Golden orb web spider spins ant-repellent silk

By Campbell Phillips 23 November 2011
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The golden orb web spider wards off marauding ants with an insect repellent spun into its silk, a new study finds.

THOUGH SPIDERS MAY SEEM safe in their nets, they can often fall prey to marauding ants. But the golden orb web spider (Nephila antipodiana) has a secret weapon: a chemical repellent to ward off ant attackers.

Researchers from the University of Melbourne and National University of Singapore (NUS) have discovered the chemical, which is spun into the spiders’ silk, is a type of natural insect repellent. The find could pave the way for a new insect repellent for humans.

“This study is among the first to show animals incorporating a chemical defence as a response to the threat of predation,” says Professor Mark Elgar of the University of Melbourne’s department of zoology. “It is particularly interesting that this only occurs in spiders that are large enough, and therefore spin silk that is thick enough, to expose them to this risk of ant invasion.”

Spinning an insect-repellent silk

Golden orb web spiders are a common sight around many backyard gardens. They spin webs of up to 1m across.

Associate Professor Daiqin Li from NUS had wondered why the webs of this species remained ant-free, despite being easy targets. He figured there must be something in the silk that deters ants.

The team set about stripping golden orb web spider silk of any chemicals to see if ants could be encouraged to walk on the web.

After narrowing down the compounds, they found one that kept the ants away.

“One of these chemicals is quite well known, but hadn’t been identified as an effective insect repellent,” says Mark. “We found that silk that had been stripped and then had the same chemical reapplied had the same effect on ants as the original silk.”

The compound known as pyrrolidine alkaloid is a common deterrent for many species of ants and moth, Mark says. It’s not lethal, but it is powerful.

“The chemical alkaloid is not very volatile. If prey were to smell it, it would probably already be too late for them,” he says.

Insect repellent silk probably common to spiders

Professor Jutta Schneider, a zoologist from the University of Hamburg, believes this phenomenon is probably not an isolated case in the world of spiders.

“I believe this study will be followed by more, similar findings,” says Jutta. “I bet it is not a unique feature of this species, or even of Nephila spiders that build permanent webs as adults. People just haven’t looked yet.”

Mark agrees that it is highly likely other species may employ the same or similar defenses; however, he points out another discovery the team made while conducting this research, which might explain why some spiders don’t need a chemical repellent.

“We found the silk of smaller spiders was actually too thin for ants to walk on,” says Mark. “The chemical simply isn’t necessary for smaller or immature spiders, but is produced in larger spiders when the silk becomes large enough to be vulnerable to ants.”

But what’s just as interesting, says Jutta, is why the spiders would expend the energy to create an ant repellent.

“The study raises interesting questions about how the spiders use the alkaloid without cost to themselves by repelling prey,” says Jutta.