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

Male-killing bacteria (MKs) 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 MKs 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 MKs with direct fitness costs can persist in host populations. This study supports the hypothesis that fitness compensation via resource reallocation can explain MK persistence.