The two best-studied varieties of these chemical alarm calls—known as herbivore-induced plant volatiles (HIPVs)—are terpenoids and green leaf volatiles (GLVs). Terpenoids are released from the whole plant, not just damaged leaves, often as late as one day after the attack. Researchers know that certain terpenoids attract specific parasitoids to their preferred hosts. In other words, these chemicals send a very clear message to parasitoids: Hey, there's a big juicy herbivore here!

In contrast, GLVs are immediately released from any wounded leaf, regardless of how it was damaged. These volatiles are responsible for the distinctive smell of a freshly mowed lawn, for instance. Researchers had never understood how parasitoids and predatory insects detected the difference between GLVs released by a leaf suffering mere mechanical damage and GLVs emitted from a leaf under siege by an herbivore.

A new study, published August 26 in Science, suggests that leaves wounded by herbivores release a different bouquet of GLVs than mechanically damaged ones do. Furthermore, the findings indicate that the plant does not directly alter its alarm calls—rather, something in the oral secretions of hungry herbivores causes this chemical change. Herbivores betray themselves as soon as they start munching, revealing their locations to any predators in the area.

"There has always been the suggestion that GLVs that immediately come off the plant are attractive to predators, too," explains Andre Kessler, an expert on plant volatiles at Cornell University who was not involved in the new study. "But it was not clear how predators would be able to differentiate between mechanical wounding and herbivore feeding, and this is what the paper resolved completely. It's a really elegant confirmation."

In the new work Silke Allmann of the Max Planck Institute for Chemical Ecology in Jena, Germany, and her colleague studied tobacco plants (Nicotiana attenuata) and caterpillars called tobacco hornworms (Manduca sexta). First, the researchers punched holes in the leaves of tobacco plants and analyzed the volatile compounds they emitted. In response to this mechanical damage, the tobacco leaves consistently gave off a characteristic bouquet of GLVs: Specifically, they released more of one kind of GLV (Z-GLVs) than another (E-GLVs).

In testing the mechanically wounded leaves, the researchers found that applying caterpillar spit significantly increased the level of E-GLVs. Further, when the researchers allowed living caterpillars to feed on the tobacco plants, they measured the same increase in E-GLVS. That is, the leaves of tobacco plants gave off a subtly different perfume when either wounded by caterpillars or exposed to their spit than when the plants suffered mechanical damage.

2 Comments

1. 1. Sarah B 02:25 PM 8/28/10

   This is absolutely fascinating! However, like most of the (popular) science I read, this portrays Nature as a system of attack and defense, which, intentionally or not, promotes justification for ego-centric systems such as territoriality, war, and greed. I hope theres another paradigm on the horizon!

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2. 2. Michael L. 07:02 PM 8/28/10

   This is interesting, indeed! The interplay of plant, herbavore and predator chemistry is so amazing, it smacks of evolutionary determinism. But if one stands back a little and stays objective, it is clear how a predator which happened to be attacted to the smell of herbavore saliva mixing with host plant GLV's could lead to this attraction being selected for with a juicy caterpilar as the "reward.

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