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Stinky-breath caterpillar has healthy addiction to nicotine

So you think tobacco is bad for your health? Try telling that to a tobacco hornworm: His stinky nicotine breath is the only thing keeping him off the evening dinner menu, scientists say.

In a study published Monday in the Proceedings of the National Academy of Sciences, researchers discovered that Manduca sexta moth larvae use a form of "defensive halitosis" to ward off ravenous wolf spiders.

As a tiny leaf-creeping caterpillar, M. sexta will gorge on coyote tobacco plants all day, consuming more than a milligram of nicotine in a 24-hour period - the rough equivalent of one cigarette.

While nicotine will paralyze most insects, the hornworm is immune to the stuff.

Through a series of experiments, researchers at the Max Planck Institute for Chemical Ecology determined that the caterpillar stored nicotine in its blood, or hemolymph, then exhaled a tiny amount - less than 1% - through its spiracles, or respiratory openings that run along the length of the insect.

In a series of experiments, senior author and biochemist Ian Baldwin and colleagues placed individual nicotine-laden caterpillars in plastic cups with individual wolf spiders that had been starved for 24 hours.

Despite the spider's hunger, they rejected the meal.

Yet there was no finicky behavior when the ravenous spiders were presented with caterpillars that had been fed nicotine-free tobacco, or tobacco that had been genetically engineered so that it "silenced" the caterpillar's bead breath genes.

The spiders quickly tore into their prey.

"Spiders usually assess their prey after capture by tapping it with chemosensory endowed legs and palps," the authors wrote. "Wolf spiders were clearly rejecting nicotine-fed larvae before penetrating their prey with their mandibles to inject their mixture of digestive enzymes and poisons."

In true scientific fashion, it took the researchers some time to figure out how the caterpillars were processing the nicotine they digested, and who it protected them from.

The realization came about, they said, at a research ranch in Utah, where they had planted regular coyote tobacco and nicotine-deficient tobacco. After taking a count of all likely predators in the area, researchers placed caterpillars on each group of crops and monitored their rate of survival.

They found that the insects disappeared much more quickly at night, and even more quickly on the nicotine-deficient plants.

The wolf spider, a nocturnal hunter, was singled out as the prime suspect. (Another hornworm predator, the so-called big-eyed bug, hunted during the day and showed no aversion to nicotine-spiced meals.)

Researchers said experiments involving the genetically modified tobacco provided additional insights into the function of specific caterpillar genes, but that they would not have been possible to conduct if it weren't for the field experiments.

"Our work demonstrates how predators can assist in the process of elucidating the function of herbivore genes," the authors wrote.

Copyright © 2014, Los Angeles Times
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