Sex ratio in ladybirds (Coccinellidae)

The sex ratio is an important fitness character of any sexually reproducing organism. In a random mating, diploid population, Fisher (1930) argued that the sex ratio will evolve towards unity providing that the two sexes require the same investment of energy. If one sex requires more effort on the part of the parents to rear to maturity, then the sex ratio should be biased towards the cheaper sex. However, in some cases the sex ratio is clearly biased even though the two sexes are equally ‘expensive’. During earlier studies on the 7-spot ladybird, Coccinella septempunctata (Holloway et al . , 1991) and the 2-spot ladybird, Adalia bipunctata (de Jong et al . , 1991) we noticed a biased sex ratio. In both studies the ladybirds were collected from the field. Ladybirds overwinter as adults (Hodek, 1973) and, whilst it is possible that local mate competition (Hamilton, 1967) may influence sex ratios, it could also be that these biases occur as a result of differential winter mortality. This seems particularly feasible considering Brakefield’s (1985) finding that lighter ladybirds have a lower chance of surviving the winter months than heavier individuals. Male ladybirds are indeed lighter than females. Another possibility has been proposed by Hurst et al. (1992) who found that cytoplasmically inherited agents, possibly bacteria, are responsible for male biased sex ratios in populations of 2-spot ladybirds derived from England. Hurst et al. found that the factor was passed on through females and resulted in up to 100% of the male eggs in some broods failing to hatch. A large experiment was carried out in the laboratory to study the quantitative genetics of life history and the defence system in the 2-spot ladybird (Holloway et al., 1993). However, since the ,design of the experiment required families to be reared it also yielded information about the sex ratio and perhaps also clues as to which