From BBC News:
Bacteria that kill off male butterflies can actually lead to increased promiscuity in female butterflies, scientists have found. The Current Biology study looked at the Hypolimnas bolina species, common to the Pacific and SE Asia. The team discovered as the bacteria caused male populations to fall, females mated more frequently to boost their chances of becoming impregnated.
The study has revealed the bacteria’s powerful effect on mating systems. The Wolbachia bacteria are passed from mother to son in some species of tropical butterfly, and kill the embryo before it hatches. The bacteria are so effective, some islands can be left with one male to every 100 females. Theoretically, an excess of females should lead to an increase of mating opportunities for males and a decrease in the average number of matings per female, as males become increasingly rare.