Increasing voltinism in a temperate moth species: A result of global change?
Climate change can have significant effects on the life history strategies of animals. For insects, it has been suggested that global warming may allow populations to increase their voltinism or the number of generations that they have per year. While the development rates of insects do rely heavily on temperature, the decision to diapause for the winter or to pupate and continue with another generation is thought to depend greatly on photoperiod. Moths in the family Limacodidae that are found in temperate regions are typically univoltine. Recently, however, we have found an increasing incidence of facultative bivoltinism for the species Euclea delphinii (Limacodidae) around Washington DC; this shift in voltinism may be associated with global warming. To test which cues E. delphinii larvae use to determine whether they pupate for the winter or emerge for a second generation, we designed a full-factorial experiment in which we tested for the effects of photoperiod, temperature and host plant quality.
We found that photoperiod had the greatest effect on voltinism, but also that larvae reared on higher-quality early-season host plants were more likely to attempt a second generation than larvae reared on late-season host plants, consistent with our previous empirical findings. Temperature and host plant quality also had significant indirect effects on voltinism. Larvae reared on higher-quality host plant species or under warmer temperatures had faster development rates. Development rate is important for voltinism because it determines the time period during which the larvae enter the dormant phase and thus whether they experience a long or short photoperiod during this crucial “decision-making” phase. Thus, while photoperiod was the ultimate cue, temperature and host plant quality also played important indirect roles in determining whether larvae entered diapause for the winter or emerged as adults in the same summer to initiate a second generation.