The parasitic wasp Dinocampus coccinellae prepares to inject a spotted ladybug with a single egg in a file picture. The ladybug has been paralyzed by the wasp's venom.
In time the egg will hatch into a larva that will develop for a few days and then chew a small hole through the abdomen of the ladybug. The larva will then spin a cocoon between the legs of the ladybug, whose body will rest on top of the cocoon as the larva undergoes metamorphosis. (See insect-egg pictures.)
In a recent study in the journal Biology Letters, scientists note that sometimes the ladybugs survive the larva's emergence, and in those cases, the D. coccinellae larva then "brainwashes" the bug into defending the vulnerable cocoon from predators, said study co-author Jacques Brodeur, a biologist at the University of Montreal.
"The parasite is taking control of the behavior of its host—that's why we call it bodyguard manipulation," said Brodeur, who worked with Ph.D student Fanny Maure.
A wormlike wasp larva emerges from its ladybug host in a recent picture.
Although most parasites eventually kill their hosts, the wasp-infected ladybugs have a more "atypical fate," according to the study—some ladybugs survive their "horrible" ordeal. For example, Brodeur's team observed in field experiments that 30 to 40 percent of the infected ladybugs lived after the young wasp hatched, including some individuals that later laid their own eggs.
The host bug can survive because the wasp larva feeds only on tissues that are not crucial for the ladybug's survival, such as fat, the scientists say.
Once a still living ladybug takes up its post on top of the wasp cocoon—as seen in a recent picture—the insect will act aggressively toward intruders, for example, by flailing its legs. The scientists suspect the twitching behavior comes from venom left in the ladybug's body after the larva emerges and builds its cocoon.
In the lab, Brodeur and his team placed predatory lacewings into petri dishes that contained either cocoons covered by live ladybugs, other cocoons covered by dead ladybugs, or cocoons that lacked ladybug bodyguards.
The results showed that the lacewings were less successful in attacking cocoons being protected by the "zombified" ladybugs.
The length of time that a wasp larva manipulated a ladybug into protecting its cocoon also varied from insect to insect. In some cases the ladybug stayed vigilant until the larva emerged from its cocoon as a young wasp. In other cases the ladybug was under the wasp's sway for just a few days, Brodeur noted.
The team found that wasp larvae that invested more time and energy into controlling their ladybug bodyguards laid fewer of their own eggs as adults than the larvae that did not.
It's the first time that scientists have shown a trade-off between host manipulation and fertility, he said.
Photograph courtesy Mathieu Bélanger Morin
A D. coccinellae wasp cocoon is seen in a closeup picture. The species is widespread throughout the world, including in Asia and Europe, Brodeur said: "It's quite cosmopolitan."
But the parasites are very haughty when choosing their bodyguards: They'll infect only ladybugs.