Effects of Aphis fabae Scop, and of its attendant ants and insect predators on yields of field beans (Vicia faba L.)

Predators (mainly coccinellid adults and larvae and syrphid larvae), although few, were important in decreasing numbers of Aphis fabae on a small plot of field beans during the early stages of infestation in a year favourable to the aphid. At the same time, ants (Lasius niger L.), attending aphids on other plants on the same plot, effectively protected the aphids from predators for about 2 weeks, enabling the attended aphids to multiply faster than the unattended. When all aphid populations started to decline, predators became more numerous and accelerated the decline on both sets of plants. Bean plants without aphids yielded fifty-six seeds per plant; those with aphids but free from ants gave seventeen; and those with ant-attended aphids, eight seeds per plant. The damage and loss of yield was caused by the large aphid populations that developed when the pods were maturing, and not by the fewer aphids present when the plants were in flower. It appears that small, temporary infestations during flowering might increase the yield of field beans.