Optimal diet theory often fails to predict a forager's diet choice when prey are mobile. Because they escape or defend themselves, mobile prey are likely to increase the forager's handling time, thereby decreasing its fitness gain rate. Many animals have been shown to select their prey so as to maximize either their fitness gain or their fitness gain rate. However, no study has yet compared directly these two measures of profitability by generating testable predictions about the choice of the forager. Under laboratory conditions, we compared these two measures of profitability, using the aphid parasitoid Aphidius colemani and its host, Myzus persicae. Fitness gain was calculated for parasitoids developing in each host instar by measuring life-history traits such as developmental time, sex ratio and fecundity. Fitness gain rate was estimated by dividing fitness gain by handling time, the time required to subdue the host. Fourth instar aphids provided the best fitness gain to parasitoids, whereas second instar aphids were the most profitable in terms of fitness gain rate. Host choice tests showed that A. colemani females preferred second instar hosts, suggesting that their decision maximizes fitness gain rate over fitness gain. Our results indicate that fitness gain rate is a reliable predictor of animal's choice for foragers exploiting resources that impose additional time cost due to their mobility.

It is well known that increasing the ambient temperature increases the metabolic rate and consequently, the foraging rate of most insects. However, temperature experienced during the immature stages of insects affects their adult size (an inverse relationship). Because body size is generally correlated to foraging success, we hypothesized that temperature indirectly influences the foraging efficiency of adult insects through developmental effects. We first investigated the role of parasitoid: host body size ratio on the handling time of Aphidius colemani (Viereck) (Hymenoptera: Braconidae), then tested the prediction that increasing temperature during immature development increases the handling time of adults. As expected, parasitoids took longer to handle large aphids than small aphids. However, large parasitoids did not have shorter handling times than small parasitoids except when attacking large (adult) aphids. Developmental temperature had the predicted effect on parasitoids: Individuals reared at 25°C were smaller than those insects reared at 15°C. Parasitoids reared at 15°C had similar short handling times for both first instar and adult aphids, whereas parasitoids reared at 25°C took longer to handle adult aphids than first instar aphids. The size-mediated effect of temperature through development on parasitoid efficiency was opposite to the more familiar direct effect of temperature through metabolic rate. We conclude that the net effect of temperature on foraging insects will depend on its relative influence on immature and adult stages.

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