The small, hooked epidermal hairs on french beans (Phaseolus vulgaris) may have a profoundly detrimental effect on colonies of Aphis craccivora. The hairs are present on the petioles, stems and undersurfaces of the leaves but are absent on the uppersurfaces of the leaves. They are most dense and therefore most injurious to the Aphids on the growing shoots. The Aphids' legs become impaled on the hooks and the results of the subsequent bleeding, starvation and exhaustion are decreased longevity and reproductive rate, high larval mortality, increased time of larval development and decreased size; this last factor is associated with a reduced fecundity. On some varieties of beans the hairs are less dense than on others and on these varieties the action of the hairs in inhibiting predators may be more important than their effect on the Aphids.
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