Coccinellids and aphids: A quantitative study of the impact of adult ladybirds (Coleoptera: Coccinellidae) preying on field populations of pea aphids (Homoptera: Aphididae)

Abstract

This paper examines the quantitative effect of predation by a ladybird beetle, <i>Coccinella trifasciata</i>, on field populations of pea aphid, <i>Acyrthosiphon pisum</i>. Field studies showed that no mathematical function, involving only the current densities of predator and prey, can predict the true predation rate. We studied the components of the predation process in detail , first in the laboratory, and then in the field. We derived a new, empirical (not theoretical) formula for predation rate, which includes predator and prey densities, predator voracity, prey age-distribution, and temperature. Temperature has a single effect on the rate of aphid development, but a double effect on the predation rate, so that coccinellids are much more effective predators at high temperatures, than at low. Field cage experiments, with known numbers of beetles, revealed that all current methods of counting adult coccinellids in the field greatly underestimate their true numbers. When this fault is rectified, the new formula correctly predicts the predation rate. The study shows that it is possible to investigate a predator∑prey relationship, in the field, in considerable detail, in order to predict the predation rate over a wide range of circumstances. The study reveals several sharp, qualitative, differences between the predation relationship observed in the laboratory, and the same relationship observed in the field. All laboratory studies must therefore be suspect, until verified in the field. In particular, arthropod predation studies must allow for effects of temperature on both predation rate and prey population dynamics. The coccinellid∑aphid relationship permits no equilibrium, or steady state, so that conventional definitions of stability do not apply. The coccinellid's functional response is inherently unstable: the relationship is stabilized solely by a numerical response. Implications for biological control are discussed.