Early this spring, Rice University evolutionary biologist Scott Egan stood in a patch of live oak scrub habitat in South Florida and scanned the trees for something he’d never seen outside his lab — a wispy, orange vine twining itself around swollen stems or pea-sized growths on the underside of oak leaves.
Rice University bioscientists have discovered the first example of a parasitic plant attacking a parasitic insect on a shared host plant. Cassytha filiformis, also known as love vine, feeds off of galls, the natal chambers of parasitic wasps.
Egan needed visual confirmation of something he and his students noticed in the lab a few months earlier: love vine, a parasitic plant, latching onto and feeding off of not the tree itself, but the tumor-like growths made by his favorite insects, gall wasps.
“I went to spots where I knew that my gall-formers and the vines were, and I just blurred my eyes across the tops of the trees,” Egan said, re-enacting the moment he scanned the forest. “And, once you have seen it, you can’t not see it. I’m like, ‘Oh. It’s everywhere. I can’t not find it, on this branch, or on this one or this one.”
For Egan, who has spent 17 years studying gall-forming insects and logged thousands of miles collecting samples from oak forests across a dozen U.S. states, it was a revelation.
“I had never seen this,” Egan said. “But the fact that no one, as far as we know, had ever documented this was incredible because biologists have studied each of these — the vines and the insects — for more than a century.”
In ecological parlance, the find was a new trophic interaction between two species, meaning that one was feeding off the other. “Basically, you have a parasitic plant attacking a parasitic insect inside of another host, a host they share,” he said.