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Male-killers are maternally inherited bacteria that kill their male host early in the development resulting in female-biased sex ratios. Their transmission is strictly vertical, however not every egg laid by an infected mother carries the bacteria. Given their imperfect transmission, the absence of horizontal transfer and potential costs associated with the infection, additional mechanisms are necessary to explain male-killer persistence in natural populations.

Different species of ladybugs are known to host male-killing bacteria. Their larvae are voracious predators that eat mostly aphids but are also known to exhibit sibling cannibalism, a behavior that is affected by aphid density. Since infected male eggs do not develop into larvae, females from the same egg clutch may benefit from the consumption of these undeveloped eggs. This resource reallocation from brother to sister in infected clutches may be one of the mechanisms explaining male-killer persistence.

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• Elnagdy, S., Majerus, M., & Lawson Handley, L.-J. (2011). The value of an egg: resource reallocation in ladybirds (Coleoptera: Coccinellidae) infected with male-killing bacteria. Journal of Evolutionary Biology 24, 2164–2172. link

• Agarwala, B.K., & Dixon, A.F.G. (1992). Laboratory study of cannibalism and interspecific predation in ladybirds. Ecological Entomology 17, 303–309. link alternative link