THE FAILURE OF A PARASITOID TO PERSIST WITH A SUPERABUNDANT HOST - THE IMPORTANCE OF THE NUMERICAL RESPONSE

Experiments on the parasitoid Aphidius ervi and its host, the pea aphi d Acyrthosiphon pisum, produced what appeared to be a paradox. In a la boratory cage, we monitored the population densities of parasitoids an d pea aphids for roughly seven parasitoid generations, over which time the parasitoid eventually went extinct. This occurred despite the par asitoid's high potential fecundity and the superabundance of its host. To investigate this apparent paradox, we performed further experiment s to quantify the parasitoid's numerical response - the number of adul t parasitoids produced per host killed. A. ervi develops as an egg and larva within its living host for approximately eight d, and we estima ted survival of parasitoids over this period between 2.8% and 6.4%. Th e estimated parasitoid survival depends on the density-dependent survi val of the pea aphids, with low survival of aphids and consequently la rval parasitoids occurring at high aphid densities. Therefore, the pop ulation 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.