Travel time affects optimal diets in depleting patches

Models of prey choice in depleting patches predict an expanding specialist strategy: Animals should start as specialists on the most profitable prey and then at some point during patch exploitation switch to a generalist foraging strategy. When patch residence time is long, the switch to a generalist diet is predicted to occur earlier than when patch residence time is short. We tested these predictions under laboratory conditions using female parasitoids (Aphidius colemani) exploiting patches of mixed instars aphid hosts (Myzus persicae, L1 and L4). The duration of patch exploitation was manipulated by changing travel time between patches. As predicted, patch residence times increase with travel time between patches. Our results provide empirical support for the expanding specialist prediction: Parasitoid females specialized initially on the more profitable hosts (L4), and as the patch depleted, they switched to a generalist diet by accepting more frequently the less profitable hosts (L1). The point at which they switched from specialist to generalist occurred later when travel times and hence patch residence times were short. By affecting the patch exploitation strategy, travel time also determines the composition of hosts left behind, the "giving up composition." The change in the relative density of remaining host types alters aphid populations' age structure.