Why Do Some Insects Kill Their Queens?

Yellow jackets revolt!

By continuously video recording yellow jacket wasp colonies, Kevin J. Loope, a postdoctoral researcher at the University of California, Riverside, has discovered some of the motivations that drive some social insect colonies to revolt and kill their mother.

Most people think of social insects as workers toiling mindlessly for the good of the queen or the colony, but it appears that workers are more calculating, and help or harm the queen depending on the circumstances they find themselves in," Loope said in an announcement from University of California, Riverside.

To explore the reasoning behind this behavior, Loope set up observation colonies of yellow jacket wasps in the lab, filmed them continuously using video cameras and then noted when matricide happened. He also collected wild colonies to increase the sample size, inferring matricide from mature, queenless colonies.

Kevin Loope, a postdoctoral researcher in the UC Riverside Department of Entomology, is seen here collecting a young yellow jacket colony. Photo credit: Barrett Klein

"The main advantage is to allow your sister workers to lay male eggs, rather than the queen, who typically stops worker reproduction by egg eating, attacking reproducing workers, and by laying many of her own eggs. By eliminating the queen, a matricidal worker allows other workers and herself to lay male eggs."

According to the study, workers do all the tasks of raising the brood. They forage for food, feed the offspring and the queen, build the nest and defend it. Only occasionally do they attempt to reproduce. Males are produced at the end of the year and mate with the new queens. Then the males die. Workers never mate but can still lay male eggs due to a quirk in their genetic system. This allows them to compete with the queen for the production of males.

Interestingly, killing the queen does not always benefit the workers. The behavioral observations and genetic analyses of wasp colonies, showed Loope that worker wasps kill queens only when they are in colonies with lots of full siblings, but not in colonies with a mix of full and half siblings.

"Workers are assessing the situation in their colony and deciding to revolt against the queen only when the genetic makeup of the colony makes it favorable to do so," Loope said.

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In a colony where the queen has mated with one or a few males, most of the workers are full-siblings. By killing the queen, a worker allows herself and her full-sisters to mother the next generation of males. Conversely, in a colony where the queen has mated with many males, the workers are mostly half-siblings. In this situation, a worker that kills her mother would provide the opportunity for less-closely-related wasps to pass on their genes too.

According to the university, the study is one of a few that suggest that workers can detect the relative proportions of full and half-siblings of their colony and respond adaptively when conflicts of interest arise over what the colony should do, for example, rear the sons of workers or the sons of the queen.

"Hence the matricide," Loope said. "Workers are not mindless automatons working for the queen no matter what. They only altruistically give up reproduction when the context is right, but revolt when it benefits them to do so."

"Kin selection theory, an extension of evolutionary theory, predicts behavior to evolve that favors close over more distant relatives; half-siblings are more distant than full siblings, so matricide is less beneficial in colonies with many half-siblings," Loope added. "Thus, this study provides rare support of kin selection theory with variation in behavior among colonies. Workers are responding to their colony makeup as predicted by kin selection theory."

The results of the study appear online in Current Biology.

Loope is now working to understand how yellow jacket wasps' interaction with other species may shape transitions in social organization.

Based on materials provided by the University of California, Riverside.