Sex ratio, local mate competition and mating behaviour in the aphid Pemphigus spyrothecae

The primary sex ratio and the selective factors associated with it were investigated in the aphid Pemphigus spyrothecae. The sexuparae gave birth to an average of 1.99 +/- 0.01 (1-2) males and 5.06 +/- 0.09 (2-6) females [+/- SE (range), n = 147]. The average weight of individual males was 16.2 +/- 0.86 (13-18) micrograms (n = 5) and of females 40.7 +/- 0.71 (38-46) micrograms (n = 15). The overall investment ratio by the mother sexuparae was therefore 1:6.4, equivalent to a proportionate investment in sons of 0.135 (interquartile range: 0.117-0.165). There was a highly significant correlation between the size of the sexupara and the number of her daughters: the number of sons was independent of sexupara size. Field observations and laboratory experiments showed that the sexuparae aggregate on poplar bark before giving birth to the sexuals: they appear to behave in such a way that there is always at least one other sexupara in the crevice with them. The average number of sexuparae forming a "foundress" group, within which offspring could potentially mate with each other, was 2.48 +/- 0.19 (1-9) (n = 80). The life-time activity patterns of the sexuals (which moult four times but do not feed) was described. Both sexes are active shortly after birth; they are then immobile for a long period but become very active again after emerging as adults (after about 42 h in males, 51 h in females). Each male can mate with up to 14 females. The females are receptive to a second male for only a relatively short period (ca. 15-20 min) after their first mating. There is considerable competition between mates during mating (just as intense between clonal sibs as non-sibs) and evidence for post-copulatory mate-guarding by the males. There is no evidence that females prefer to mate with non-sibs rather than sibs. The observed sex ratios and the mating behaviour of the aphids are discussed in relation to models of local mate competition assuming variable female fecundity, as developed by Frank (1987 a,b) and Yamaguchi (1985) (the constant male hypothesis) and by Stubblefield and Seger (1990).