Print PageSpatially distributed predator-prey systems can display a range of self-organizing behavior that originates, in part, from the spatial foraging movements of the predators. Here we present an analysis of movements of two coccinellid predators foraging among multiple patches of Aphis helianthi located on isolated racemes of Yucca glauca. Our previous theoretical work indicated that foraging movements of the coccinellids are likely key to the observed rapid self-organization of this spatially distributed predator-prey system.We performed fifty-nine 20-minute observations at individual Yucca glauca racemes and recorded the rate of arrival and departure of Coccinella septempunctata and Hippodamia convergens as a function of conditions observed on the focal raceme and its nearest neighbor. The conditions included aphid population size, number of ants, and number of resident coccinellids of each species. In the second study we traced 245 flights of H. convergens and compared the conditions on the racemes the coccinellids decided to land on with conditions on all available plants within the field site.
Results/Conclusions
Our results indicate that the arrival rates of H. convergens showed a strong self-aggregative response: arrival rate was significantly higher to racemes that already hosted large numbers of H. convergens (R2=0.378, n=59, p<0.0005). This self-aggregation was confirmed in our second study that traced coccinellid flights. H. convergens preferentially landed on plants that had more H. convergens already present, however the attractiveness of conspecifics tended to saturate at high resident conspecific number (logarithmic function R2=0.683, n=10, p=0.011). Per-capita departure rates decreased with increasing numbers of resident aphids (R2= 0.600 n=59, p<0.0005). The conditions on the nearest neighbor significantly influenced foraging movements of H. convergens. H. convergens arrival rates were significantly higher when the neighboring raceme had large numbers of H. convergens (R2=0.552, n=59, p<0.0005), indicating significant local diffusion of coccinellids. Large numbers of H. convergens on neighboring raceme also lead to higher per-capita departures (R2=0.576, n=59, p<0.0005), indicating that the attractiveness of coccinellid populations can be perceived from a distance. In contrast, the movements of C. septempunctata were relatively simple: C. septempunctata appeared to arrive at racemes randomly, but the per-capita departure rate decreased with increasing aphid abundance on the raceme. We did not detect any influence of the conditions of the nearest neighbor on the arrival and departure rates of C. septempunctata.
