Assassin Bug *Pahabengkakia piliceps* Uses Plant Resin to Trap Stingless Bees

An insect that gathers and alters plant resin to trap prey adds to mounting evidence that the ingenuity of invertebrate animals may have been underestimated.

Assassin bug resin use to catch stingless bees

The assassin bug (Pahabengkakia piliceps), which occurs throughout Thailand and China, coats its legs in plant resin for a purpose beyond simple stickiness. The scent released by the sap draws prey into the ideal position for an ambush, substantially raising the bug’s hunting success.

"We empirically demonstrate how an invertebrate predator adapts to the colony defense of social insects via tool use behavior," writes a team of researchers, led by entomologists Zhaoyang Chen and Li Tian of China Agricultural University, in a new paper.

"Our findings offer a new model for studying the adaptive functions and underlying mechanisms of tool use behaviors in animals."

In recent years, researchers have increasingly found that non-human animals may use tools more often than previously recognised – and that this ability is not confined to vertebrates.

Bees and ants are among the insects that use tools to simplify their lives. Meanwhile, a 2023 study of Australian assassin bugs showed that these resourceful invertebrates apply resin to make their bodies sticky, helping them capture prey more successfully.

P. piliceps is a separate species found in another part of the world, with a far more specialised way of life than the assassin bugs of Western Australia. It depends on highly specific prey: six species of stingless bee. The bug makes its nest close to stingless bee hives and waits around their entrances to hunt the bees.

How bee-hive resin becomes an ambush tool

Because the bees have only degenerate stingers, they must take inventive approaches to defence. They place droplets of resin around their hive entrances; if an intruder attempting to enter becomes stuck in the resin, guard bees quickly emerge to attack and disable the threat.

This is where P. piliceps comes in. It immerses its legs in resin laid down by worker bees, spreading an even layer over its front and middle legs. It then waits beside the hive entrance and seizes the guard bee that rushes out to investigate the disturbance. With its prey secured, the assassin bug withdraws to a safe spot nearby.

There is a further twist. Although resin surrounds the hive entrance, guard bees head directly towards the waiting assassin bug. Something other than the resin’s adhesive quality appeared to be influencing them.

To explore this, the researchers carried out field tests examining the resin’s stickiness and its contribution to hunting. They coated the hind legs and hindquarters of some assassin bugs with bee resin, then observed the results.

Even bugs carrying resin on their hind legs and hindquarters – rather than on the legs used to grasp prey – achieved greater hunting success than bugs without resin. Unlike their Australian relatives, this indicated that stickiness was only partly responsible for their effective predation.

Resin scent draws guard bees towards P. piliceps

The team therefore designed experiments to determine the reason. They proposed that drying resin releases volatile compounds more slowly, and that spreading the resin across their legs enables assassin bugs to increase the release rate of these compounds.

Chen and his colleagues put resin in a ventilated setting and measured its emission rate before and after it was smeared. As expected, the rate at which volatiles were emitted increased following smearing.

The findings suggest that by working the resin, the bugs signal to bees that a problem needs attention. They then turn into the larger problem themselves.

According to the researchers, this behaviour qualifies as tool use.

"To be classified as tool use, a behavior must satisfy three criteria:" they write in their paper, "1) exerting control over a manipulable external object closely linked to a specific goal; 2) altering the user's or another organism's physical properties through a dynamic mechanical interaction; or 3) mediating the flow of information between the user and the environment or other organisms in the environment."

Applying the resin – an external object – changes the assassin bug’s physical properties in pursuit of a particular aim. It therefore meets the stated criteria. Notably, however, this use of a tool differs from how Australian assassin bugs employ a closely comparable one. The bee specialisation of P. piliceps may be involved, although further research is needed.

"While the role of food specialization in the evolution of resin use warrants future multi-species comparative studies within a phylogenetic framework," the researchers write, "this stingless bee-specialized assassin bug system offers a valuable model for investigating adaptive evolution and tool-use behaviors, with fewer ethical constraints than vertebrate-based studies."

The research was published in PNAS.