CREDIT: JOHN TOOKER

When insects target a leaf for a meal or to lay eggs, plants can respond with chemical defenses. Now researchers say they've found a wasp in which females can change the host plant's chemistry for sexual purposes.

Each summer, the wasp Antistrophus rufus lays its eggs inside the stems of 4-meter-tall flowering compass plants on the Illinois prairie. In spring, males emerge first and go looking for the still-slumbering females.

John Tooker of the University of Illinois, Urbana-Champaign, and colleagues wondered how males locate the plants with the concealed females. The team found that plants hosting female larvae had different chemical compositions than those that didn't. It appeared that the females were altering the ratios of monoterpenes, chemicals that generate the plant's smell. The researchers then exposed male wasps to filter paper soaked in compounds taken from plants harboring females. The wasps excitedly waved their antennae over the treated paper while ignoring untreated paper, the team reported online last week in the Proceedings of the National Academy of Sciences.

Ian Baldwin, director of Germany's Max Planck Institute for Chemical Ecology, says the study is the first to identify an insect that manipulates plant chemistry for sexual signaling.

But to really nail down the case, Joachim Ruther and Monika Hilker of the Free University of Berlin suggest, the researchers should also test male wasps' reactions to the smell of stems that didn't harbor females.