Cops with six legs: law and order among insects

Science News, March 19, 2005 by Susan Milius

In a colony of tree wasps, workers on nursemaid duty crawl this way and that along the bottom of their nest, tending the youngsters in the comb. Most of the workers dutifully look after the queen's offspring, stopping only to spit a runny meal into the mouth of a pale, lumpy larva snug in its cell. But one of these workers is up to no good. This selfish worker stays still for a minute or two in a suspiciously crouched position. She's laying her own egg in an empty cell.

Such rogue egg laying is a crime against insect society. The wheels of justice, however, don't require a special caste of investigators and prosecutors. Punishment among insects is meted out by ordinary workers--and sometimes the queen herself--says biologist Tom Wenseleers, who has watched dozens of hours of black-and-white videos from infrared security cameras that he's trained on nests of tree wasps.

In the most dramatic episodes, the egg sneak finds herself surrounded by a posse of vigilante workers. "They're grabbing on to her; they try to sting her," says Wenseleers of the Institute for Advanced Study in Berlin.

Insect criminologists initially reported punitive action, among honeybees, in 1989, but researchers have found examples in 15 species since then, 5 of them documented last year. With the recent surge of police reports in insects, researchers see a broader range of both crimes and responses. For instance, queens do some of the rough stuff, such as killing illicitly laid eggs, even though they're her kin. What's more, queen wannabes can end up as police targets themselves.

Most biologists who have considered insect societies see them as models for studying altruism, with the workers looking out for the common good. But according to Wenseleers, the new work suggests that the more appropriate image is that of oppressed workers in a police state.

CRIMINAL INTENT Honeybees are extremely law-abiding, so when Francis L.W. Ratnieks, of the University of Sheffield in England, wanted to see whether colonies would police a crime, he had to commit the offense himself.

To obtain illicit eggs that he could sneak into his test colony, Ratnieks enlisted accomplices in a neighboring hive. He and his colleague, P. Kirk Visscher, who's now at the University of California, Riverside, divided a bee colony with a screen that trapped the queen in one zone but still let the colony odor waft throughout. In the queenless zone, workers soon began laying eggs prolifically, as they do when their queen dies. The researchers collected eggs laid by the deceived workers along with bona fide queen-laid eggs from the other part of the colony.

The team then created a major crime wave by planting this booty in the test colony. After 24 hours, Ratnieks and Visscher could find only 1 percent of the transplanted worker-laid eggs, yet 59 percent of the introduced queen-laid eggs had survived. Thus, workers weren't killing eggs just because they had the scent of another colony. The vigilantes were especially keen on eradicating other workers' eggs.

In 1989, Ratnieks and Visscher published this first report of insect policing. However, Ratnieks had predicted such a phenomenon in a mathematical model 3 years before, arguing that honeybee workers have incentives to destroy the eggs of other workers.

In honeybees, as in wasps and ants, workers don't mate but can develop working ovaries and lay eggs. Their unfertilized eggs hatch and grow, but only into males.

The females in these species, the vast sisterhood that does the work of the colony, come only from eggs laid by the queen. These eggs get fertilized with sperm that the queen stores when she's young. In honeybees, that youthful bout often involves 10 males. All workers are therefore sisters or half-sisters.

Occasionally, the honeybee queen also releases an unfertilized egg. It grows into a royal son. Ratnieks pointed out that a royal son is more closely related to the average female honeybee worker--he's her brother or half-brother--than the son of some other worker would be. Thus, for the best chance of spreading her own genes to the next generation, the worker honeybee should favor her brother and kill the other workers' sons. From her point of view, the less-related male just drains colony resources without much genetic payoff.

But what if the queen mated with one male only? That might change everything, Ratnieks predicted. Given that males come from unfertilized eggs, a worker of a singly mated queen would on average share more genes with a nephew--the son of one of her sisters--than with a brother. Workers of the saxon wasp, Dolichovespula saxonica, for example, tend not to police their coworker's eggs if the queen mates once, but tend to do so if the queen has multiple mates, Ratnieks and his colleague Kevin Foster reported in 2000.

Relatedness isn't the whole story, however. Last year, another lab's survey of policing reports from a broad range of species highlighted ones with singly mated queens that don't conform to the simple relatedness model (SN: 8/28/04,p. 142). Ratnieks argues that the survey doesn't disprove his original model; it had predicted that such factors as colony efficiency might change the incentives for policing. For example, a worker that starts laying eggs often stops working.