One way to control pests without pesticides (which can be dangerous for humans and more desirable bugs) is to introduce sterile males – insects who have had their reproductive systems damaged with radiation – into the region. The sterile males compete with wild insects to mate with the females, but their couplings don’t produce offspring. It's been used to eradicate pesky species before, but the more insects you need to sterilize, the more expensive that method becomes.
So Czech scientists modeled what would happen if they introduced strains of bacteria known to target male insects into the population first – to reduce the number of males the sterilized bugs would have to compete with.
They showed that some of these microbial killers could eliminate the entire group of insects on their own. Each species of the bacteria lives alongside a matching insect species, so one day scientists could supplement other pest-control methods with male-killing bacteria to kill off pests like disease-carrying mosquitoes.
“To reduce the cost you can reduce the natural population of males with the male-killing bacteria first,” said Ludek Berec, researcher at the Institute of Entomology at the Academy of Sciences of the Czech Republic and chief author of the study, recently accepted to the Journal of Applied Ecology. This is the first study that’s modeled how quickly the bacteria could eliminate pests, said Berec, and it shows that in some circumstances, the bacteria are such efficient assassins that sterilization is unnecessary.
Scientists don't have to tinker with bacteria in the lab to induce this manslaughter. There are plenty of bacteria that have evolved to target male insects of different species – and kill them in different ways. In ladybugs, for example, a mother infected with Wolbachia will lay eggs filled with the stuff. Once the eggs are fertilized, any male embryos are swiftly killed by the bacterial infection. This leaves the females with less competition for their first meal – and a lot of dead brothers to snack on – giving them a better chance of survival, and giving the bacteria in their eggs a better chance of being passed on to another round of offspring.
Eventually, if there were no more males left to fertilize those infected eggs, the whole population could die off.
Scientists still aren't sure how the bacteria can tell the males from the females, said Francis Jiggins, who studies infectious diseases and “feminist bacteria” at the University of Cambridge in the United Kingdom. “There’s lots of different bacteria that cause male killing, so there's probably lots of different ways they can do it.”
But however they manage it, these bacteria are often very good at what they do. Jiggins, who wasn't involved in the new study, has seen populations of butterflies in Africa that were 80 percent to 90 percent female thanks to the microscopic misandrists.
And a world with few men isn’t usually a paradise for female insects. In uninfected butterfly species, for example, the males will typically wait in large groups at the tops of hills for the females to come along and take their pick. But in species infected with male-targeting bacteria, the behavior switches and the females start waiting around for the few remaining males.
“The males themselves can become quite exhausted from the excess of females with which to mate,” Jiggins said. “But that only happens in the few cases where there's an extreme prevalence of the bacteria.”
Jiggins doubts that male-killing bacteria alone could eradicate a pest population; evolution catches up and if males emerge who are immune to the bacteria, that trait would quickly spread throughout the species. But he agreed that male-killing bacteria could be effective when combined with sterilization. Plus, specific bacterium target specific species, so to kill off only the mosquitoes that carry the Zika virus, for example, one could potentially introduce the matching boy-crazy bacterium.
Berec cautioned that the model is still in its infancy, and in a perfect world it would still take five to ten years of theoretical and experimental research for male-killing bacteria to end up on the market for pest control. And many in the field still embrace pesticides as the best method to get rid of dangerous bugs.
“Ecologists are not that open to new techniques,” Berec said. “But I hope people will see this new method and want to collaborate.”