Tasty Pesticides

A while back I was musing on the fact that some of the plant-derived substances many people enjoy on a daily basis are basically natural pesticides.

(One term that will come up in the post is “LD₅₀”; this is the median lethal dose, the amount of chemical per unit of body mass at which roughly half of a population will be killed by said chemical. It's a way of comparing toxicities between different substances.)

One such natural pesticide consumed by pretty much everyone in developed countries in one form or another is good ol' caffeine. In the wild, caffeine paralyzes and kills certain insects that feed upon the plants that make it. It can also have toxic effects in mammals (including humans), although the toxicity is fairly low: it's estimated the average adult would need to drink 80 to 100 cups of coffee in a short period of time to receive a lethal dose. (The  LD₅₀ is estimated to be about 150-200 milligrams per kilogram of body weight; the amount in a cup of coffee ranges from 80-125 milligrams [typical soda about half that], while average adult body mass is on the order of 60-100 kilograms.) Obviously drinking that much coffee in a short enough time is impossible, but there have apparently been reported deaths after overdosing on various caffeine pills that give extremely high doses at once.  For the majority of my readers that shouldn't be a concern though I imagine.

Another cocktail blend near and dear to my heart is peppermint oil. Peppermint is my panacea of choice and I love the taste, which comes from a complex blend of various chemicals. These chemicals, again, act as natural pesticides against various browsers. Keeping that mind that everything is poisonous at a high enough dosage, I didn't find anything specifically about peppermint toxicity in mammals, nor an LD₅₀. Probably the fact that it's a mixture of chemicals rather than a single specific one has something to do with that.

Another natural insecticide that many people enjoy is nicotine. It's no secret that nicotine is toxic, and it is definitely quite dangerous to mammals. For comparison, remember that the LD₅₀ of caffeine is estimated to be about 150–200 mg/kg; that of nicotine is a mere 0.5–1 mg/kg, around 75–200 times more toxic. (Or if you want, you can flip it on its head and tell people that caffeine is 75–200 times less toxic than nicotine.) Although it's unlikely anyone could get that amount from most of the various recreational sources of nicotine (though it is possible to overdose), pure nicotinic acid is highly dangerous because of how easily and quickly it get absorbed by the skin, and there are reports of people getting deathly sick simply by spilling a tiny amount of high-concentration nicotine solution on their skin.

Another chemical used by plants for defense against mammals, capsaicin has ironically enough become one of the main reasons peppers are cultivated around the world. Capsaicin is an incredibly nasty chemical. I should know, having had a lot of exposure to it growing up since my father is a pepper breeder. Pure, isolated capsaicin is such a hellish material that it requires proper respiratory apparatus, face protection, and strict adherence to hazardous materials guidelines.  I couldn't find an  LD₅₀ for humans; for mice it's about 47 mg/kg, which puts it somewhere between caffeine and nicotine in toxicity.

Edit: according to my dad, the LD₅₀  of Tabasco Sauce is 24 milliliters per kilogram of body weight. It's not straight capsaicin, but it definitely contains a lot of it. To put it in perspective, a fatal dose for the average person would be about 3 pints (about 1.5 liters), though I'd imagine you'd never want to see the stuff ever again long before you managed to down that much in a single sitting...

Finally, another natural pesticide is allicin, the active ingredient in garlic. Allicin, however, mainly targets bacteria and viruses rather than multi-cellular organisms (although its pungent taste can certainly drive them away). I couldn't find an  LD₅₀ for it for mammals, but it's incredibly effective at killing pretty much any bacteria brought in contact with it, even ones resistant to other antibiotics.

These five chemicals are just the ones I could think of off the top of my head. I'm sure there are plenty more out there if I were to do some digging, but these are common ones that many people will be familiar with (the form they come in, if not the chemical itself). So the next time you consume some “pesticide free!” organic food, just remember, some of the potentest pesticides in nature come from within.