Intra-specific mechanisms in relation to the natural regulation of numbers of Aphis fabae Scop

SUMMARY Parthenogenetic virginoparous apterae of Aphis fabae Scop. on field beans (Vicia faba) reproduced faster initially in populations of eight colonizing apterae than in those with 2?4 or 16?32 per plant. The aphids were at first mutually benefited but were quickly affected by competition as numbers rose above the critical density represented by about eight apterae and their first progeny. This is because the aphids remained densely aggregated and seemingly created a local shortage despite abundant food and space elsewhere on the plant. Such self-induced competition provides the basis for self-regulation of numbers of A. fabae in relation to (1) food and space provided by the growing plant and (2) mortality from natural enemies and from other causes including insecticides. As competition increased, the multiplication of A. fabae populations slowed, newly formed adult apterae emigrated and increasing numbers of alatae were formed. The mean weights of apterae decreased from about 1·8 mg. to 0·3 mg. and of alatae from 0·9 to 0·2 mg. Such decrease probably favours production of many adults that might otherwise fail to mature. Experiments in a glasshouse and in field cages indicated the success with which an A. fabae population adapts to and exploits a growing plant. Field bean plants sown in mid-March and infested as in the field produced an average of 15,000?17,000 A. fabae emigrants per plant of which 78?84% were adults (mostly alatae). This is equivalent to about 1600 million alate emigrants from 1 acre (0·4 hectare) of an infested field bean crop.