Assassin bugs cover themselves in sticky plant resin to trap prey

A bug improves its hunting success by slathering itself in the sticky resin of a grass, in a rare example of tool use by insects.

Australian assassin bugs, from the genus Gorareduvius, are often seen resting on the blades of spinifex grass. This grass, a characteristic feature of dry regions of Australia, produces sticky resin that was quite popular with the first human inhabitants of Australia for toolmaking.

Biologists thought several species of assassin bugs might be using the spinifex resin for capturing prey, but this had never been tested in experiments, says Fernando Soley at Macquarie University in Sydney.

He and his colleague Marie Herberstein, also at Macquarie University, collected 26 assassin bugs in the Kimberley region of Western Australia and brought them to their laboratory inside a tent set up in the field.

They noticed that males, females and immature nymphs scraped the resin off the leaves of spinifex and meticulously applied it over the body, especially their forelegs.

Each bug was placed in a glass jar and offered two prey, a housefly and an ant, one at a time. Then the researchers removed the resin from the bug’s body with makeup remover pads and repeated the experiment.

The bugs were 26 per cent more successful at capturing prey when equipped with resin than without it. Without resin, the flies were 64 per cent more likely to escape.

Although the resin didn’t guarantee success, it appeared to slow down the prey just enough for the assassin bugs to grasp and stab it.

Soley and Herberstein say this is a definitive example of tool use by insects, which is quite rare. This behaviour appears to be hardwired into the bugs, as even freshly hatched and isolated nymphs were found smearing the resin over themselves.

“I find the use of the term ‘tools’ appropriate in this context,” says Christiane Weirauch at the University of California, Riverside. “It is the same as insects camouflaging themselves with pieces of debris or ant corpses.”

Although tool use is often thought of as a sign of high intelligence, this isn’t always the case, says Weirauch. “I’d argue that tool use could be genetically hardwired as well as have some element of learning. We are looking at a gradient, with some animals such as assassin bugs being closer to the genetically hardwired and others, such as primates and octopuses, incorporating more learning into their tool use.”