A Plant Can’t Run
by Laura K. Zimmermann
Coyote tobacco plants (Nicotiana attenuata) grow with roots nestled in the sands of the Great Basin Desert. The one we’re observing lives near a sagebrush that is currently being attacked by a hungry grasshopper. As the grasshopper munches, the bush releases a volatile scent, a warning, and our coyote tobacco responds. If insects come, it will greet them with its signature poison, nicotine. Which will definitely help, unless the insects are hornworm caterpillars.
Big-eyed Backup
Hornworms caterpillars are immune to nicotine. But the coyote tobacco is ready. It recognizes the hornworms as soon as they begin to chomp. Their spit triggers a chemical SOS that draws big-eyed bugs who devour caterpillar eggs and attack young caterpillars. The big-eyed bugs eliminate many hornworms, but sadly, not all. Some survive the initial attack and continue to graze.
The Trichome Trap
Luckily, the coyote tobacco’s not done yet. It has sugary hairs on its leaves called trichomes—a sweet treat the hornworm caterpillars can’t pass up. So, they don’t. They crawl right in and gobble them down, which gives them a unique scent, that they don’t mind at all, until the predatory ants show up. Predatory ants are attracted to the caterpillars’ new body odor. They follow the smell, collect the caterpillars, and take them back to their nest.
A Pollinator Paradox
Unfortunately, there’s another problem. The caterpillars’ mothers keep laying more eggs. But don’t worry, the coyote tobacco plant still has a trick up its stem. To understand this trick there’s something you need to know. The hornworms’ mothers are hawkmoths, nighttime pollinators of the coyote tobacco. No moths, no pollination. No pollination, no new plants. The solution? Attract a new pollinator. So, the coyote tobacco changes how its flower smells by turning off the release of benzyl acetone, the chemical scent it uses to attract moths. It also changes the shape of its flowers, making them more tube-like, adjusts the amount of sugar in its nectar, and begins to bloom in the morning. The moths stay away and the plant attracts hummingbirds, who don’t leave a single egg behind.
But the question remains. If hawkmoths are such a problem (a single moth can lay as many as 200 eggs), why attract them at all? No one really knows for sure. It may be that hawkmoths are better pollinators than hummingbirds travelling farther and visiting more plants. It is also possible hummingbirds aren’t as reliable as a season-long pollinator. But whatever the reason, once the caterpillars are gone the plants change back and start attracting hawkmoths again.
References
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Weinhold, A., & Baldwin, I. T. (2011). Trichome-derived O-acyl sugars are a first meal for caterpillars that tags them for predation. Proceedings of the National Academy of Sciences of the United States of America, 108(19), 7855-7859. https://doi.org/10.1073/pnas.1101306108