Some populations of insects and other invertebrates have more females than males. Researchers studying the female-biased sex ratio in lacewings reveal that the likely culprit is a bacterium that’s passed from mother to offspring. And it kills males only. For a maternally-transmitted pathogen, killing off male hosts and distorting the sex ratio toward females helps increase its chances of transmission. The findings were published in PLOS One earlier this month.

Unlike vertebrates, invertebrates harbor massive amounts of microbes within their cells. And these so-called cytoplasmic elements are only transmitted to offspring from their mothers. That means male hosts aren’t nearly as useful, and these selfish microbes have been known to bias the sex ratio in favor of more female invertebrate hosts. Microbe-induced male-killing has been found in four insect orders and at least one arachnid. 

A team led by Daisuke Kageyama from Japan’s National Institute of Agrobiological Sciences focused on green lacewings (Mallada desjardinsi). Their larvae are voracious eaters of aphids, and they escape from aphid-tending ants by carrying aphid carcasses on their backs. The team collected 64 adult lacewings by sweeping a net under street lamps near trees and buses on the Chiba University campus at nighttime in 2011. They reared 35 broods in the lab, and they used DNA sequencing, microscopy, and antibiotics to figure out the underlying cause of the skewed sex ratios they observed.

The researchers found a strong female bias created by a maternally inherited, male-killing bacterium that’s closely related to the plant pathogens Spiroplasma phoeniceum and Spiroplasma kunkelii. Twenty-one broods (60 percent) were infected with Spiroplasma bacteria and consisted of only females – that’s 940 individuals. Of the broods that consisted of both males and females, four of them were doubly infected with Spiroplasma and another bacterium called Rickettsia, six were infected with Rickettsia, and three broods were uninfected.

After the all-female broods were treated with antibiotics, their embryo and larva mortality was significantly reduced, and the male-to-female sex ratio was restored to one to one.