Answer: Caffeine

You most likely know caffeine as the substance that gives you a boost in your morning cup of coffee (or, perhaps if it doesn’t sit well with you, gives you a bad case of the jitters). The fact that caffeine does anything at all to the human body is merely a curious biological coincidence, however, and caffeine’s role in the natural world is that of a pesticide.

You read that correctly: although caffeine is the most widely consumed psychoactive drug in the world, it isn’t a pick-me-up for plants like it is for humans and, in fact, exists in plants to ward off predators. Although most closely associated with coffee plants, caffeine is present in approximately sixty plants including the leaves of the tea bush, kola nuts, yaupon holly leaves, and seeds from Amazonian maple guarana berries. While the substance acts as a stimulate in humans, in insects it causes paralysis and even death, and functions as an effective pesticide for the diverse number of temperate climate plants around the world that produce it.

On a related note, another widely consumed drug is also a pesticide (insecticide): nicotine. The active ingredient in cigarettes is used by tobacco plants as a pesticide just like caffeine. Historically, nicotine was even sold in powdered form for use in farming, but is no longer on the market due to toxicity concerns.