Scientists Reveal Why Asp Caterpillar Stings Are So Excruciatingly Painful

A toxin in the insect’s venom, which can punch a hole in cell walls, could inspire new drug-delivery methods in humans

Caterpillar on a leaf
The so-called puss caterpillars have sharp spines that can inject powerful venom into humans. University of Queensland

Asp caterpillars spend most of their time innocently crawling along the branches of oak and elm trees in North America. But when these hairy creatures come into contact with humans, they use their concealed, prickly spines to inject a painful venom into their victims.

The sensation is so excruciating that one sting-sufferer in Texas said it felt like she’d broken her ankle—the feeling was “staggering” and “intense,” as Jordan Meredith told KTRK in 2020. After being stung by one of these caterpillars—which grow into moths, such as the southern flannel moth and the black-waved flannel moth—people often go to the hospital.

But how can such a small creature deliver such an effective blow? Scientists at the University of Queensland in Australia decided to find out.

The team revealed that the venom of asp caterpillars, also known as puss caterpillars, contains a special protein that can form into a ring shape and poke holes in cells. In turn, the cells send powerful pain signals to the brain. This unique shape-shifting, hole-punching protein is what makes the caterpillar’s sting so effective, report the scientists in a paper published Monday in the journal Proceedings of the National Academy of Sciences.

Caterpillar venom study reveals toxins borrowed from bacteria

This discovery is significant for several reasons. For starters, it helps explain the mystery of the caterpillar’s agonizing sting, which people living in many parts of the American South have long endured.

But, beyond that, it may help medical researchers develop effective drugs for human medicine. They might be able to use the same hole-punching mechanism to “get drugs inside cells where they need to work,” says study co-author Andrew Walker, a molecular bioscientist at the University of Queensland, to the Australian Broadcasting Corporation’s Antonia O’Flaherty.

Additionally, “we might be able to engineer these kinds of toxins to target cancer cells or to target pathogens while leaving human cells alone,” he tells the publication.

Person's hand with red welts
Getting stung by an asp caterpillar feels like touching hot coals or getting hit by a baseball bat. University of Queensland

The finding also offers new insight into a rare process known as horizontal gene transfer, reports Live Science’s Sascha Pare. The researchers believe the caterpillars’ unique venom-delivery adaptation developed some 400 million years ago, when bacteria transferred the special cell-infiltrating gene into an asp.

For this to have happened, the bacteria—which probably belonged to the Gammaproteobacteria group, which includes strains of salmonella and E. coli—would have had to infect the caterpillar, then insert its DNA into the nuclei of the insect’s reproductive cells. Then, when the caterpillar turned into a moth and mated, it would have passed along this gene to its offspring.

hairy caterpillar against white background
They may look innocent, but a brush with one of these furry caterpillars can send a person to the emergency room. University of Queensland

Scientists still don’t fully understand how or why horizontal gene transfer happens, and it occurs so infrequently that it’s challenging to study. But the discovery that this process might have occurred with caterpillars is another data point for them to contemplate.

Perhaps above all else, the findings further reinforce that “evolution and life are weirder and more complex than we usually assume,” as Walker writes in the Conversation.

Get the latest stories in your inbox every weekday.