DEADLINES AND DELAYS AS FACTORS IN APHID SEX ALLOCATION

In this paper we use the concept of the evolutionary individual as the basis for an attempt to characterise sex-allocation patterns in aphid s. We then examine the various selection pressures involved in the evo lution of aphids' sex ratios, and propose a novel explanation for bias ed sex allocation in host-alternating aphidines, in which inbreeding i s impossible. Their production of gynoparae (females that migrate from secondary to primary hosts to produce the sexual females) before male s is the clonal equivalent of sex reversal - sequential hermaphroditis m. Selection on the timing of the reversal, and thus the overall sex-a llocation ratio, should depend largely on the rate of decline of the p opulations on the secondary hosts in autumn and on how long it takes o viparae (sexual females) to reach maturity. The longer the nymphal per iod of the oviparae, the sooner investment in gynoparae becomes futile , since both gynoparae and oviparae must mature and oviparae must ovip osit before leaf-fall. It is the combination of a deadline - leaf-fall - and a delay - the two generations that must be completed between al location to gynoparae and oviposition - that determines the selection on the allocation ratios of host-alternating aphidines. Data on Rhopal osiphum padi in Scotland and Sweden show a strong female bias, as the model predicts.