Did you know that caffeine is an insecticide? Moreover, that the coffee plant (Coffea arabica and others) produces it in order to protect itself from predatory insects? When under attack by unwanted invertebrates, moreover, the coffee plant increases the dose of caffeine, often producing enough to kill the intruder.

Caffeine extracts applied to various insects (milkweed bugs, caterpillars, mosquito larvae, etc.) cause agitation, reduce appetite, inhibit reproduction, and can even lead to death. Caffeine-treated mosquito larvae, for example, become so poorly coordinated that they can no longer swim and end up drowning.

Several companies are working on deriving an organic pesticide from caffeine and other similar substances.

Note that theine, produced by the tea plant (Camellia sinensis) and present in tea, is actually just caffeine: the two are chemically indistinguishable. And the caffeine/theine in tea plants serves the same purpose: to repel or kill insects that attack the plant. Theobromine, present in the cacao tree (Theobroma cacao), plays a similar role. Both caffeine and theobromine are xanthine alkaloids.

Are Caffeine and Theobromine Toxic to Humans?

It’s said that it’s the dose that makes the poison and so is it with caffeine and theobromine: the amount present in a cup of coffee, tea or hot chocolate (or a glass of Coca Cola or Pepsi) is insufficient to kill a human. Moreover, some adults drink three or four cups a day without suffering any major sequel. As a result, most people think of caffeine and theobromine as being rather innocuous. Our body simply digests these alkaloids as with so many other products we ingest.

In addition, several studies indicate that coffee, tea and yes, even chocolate can be beneficial for your health, at least in case of adults. Drinking one or two coffees a day seems to help protect against Parkinson’s disease, diabetes and various liver diseases including cancer, improves cognitive function and reduces the risk of depression. Note that we don’t know why coffee seems good for our health and that these benefits don’t necessarily come from caffeine: coffee contains about 1,500 chemicals and only very few have been studied.

However, caffeine isn’t always as innocuous as we like to think. The amount of caffeine found in energy drinks and diet pills is much greater than in a typical cup of coffee or tea (there are about 100 ml of caffeine in the average cup of coffee, but 300 ml in a typical energy drink) and either product can lead to an overdose and even death, especially in adolescents, if abused.

There is also the risk of developing a physical dependence on caffeine. As little as one cup of coffee or two of tea a day (about 100 ml of caffeine) can lead to physical dependence and, if you stop drinking them, sometimes serious withdrawal symptoms. It’s estimated that nearly one quarter of adults worldwide are “hooked” on caffeine.

Very Toxic to Animals

Caffeine is much more harmful to pets than to humans. Don’t ever let them drink coffee, tea or hot chocolate or eat coffee grains or chocolate bars. Even chewing a leaf or two on from a coffee plant, found in many homes as a houseplant, can send a cat to the vet’s.

What About Coffee Grounds?

The Internet is full of websites extolling the merits of coffee grounds as a handy-dandy insect repellant for the garden, claiming that if you apply them to the base of plants, it will keep undesirable insects away. Unfortunately, no serious study has ever found any truth to that, and for good reason. Because there is no longer enough caffeine left in coffee grounds to repel insects: you have already drunk the insecticidal part!