Some Experiments On Sex Ratio and Sex Regulation in the Pteromalid Lariophagus Distinguendus

Abstract

The pteromalid Lariophagus distinguendus is able to exploit successfully a wide range of developmental instars of its host (the granary weevil, Sitophilus granarius). On young hosts (L II) only males reached maturity; an increasing proportion of females was found with increasing age of the hosts. At the same time the total yield (numbers of weevils and wasps) of bottles with parasitised young hosts was considerably below that of controls; total yield with older hosts equalled that of controls. It could be proved that the differences in sex ratio as found did not result from a differential mortality of the sexes, but were the outcome of a genuine sex regulation mechanism, operated by the female parent. Rate of oviposition, host density, and degree of superparasitism did not affect the experimental results. There was a considerable variability in size of the adult wasps which could be related to age (size) of the host on which they had developed. Size of the adult female wasp was found to be related to longevity and fecundity. Small females are very probably unable to exploit the different host stages to the same effect as large wasps do. Likewise, the developmental period of the parasite was related to the age of the host on which it had fed; very small adults emerged later than expected, presumably because escaping from the grain took far more time than in normal adults. Some properties of a sex regulation mechanism were discussed. It seems to be improbable that an entirely fixed fertilisation threshold is involved. More likely there is at least an additional mechanism to guarantee the production of a minimum number of males under optimal host conditions. Some arguments were given in support of a fertilisation threshold that, instead of being fixed, may vary according to some quantitative characteristics of hosts dealt with previously.