Polyandry, genetic diversity and fecundity of emigrating beetles: understanding new foci of infestation and selection

The frequency of polyandry and its ecological (and therefore selective) advantages remain unclear, especially in the field, where such tests are seldom conducted. We quantified the fecundity and level of polyandry in field populations of two stored grain beetles, Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.), by comparing resident with dispersing individuals at common sites. Almost all females caught in flight had mated, with paternity analysis from 110 parent–offspring arrays revealing that most females had mated with more than one male (T. castaneum: 90.6, R. dominica: 70.2%). No difference in the extent of polyandry between beetles collected from grain in storage and beetles caught in flight was detected for either species, and the degree of polyandry did not impact overall fecundity. Levels of polyandry were similar to those in recently established (first generation) laboratory cultures of both species. Our results provide the first direct evidence of polyandry in wild populations of T. castaneum and R. dominica, and that females that have mated several times have the potential to produce offspring of multiple genotypes in the field. Females thus invade resource patches (whether previously colonised or not) with considerable genetic diversity to distribute across their offspring, an advantage in a newly colonised site because it increases the chances of her offspring establishing there. This advantageous consequence of polyandry to dispersing individuals, and the fact that most of the individual beetles sampled in the field had mated multiple times, provides justification as to why polyandry should not be viewed as an evolutionary paradox