Natural pesticides: what have I been eating?!?!

Several months ago, there was a thread on the GMO Skeptiforum on Facebook about natural pesticides. It's one of the threads I learned from the most, so I thought I'd share some of it here along with the papers to back it up.

So, what are "natural pesticides"? Plants and animals have evolved mechanisms to fight against their predators. Some of them are mechanical, like thorns or spines on a puffer fish, but some are chemicals or natural pesticides.

It's important not to let the term "pesticide" confuse you. When the spouse read this article, he said that he didn't get why I used the term "pesticide" to describe a component/chemical in a plant. We're used to thinking of pesticides as the stuff we spray on plants or around our house to get rid of bugs. But the term "pesticide" is much broader than that: it's any substance that gets rid of or repels a pest. The term encompasses many different -cides: herbicides (to get rid of plants), fungicide (to get rid of fungi), insecticides (to get rid of insects), etc, etc. A natural pesticide can be toxic to the pest that its evolved to target, so I use the term "toxin" in this piece as well. These pesticides or toxins can be very specific in the organisms that they target: for example, the Bt-toxin which is found in different GMOs is actually from a bacteria in the soil and it is toxic to various insects, but the way it works doesn't impact mammals; chocolate is toxic to dogs, but not to humans; etc.

I first became aware of natural pesticides in my teens when my mom told me that I shouldn't buy green potatoes because they can make you sick. Back when we lived in Venezuela, every Friday morning my mom would go to the market to buy our fresh produce for the week. In my last two years of high school, I was lucky enough to not have morning classes on my schedule, which turned out to be unlucky for me because I'd get dragged out to the market. Unlike the fru-fru-shee-shee farmer's markets we've got in California, the market in Barquisimeto, Venezuela was dirty and really crowded. I always got stuck buying the potatoes, tomatoes, and passion fruit, while my mom bought the greens, papayas and melons. The potatoes were caked with dirt, so I'd have to smack them to see if they were green or not. Most of the time they were. As I got older, I wondered if my mom's advice was a mythical Persian legend or if it was legit, and Wikipedia helped me find the answer.

All plants should bear a warning symbol
given the amount of toxic substances they contain

Potatoes are a member of the nightshade family which have a poison called solanine present in different parts of the plant. This paper from Lancet published in 1979 states that potatoes have small amounts of solanine in the peel and none in the flesh, but when the potato starts to green or sprout (i.e. the 'eyes' start growing), then the amount increases significantly. Solanine levels also increase in potatoes when they're diseased, such as with the blight, and is probably part of the plant's defense system. The Lancet paper documents several cases of solanine poisoning from eating potatoes, but they were not typical cases (for example, individuals may have been malnurished). Current guidelines from the NIH state that eating solanine in very small amounts can be toxic and recommends throwing out spoiled potatoes or those that are green below the skin.

But solanine is just the tip of the iceberg when it comes to natural pesticides. Here are a few others:

• Caffeine: This paper from 1984 suggests that caffeine inhibits insects from eating the plant. It "appears to suppress certain enzymes in insect nervous systems". Attempts have been made at making a caffeine-based pesticides with mixed results. The chemical compound also magically makes my husband a much more agreeable person in the morning.
• Nicotine: Also present in the nightshade family of plants, nicotine is one of the very first agricultural pesticides developed, particularly since it is "toxic to most herbivores". It was phased out of farming in the US as of 2014. 
• Capsaicin: It's what makes peppers hot. Does it really surprise you that it's a natural pesticide? What animal in its right mind would want to punish themselves by eating terribly spicy foods? Apparently Homo sapiens... The first pesticide containing capsaicin was registered with the EPA in 1962. As you know, it's an irritant to most mammals, but it can also repel or kill insects. 
• 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (abbreviated as DIMBOA): a pesticide found in corn, wheat, rye, other grasses. It is released in these plants when a tissue is damaged and it's toxic to a wide range of  insects and bacteria.
• The chemicals to which we attribute many of the flavors in herbs and spices:
• Tetradecanoic acid in nutmeg: has been tested as a potential larvicide against the yellow fever mosquito.
• Pulegone in peppermint and catnip: found to be a potent insecticide.
• Carvacrol in oregano and thyme: has antibacterial properties by making the cell membrane of bacteria permeable, including E. coli.
• Eugenol in cloves, nutmeg, basil and others: used as a bait in many pesticides, data suggests that it can also cause liver damage.

The list is virtually endless. In 1990, Bruce Ames published a paper entitled "Dietary pesticides (99.99% all natural)". In it, he and his coauthors outline that we eat an estimated 1.5 grams of natural pesticides a day, "which is about 10,000 times more" than the amount of synthetic pesticide residues we eat. This amount would be significantly higher in vegetarians and vegans. As an example, the authors provide a list of 49 different pesticides found in cabbage alone. The concentrations of these pesticides are in parts per thousand or parts per million, whereas the amount of synthetic pesticides we find on our food are in the parts per billion range.

Despite the vast amount of toxins in our diet, only a handful of these have ever been tested (note that the paper was written in 1990, but the point still stands). The paper highlights that of all the chemicals tested for chronic cancer tests in animals, only 5% have been natural pesticides and half of these were carcinogenic.

Think about that for a moment. While there's an uproar about parts per billion amounts of synthetic pesticide, there are more concentrated compounds in fruits and veggies known to cause cancer (at much higher doses). In addition, some pesticides used in agriculture have mechanisms of action that are specific to the pests their targeting. I've already given the example of Bt-toxin, but glyphosate shuts down a biochemical pathway in plants that simply doesn't exist in mammals. For some reason, we're far more concerned about these two compounds than we are about natural formaldehyde in pears. Check out the FANTASTIC graphic at the end of this article that highlights this point: we fear anything that's synthetic because we assume that it's "bad for us", but there's plenty of stuff that's "natural" that can be harmful at the appropriate dose (I wanted to entitle this article "The Hidden Carcinogens in Your Food", but it seemed too click-bait-y).

I've read  a lot of arguments from anti-GMO groups about how transgenic crops that have the Bt-toxin will kill us all, because it's a registered pesticide with the EPA. "Do you want to eat something that's a pesticide???" is what I've read time and time again. There are plenty of "natural chemicals" that are registered pesticides, as I've noted above, but no one seems to be freaking out about basil and mustard seeds. Additionally, what many GMO-advocates will point out is that the cross-breeding and "natural" hybridizations we've been doing for centuries has undoubtedly impacted the levels of some of these pesticides by unknown amounts, because no one examines them. Going back to solanine, in the '60s a new strain of potato known as the "Lenape" potato was developed through "natural" methods, but was found to be toxic due to increased levels of solanine: it had ~2-4x the amount of solanine found in other potato varieties and it had to be pulled off the shelves. But no one seems to be screaming about "unintended consequences" of traditional crossbreeding.

Well, 'tis my bed time. I hope all had a great thanksgiving. You can probably thank natural pesticides for half the flavors in the delicious food you ate!!

http://www.compoundchem.com/2014/05/19/natural-vs-man-made-chemicals-dispelling-misconceptions/