Effects of prey quality, prey quantity and temperature on two insect predators.

摘要

Prey quality, prey quantity and temperature are the three main factors that affect performance of insect predators. Combinations of these three factors may elicit different responses by predators than predicted by the results of each factor alone. This dissertation examines how two common ‘old-field’ predators, lacewings (Chrysoperla carnea) and lady beetles (Coleomegilla maculata), are affected by the combination of differences in prey quality, prey quantity and temperature. Most of the variation in prey quality is due to plant allelochemicals found in prey. Therefore, low quality prey included milkweed bugs fed milkweed seeds and milkweed aphids, both of which sequester and store cardenolides from milkweed. The intended high quality prey were milkweed bugs fed sunflower seeds, goldenrod aphids and fruit flies, all of which lack plant allelochemicals. Predators were given either an abundant or scarce level of prey, with levels reflecting the natural range of prey availability. The predators were raised at a thermal regime of 26:15°C, representing midsummer, or 21:10°C, representing late spring or early fall. Survival, growth and developmental rate of the predators were determined. Results indicated that prey quality was the most important factor determining the performance of these two predators. For lacewings, mortality was so high on the poor quality prey that temperature and prey quantity effects were not detected. For lady beetles, the effect of prey quality on performance was dependent on thermal regime and prey quantity. In addition, the quality of prey for each predator was not just a function of plant allelochemicals, but also determined by prey size and behavior. As a result, goldenrod aphids, which were intended to be high quality, were actually low quality prey due to their relative size and defensive behavior. Additional experiments revealed that a mixed diet can be beneficial to generalist insect predators, but only under certain conditions, depending on the quality of prey in the mix, temperature and prey quantity. Prey diet effects were often only observed at the ‘summer’ thermal regime and with abundant prey; that is, at the spring or fall thermal regime, response to diets was similar regardless of prey quantity. Overall, prey quality was the determining factor of predator performance, with temperature and prey abundance modifying that effect but not always as predicted based on the individual effects of these factors.