold weather slowed larval growth.
Our results clearly demonstrate a synergistic interaction between host plant quality and temperature in the induction of supernumerary instars, which can be interpreted as a compensatory response to larval stress. As our climate changes, the likelihood of increased larval durations may impact the ability of caterpillars to reach an appropriate stage in time to survive harsh winter conditions.