Gardening Tips: July 16, 2014
This is the time of the season when our gardens come under attack from all sorts of pests ranging from insects and mites, to slugs and diseases. It would be great if nature took care of pests in the same way that things usually work out in our forests. There is often a kind of dynamic harmony that exists in forest ecosystems in which pests are kept in balance by other factors.
Last weekend I was hiking in the Town of Durham and I noticed that every single gypsy moth caterpillar was dead or dying from a fungal disease. They were hanging on the tree trunks in an inverted V form, indicating they had been infected by a fungal disease that had been imported to the US back in 1890, when the caterpillars first started causing massive defoliation in Massachusetts. This disease seems to have effectively prevented the boom and bust cycles we had experienced from the late 1960s to the late ’80s. Most ecosystems are remarkable resilient, given enough time to adjust. I lose little sleep worrying about the latest “threat” to our forests because, inevitably, the forest will endure as long as it is not paved over.
Backyard and landscape gardens are a different story however! Most of the food we grow in our vegetable gardens are exotic plants that we have domesticated for short-term food production. They have not developed natural resistance and we usually have to deal with their pests on a regular basis. Most of us will employ some sort of pesticide to control pests. Most pesticides are poisons, whether they are labeled as “organic” or not. They are designed to either repel or kill targeted species. Most of them will leave some sort of residue on the crop on which they are sprayed. It is the residual poison that concerns many people.
Many years ago I learned how it is determined what amount of pesticide residue is acceptable on the food we eat. The process begins with determining the LD 50 of any substance. LD 50 stands for lethal dose required to kill 50 percent of a population of some test animal, usually rats or rabbits. The test animals are subject to both oral and dermal applications of the substance. The lethal dose is determined in parts per million or milligrams per kilogram of body weight. The lower the LD 50 is, the more toxic the substance is. An “organic” pesticide, such as rotenone, may actually have a much lower LD 50, than a chemical poison. This is the case for rotenone since its LD 50 is 50 to 75 parts per million (ppm) whereas carbaryl (sold as sevin) has an LD 50 of around 500 ppm. Rotenone is 10 times as acutely toxic as carbaryl, yet it is generally preferred by organic gardeners because it is derived from naturally occurring plant parts. “Natural” poisons, like arsenic, or lead are poisons nevertheless, but many people consider them “safer” then chemicals. The debate between “organic” versus “chemical” is far more complex than I have time to go into now, but I may revisit it in the future.
Once the LD 50 has been determined, the next step is to develop the “NOEL” level. “NOEL” stands for No Observed Effect Level. Test animals are fed smaller and smaller dosages of the pesticide for generations until it is determined that a certain dietary amount seems completely harmless. This is the NOEL level and is also expressed in ppm. That level is then divided by 1,000 to determine the acceptable residue of that substance you may have on your food. That means that when you buy and eat an apple or a green bean, it may very well and probably does, have a pesticide residue on it. For substances with high LD 50s that “tolerable” amount will be much higher than for substances with lower LD 50s. Therefore, you may be eating much more carbaryl residue than rotenone, since rotenone is far more acutely toxic.
This seems simple enough, but as is usually the case, the Devil is in the details. Acute oral toxicity is only one measure of how a substance can harm you. It does not encompass long-term carcinogenic or mutagenic effects, which are much harder to calculate. To try to get a handle on this, for some time, scientists used what is known as the “Ames” test. This was named after its inventor and is only designed to see what substances cause mutations, at any concentration. It is used on single celled bacteria, which are easier to monitor.
Today the Ames test is rarely used, because, as it turned out, almost everything causes mutations at some concentration to single celled bacteria. Many common substances that have a long history of safety and efficacy flunked the Ames test. Even Professor Ames does not suggest its use as a tool today in most circumstances, because it is much too broad in scope.
So these are the tools and techniques used today to try to monitor our food supply. It is surely not a foolproof system, but it is the best we have right now. Of course you can be confident that the fruit and vegetables you eat are pesticide free, if you grow your own and do not use any pesticides at all!