INSECTS AND TEMPERATURE—DIFFERENTIAL EFFECTS OF EXPERIMENTAL CONDITIONS ON GROWTH AND DEVELOPMENT

Abstract

AbstractCohorts of Pieris rapae (L.) (Lepidoptera: Pieridae) were reared at different temperatures on three substrates: plants in growth chambers, plants in field cages, and artificial medium. Pupal weight and larval development times were measured. The data were analyzed to determine the effect of temporal changes in substrate on pupal weight. When cohorts were reared on whole plants, average pupal weight declined with increasing time to pupation. For cohorts on potted plants in growth chambers, the rate of decline decreased as temperature increased from 15 to 27 °C. But, for cohorts on plants in field cages, the rate of decline increased as temperature increased from 16 to 25 °C. We speculate that the former trend resulted from plant deterioration, especially at low temperatures, and the latter from larval competition at high temperatures. When larvae were reared on potted plants to the late II or early III instar and then transferred to artificial medium, deterioration of the medium at temperatures below 20 and above 27 °C caused pupal weight to decline significantly with increasing time to pupation, whereas between 20 and 27 °C, pupal weight was constant or increased with increasing time to pupation. Development times of larvae from cohorts reared on plants in growth chambers showed no consistent differences from those of larvae from cohorts reared on artificial medium, at any temperature (overall averages: 187 and 184 day-degrees above 10 °C, on plants versus medium). Pupal weights are determined by the ratio of growth rate to development rate (Appendix). Therefore, the observed differences in pupal weight reflect differences in growth rates rather than differences in development rates. The results show that, depending on temperature, substrate deterioration may seriously affect laboratory studies of herbivore growth and that there are important interactions between temperature and host-plant growth conditions that may make extrapolation of laboratory estimates of herbivore growth to field conditions tenuous.