Which makes sense. To do so is energetically expensive and is unlikely to deter a rampaging cow or combine harvester. Also, some defence compounds are very toxic and can damage the plant. Compounds in the insect's saliva probably give its presence away. Takabayashi's team are not the first to spot plant defence.
Some plants produce weapons tailored to their attackers.
One South American potato releases a volatile that mimics the alarm call of infesting aphids, making them flee in panic. Or it could be that predators have simply learnt to associate volatiles with particular insects. Takabayashi's group is now looking to see if plants downwind from an insect-damaged plant produce volatiles to recruit predators as well as switching on their internal defences in preparation for being attacked. They hope that improved understanding of these airborne communications systems among plants will suggest new, more environmentally friendly methods of insect control, as they explains in Nature 1 . Arimura, G. et al.