Plant toxins: a double-edged sword.

Secondary plant chemicals -- a wide variety of compounds, including opium, caffeine and strychnine, produced by plants but not essential to their primary metabolism -- often poison or repel insects and other herbivores. However, in an ironic twist, some plant-eating insects exploit these toxins to protect themselves against predators. New field research in the Sierra Nevadas of California shows that production of secondary plant chemicals can actually work against plants by this circuitous route.

Salicin, a toxin produced by willows, so improves one kind of willow-eating beetle's survival that the willows that produce the most salicin end up suffering the most beetle-inflicted leaf damage, report researchers at the University of California at Irvine in the Aug. 16 SCIENCE.

"This is the first field demonstration that a plant can be harmed by the chemicals that it originally produced itself as a defense against herbivores," says insect ecologist John T. Smiley, who led the study. Although other research has shown that glutathione, an antixodiant secreted by many crop plants, attracts and invigorates Mexican bean beetles (SN: 4/20/85, p.247), glutathione is a primary plant chemical and is secreted in response to stress.

The willow-leaf-eating beetle, Chrysomela aenicollis, belongs to a group of beetle species known to prefer plants rich in sailicin, which the beetles convert to salicylaldehyde--a bitter, irritating chemical. Secreted from surface glands by the beetle larvae, salicylaldehyde deters ants and probably other arthropod predators such as wasps from attacking the larvae, say the researchers.

Smiley and two graduate students placed beetle larvae, drained of their salicylaldehyde secretions, on willow shrubs that were either high or low in salicin. (The salicin content of the willows varied 100-fold at the study site.) After 10 days, all the larvae on high-salicin willows could secrete salicylaldehyde, but none of the larvae on low-salicin willows could. Even more telling, 60 percent of the larvae on low-salicin willows had disappeared--and presumably been eaten--compared with only 10 percent of the larvae on the high-salicin willows.

By measuring both leaf damage and salicin content in individual willow shrubs, the researchers discovered that the more salicin a willow produced, the more heavily it was damaged.

Salicin does repel herbivores less specialized than these beetles: Fewer kinds of herbivores eat salicin-rich plants than eat salicin-poor plants, says Smiley. But because Chrysomela aenicollis developed the ability to exploit salicin, an "evolutionarily unstable" situation has arisen, he says. To whatever extent the willows are debilitated by the beetles, the willows will exprience selective pressure to not make salicin. In fact, the researchers note, most of the low-salicin willows in the study were at the lower elevations, where there are more beetles.

The researchers suggest that beetle-mediated natural selection may account for the great variation in salicin content of the willows. They also suggest that, more generally, repeated exploitation of secondary compounds by plant-eating insects may account for the great diversity of secondary chemicals that plants produce: That is, every time an insect herbivore adapts to a plant's toxins, the plant is forced to produce a new one. Some scientists have suggested that insects' remarkable capability to develop resistance to synthetic pesticides is a knack they have evolved in response to the constant production of new pesticides by plants.