Feeding, growth, and the thermal environment of cabbage white caterpillars, Pieris rapae L.

Laboratory studies of temperature effects on short-term feeding and growth rates were combined with field data on thermal environments to explore the consequences of temperature variation for growth of caterpillars of the cabbage white butterfly, Pieris rapae. Mean short-term (24-h) consumption and growth rates of fourth-instar P. rapae feeding on collard leaves increased continuously with increasing temperatures between 10 degrees and 35 degreesC, peaked at 35 degreesC, and declined rapidly with temperatures above 35 degreesC. Physical models can mimic temperatures of real fifth-instar caterpillars under collard leaves within 1 degrees -2 degreesC in sunny summer conditions in Seattle, Washington. Continuous recordings of operative temperatures of model caterpillars in a collard garden suggest that, at the timescale of the duration of the fifth instar (5-8 d in the field), P. rapae caterpillars frequently experience temperatures spanning a 25 degreesC range, they spend most of their time at temperatures well below those that maximize growth, and they encounter substantial variation in the frequency distribution of operative temperatures between time periods. Combining these data on growth rate as a function of temperature and the distribution of operative temperatures in the field, I illustrate how growth rates at higher temperatures can make disproportionate contributions to the overall mean growth rates even when higher temperatures are relatively infrequent. Fluctuating thermal conditions may generate variable patterns of selection on reaction norms for growth rate in the field.