How some sap-sucking insects fling their pee

Some sap-sucking insects can “make it rain,” flinging droplets of pee while feeding on plant juices. Now scientists have explained how the insects, known as sharpshooters, create these sprays using tiny catapult-like structures that propel the waste at extreme accelerations.

A tree infested with sharpshooters exudes a steady pitter-patter of pee. “It’s crazy just to look at,” says engineer Saad Bhamla of Georgia Tech in Atlanta. That intriguing process — which can dampen unsuspecting passersby — got Bhamla and colleagues hooked on studying how the insects release their waste.

The researchers took high-speed video of two species — the glassy-winged sharpshooter and the blue-green sharpshooter — feeding and then flinging their pee. Those videos showed that a tiny barb called a stylus at the insect’s rear end acts like a spring. Once a drop collects on this structure, the “spring” releases, and the drop flies off as if hurled from a catapult.

What’s more, tiny hairs at the end of the stylus increase its flinging power, Bhamla and colleagues suggest, much like the sling at the end of certain types of catapults. As a result, the stylus launches liquid waste with a maximum acceleration 20 times that of Earth’s gravity, the scientists report in a video published online in the American Physical Society’s Gallery of Fluid Motion, as part of the APS Division of Fluid Dynamics annual meeting held November 18–20 in Atlanta.

It’s not clear why the insects fling their pee. Perhaps the practice allows the creatures to avoid being bathed in a fluid that could attract predators, Bhamla says.

Sharpshooters can do serious damage. The pests slurp hundreds of times their body weight daily and can transmit bacteria that cause diseases in plants. Glassy-winged sharpshooters, for example, have spread beyond their native southeastern United States, sickening plants in California’s vineyards and wreaking havoc in Tahiti by poisoning spiders that eat the insects (SN: 4/8/06, p. 221).