Bed bugs are extraordinarily good at tormenting us. They squeeze into mattress seams, furniture, and cracks barely wider than their flattened bodies. They emerge while we sleep, drink our blood, then disappear again. They’ve been with us since we built our first cities and they’ve proven to be notoriously resilient.

Many populations have also evolved resistance to insecticides, making infestations even more difficult to eliminate.

But now, researchers may have found an unexpected vulnerability: water.

Weakness of bed bugs. Image credits: Pixabay/Pexels

It turns out, bed bugs instinctively avoid water. In controlled laboratory experiments, the insects turned away from wet surfaces long before making contact, often making abrupt U-turns and speeding away. 

This discovery was made almost by accident, revealing a previously unknown behavior that could help researchers develop more effective ways to control infestations.

An interesting encounter with moisture

The discovery started during routine work at the University of California, Riverside (UCR) where researchers maintain colonies of the common bed bug (Cimex lectularius) inside small laboratory vials. 

To feed them, they place an artificial blood feeder over the container, allowing the insects to pierce a thin membrane and drink. During one feeding session, the membrane developed a small leak. 

“The leaked blood was slowly soaking the paper from the top of the vial. I thought the bed bugs would be happy to drink the blood from the paper. But what I saw was very different. They were actively avoiding the part of the paper that became wet with blood. They wouldn’t even walk near the wet areas,” Dong-Hwan Choe, one of the study authors and a professor of Entomology at UCR, said.

To find out whether blood or moisture was responsible, the team repeated the experiment using deionized water, a purified form of water with most dissolved minerals and salts removed. 

Surprisingly, the insects stayed away from the wet areas again, suggesting that moisture, not blood, was triggering the response. This unexpected observation inspired a series of controlled experiments. 

Bed bugs have a water ick

Since young bed bugs are only about two millimeters long, the researchers used an infrared camera equipped with a magnifying lens to record their movements. Motion-tracking software then measured where the insects walked, how far they traveled, how fast they moved, and exactly when they changed direction.

The experimental arenas had filter-paper floors. One side remained dry, while the other was treated with deionized water at one of three application levels.

The results were clear.

Adult males, adult females, and nymphs all spent less time on the wet side of the arena and traveled shorter distances across it. They avoided the wet surfaces, and the wetter the paper, the more they avoided it.

“On average, bed bugs made turns when they were 0.58 cm away from the wet zone. The minimum distance from the wet zone before making a turn was 60% longer for nymphs compared to adults,” the study authors added.

This indicates that younger insects are even more sensitive to moisture.

A hidden weakness with practical consequences

This is the first time that this water avoidance was documented scientifically in bed bugs, but it makes sense.

Bed bugs have extremely flat bodies and breathe through tiny openings along the sides of their abdomen. If they become trapped by water’s surface tension, those breathing openings can become blocked, potentially increasing the risk of drowning. Rather than taking that chance, the insects appear to treat moisture itself as a warning sign and retreat before getting too close.

The findings could have important implications for pest control because many insecticides are applied as water-based sprays. If those products do not kill bed bugs immediately, the insects may simply avoid freshly treated surfaces and disperse into nearby hiding places instead. 

The researchers say this newly discovered behavior should be considered when developing and applying liquid-based control methods.

The study is published in the Journal of Ethology.