The Failure of a Parasitoid to Persist with a Superabundant Host: The Importance of the Numerical Response

Abstract

Experiments on the parasitoid Aphidius ervi and its host, the pea aphid Acyrthosiphon pisum, produced what appeared to be a paradox. In a laboratory cage, we monitored the population densities of parasitoids and pea aphids for roughly seven parasitoid generations, over which time the parasitoid eventually went extinct. This occurred despite the parasitoid's high potential fecundity and the superabundance of its host. To investigate this apparent paradox, we performed further experiments to quantify the parasitoid's numerical response - the number of adult parasitoids produced per host killed. A. ervi develops as an egg and larva within its living host for approximately eight d, and we estimated survival of parasitoids over this period between 2.8% and 6.4%. The estimated parasitoid survival depends on the density-dependent survival of the pea aphids, with low survival of aphids and consequently larval parasitoids occurring at high aphid densities. Therefore, the population growth rate of A. ervi will be the lowest when aphid densities are high, and this will limit the ability of A. ervi to control aphid outbreaks.