Parasitic Vines That Feed on Parasitic Wasps That Feed on Trees

By Veronique Greenwood


It’s parasites all the way down.

Oak tree galls — compartments of plant tissue containing wasp larvae — with a parasitic vine attached by tiny wooden suction cups.CreditCreditBrandon Martin/Rice University

Parasites have a leisurely lifestyle — set up camp at someone else’s place, live off their food, profit (evolutionarily speaking). But new research shows that sometimes the parasite gets a taste of its own medicine, and from an unexpected source.

Scientists studying wasps that target oak leaves found that a second parasite, a vine, can get its tendrils into the homes set up by the wasps, called galls, subverting their diversion of the host’s resources. After that, things don’t go so well for the wasp.

Galls are like in-law apartments for parasites: swollen masses of plant tissue that route nutrients to wasp larvae, which grow and develop safely within until they are mature enough to leave.

The researchers behind the new paper, published last week in Current Biology, study gall-forming wasps all over the Southeast. But when they first encountered a gall with small woody suction cups dug into it, it looked so strange they wondered if it had been collected by mistake.

“I thought it was maybe a seed or a fruit,” said Scott Egan, an assistant professor of biosciences at Rice University in Houston and the study’s lead author.

But cutting open the growth revealed the body of a wasp inside, confirming its gall status. The scientists looked through their sacks of samples gathered in the field and found this gall was not the only one to be compromised.

Belonocnema treatae, the oak leaf gall wasp. CreditAndrew Forbes

So, Dr. Egan went back to the Florida sand live oak forest where his collaborator, Glen Hood, first found the gall. Dr. Egan walked through the trees and kept his eyes open. Soon, he realized that in one patch, the oaks and their galls were threaded with a plant called the parasitic love vine. There the researchers found numerous instances of the vine entering the galls.

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The connection did not seem harmless. When the researchers dissected 51 love-vine-infested galls from one wasp species, they found that 45 percent contained a mummified adult wasp, compared with only 2 percent of uninfested galls.

That suggests that the love vine interferes with the wasp’s nutrition such that it develops fully but is not able to leave. And the host tissue within dissected galls was twisted toward the vine's entry points, hinting that it was co-opting the gall's nutrients.

The vine isn’t just randomly attacking the galls, the researchers conjecture. It usually attaches itself to stems, buds, and the tops and sides of leaves, but never to the undersides of leaves.

That is where galls of this particular wasp species form, however. And when a gall is present, the vine’s aversion to undersides seems to disappear.

A closer view of the suctions cups which the parasitic love vine uses to attach itself to the galls.CreditScott Egan and Matt Comerford/Rice University

“Maybe they are actually targeting these tissues or are attracted to these tissues,” said Dr. Egan. Or, he added, it could be that the gall changes the surface of the leaf enough that attachment becomes possible.

Gall-forming parasites have been studied for hundreds of years, as have parasitic plants. But this is the first time that a parasitic plant feeding on a parasitic insect feeding on a host plant has been reported, said Dr. Egan. Additional research will confirm whether it is more common than scientists had realized.

And what of the sand live oak, the setting for this parasite death match? How is it doing?

“In general, gall wasps can have fitness consequences for their hosts — they can reproduce less, generate fewer acorns,” said Dr. Egan. The same is true for parasitic vines.

But at least on first glance, the trees don’t seem to be suffering too overtly. The team has been visiting some wasp-infested trees for ten years, and they are green and flowering, producing acorns. The hosts can take a certain amount of beating on a regular basis, it seems, and still survive.