Natural selection as karma: wasp parasitises tree, vine parasitises wasp
Researchers find that an insect living off a tree is itself a target for a very different type of predator. Tanya Loos reports.
A parasitic plant known as the love vine grows on oak trees, and also feeds upon gall wasps. The researchers who discovered this behaviour are calling it “botanical parasitism of an insect by a parasitic plant” – a peculiar type of interaction never previously recorded.
The wasps induce the Sand live oak tree (Quercus geminata) to create tumour-like growths called galls, which protect their developing young. The love vine (Cassytha filiformis) actively seeks out these growths and attaches their specialised roots to their walls, leaching out the moisture and nutrients within. Inside a normal gall, a young wasp grows into an adult and exits through a small hole in the wall. When a love creeper feeds on a gall, the wasp still develops into an adult, but ends up a mummified corpse trapped inside.
“I've been studying gall wasps and their interactions with their hosts and natural enemies from the southern tip of Florida to the southern edge of Texas for over a decade but had never observed this interaction between the wasps and parasitic vine,” says lead author Scott Egan, from Rice University, Houston, US.
“We have discovered a new interaction between plant and insect parasites when the two exist on a shared host. Most notably, the vine parasite is directly influencing the fitness and survival of the insect parasite.”
The wasp most commonly attacked is a species called Belonocnema treatae, which forms spherical single-chambered galls, but the love creeper vine was also found attached to those of several other species, suggesting that the behaviour made be widespread.
This means that the potential is global in scale. Parasitic species comprise 1% of flowering plants, or angiosperms, representing some 4500 species in 20 families. Gall formation is described in more than 13,000 species in six insect orders.
The next goal for Egan and his colleagues is to establish how the love vine finds the galls.
“We know that the vine's root structures are targeting the gall tissues specifically because they're attaching to galls located in regions of the tree the vine normally doesn't attach to,” he says.
“This suggests that the vine may have some kind of searching mechanism or that there's something special about the gall that's drawing them in.”
How the vines detect galls may have medical applications, particularly in cancer research.
The research is published in the journal Current Biology.