Multistage Functional Responses in a Ladybeetle-Aphid System: Scaling up from the Laboratory to the Field

Abstract

Experiments and models are described that quantify the functional responses of the larval and adult stages of the sevenspot ladybeetle, Coccinella septempunctata L. toward mixed stage populations of the cotton aphid, Aphis gossypii Glover, in cotton. In the laboratory, functional responses were measured for five beetle stages and three size groups of prey at five temperatures (15 to 35°C). The 75 resulting functional responses were each characterized by a search rate (cm2/predator/d) and a handling rate (prey/predator/d). Both search and handling rates increased with predator stage. Search rates increased, while handling rates decreased, with prey size. Search rate increased linearly with temperature, while handling rate showed an optimum response to temperature. Field cage studies were conducted to verify whether the search rates determined in the laboratory were valid under field conditions. Search rates of predators derived from observations in field cages yielded parameter estimates that were similar to those found in the laboratory. A comprehensive model, allowing for effects of temperature, stage distribution of the aphid population, and plant leaf area on predation rate, was then constructed to calculate predation by larvae and adults of C. septempunctata on multi stage populations of prey. This model gave good correspondence to the cage observations if two-sided leaf area was input into the model as search substrate for the predator. The model appears suitable for calculating predation rates of C. septempunctata on A. gossypii under field conditions. A sensitivity analysis of the functional response model shows the crucial effect of crop leaf area on predation.