When it's time to woo, tiny wasps get a little help--from the plants they live in. They apparently coax the plant to change its chemistry, creating a proxy for a wasp sex pheromone, a new study suggests. The work adds an intriguing twist to what's known of the complex chemical interactions between plants and insects.
When insects bite a leaf or lay eggs in a stem, plants may release airborne chemicals and set in motion elaborate ecological dramas. The chemicals may attract parasites of the insect to protect the plant, tell competing insects that the plant is already taken, or induce it and its neighbors to beef up defenses. Now researchers say that they've found a wasp that manipulates its host plant's chemistry in order to find mates.
Larvae of the wasp Antistrophus rufus live inside the stems of the 4-meter-tall Illinois prairie plants Silphium laciniatum and S. terebinthinaceum. They remain in the fallen stems all winter; when spring arrives, males hatch first, explore the dead plant material, and locate hidden females. Males then battle each other for rights to mate with the female when she emerges. John Tooker of the University of Illinois, Urbana-Champaign, and colleagues wondered how males found their concealed mates.
The researchers determined that ratios of plant-produced chemicals differed depending on whether the plants contained wasp larvae: Somehow, the presence of wasps altered the chemical composition of the plant's aroma. The researchers isolated the compounds in question and tested the extracts; male wasps responded to compounds taken from plant tissue where females lived, but didn't react to a solvent control without those chemicals. Tooker's team concludes that male wasps follow plant odors altered by the presence of females inside stems. The study appears online this week in the Proceedings of the National Academy of Sciences.
Some entomologists praise the work, but others aren't convinced. Because the wasps weren't offered chemicals from stems without females, the males might be responding to plant compounds in general, say Joachim Ruther and Monika Hilker of the Free University of Berlin, Germany. But Tooker argues that extensive field observations make it clear that males in the wild discriminate between stems with and without females, suggesting that the altered compounds are the real attraction.