The value of an egg: resource reallocation in ladybirds (Coleoptera: Coccinellidae) infected with male-killing bacteria

Male-killing bacteria are thought to persist in host populations by vertical transmission and conferring direct and/or indirect fitness benefits to their hosts. Here, we test the role of indirect fitness benefits accrued from resource reallocation in species that engage in sibling egg cannibalism. We found that a single-egg meal significantly increased larval survival in 12 ladybird species, but the value of an egg (to survival) differed substantially between species. Next, we tested the impact of three male-killing bacteria on larval survival in one ladybird species, Adalia bipunctata. Spiroplasma reduced larval survival, whereas Wolbachia and Rickettsia had no effect. However, Spiroplasma-infected larvae showed the greatest response to a single-egg meal. The indirect fitness benefit obtained from a single egg is thus so large that even male-killing bacteria with direct fitness costs can persist in host populations. This study supports the hypothesis that fitness compensation via resource reallocation can explain male-killing bacteria persistence.