Laboratory studies on aggregation, size and fecundity in the black bean aphid, Aphis fabae Scop.

Adults of Aphis fabae Scop. can weigh from as little as 200 μg to as much as 1 800 μg (Way & Banks, 1967), a remarkable range in size for a single species. An extensive range in size also occurs among the adults in other species of aphid (Murdie, 1969 a , b ; Dixon, 1970). Small individuals result when aphids develop either under crowded conditions or on a mature host-plant; large aphids result when the nymphs are reared in isolation or on young or senescent host-plants (Way & Banks, 1967; Murdie, 1969 a , b ; Dixon, 1970). A small adult produces fewer nymphs and also smaller nymphs than average (Murdie, 1969 b ; Dixon, 1970). When aphids are overcrowded or feed on host-plants of poor quality, many aphids reach maturity that might otherwise fail to mature because of the ability of the species to produce viable adults of a great variety of size, including very small adults (Way & Banks, 1967).