Throughout the Seed 2 Table site you will see recommendations for particular pest controls, very few of which involve the spraying or dusting of pesticides. Nonetheless, a basic understanding of what pesticides are and how they work is invaluable. Pesticides run the gamut from homemade remedies to high-tech and highly toxic chemicals, and in one form or another they have been in use since humans first began cultivating plants. The ancient Sumerians and Chinese, for instance, used mineral and botanical compounds to control various pests.
There are four main categories of pesticides:
- fungicides, and
We will concern ourselves mostly with insecticides because, aside from fungicides like Captan or Thiram, most vegetable garden use of pesticides is for the control of insects. There are four basic types of insecticides using three general methods of action. Pesticides can be biological, botanical, mineral (elemental), or synthetic, and they generally kill either by infection or poisoning, though some pesticides act mechanically. We will discuss them in terms of their origin, stating the method of action for each class.
Biological pesticides are those that kill by infection, and they essentially involve the destruction of one organism by another. The best example is the microbial insecticide Bacillus thuringiensis, or Bt, which is a naturally occurring bacterium that infects the gut of soft-bodied caterpillars. By applying a solution of this bacteria to the edible parts of affected plants, we guarantee that the pest caterpillar becomes infected with the bacterium, which produces a toxin that poisons the caterpillar. Other bacterial pesticides have been developed; in most cases they are quite specific to the pests for which they are applied, and thus nontoxic for humans and other creatures. A problem may arise with the widespread use of bacterial sprays, however, if the pest species develop genetic resistance to the diseases they cause.
Botanical pesticides are those derived from plants. Pyrethrum and rotenone are two well-known and widely used botanical dusts. Both are contact poisons and, as such, are relatively toxic to a wide range of organisms, including humans. They are, however, short-lived in the environment, and not responsible for the kinds of long-term health and pollution problems associated with synthetics pesticides. Sprays made from hot peppers or garlic juice have also been used since time immemorial as pest and disease controls, respectively. Garlic juice depends on its sulfur content for its effectiveness, as sulfur is toxic to all kinds of bacteria and fungi, while the former depends on capsaicin, the essential compound found in hot peppers. Other botanical (as well as some petroleum) oils are also used as pest controls, primarily on shrubs and trees. They work by coating the pest with oil, usually during its dormant stage, so that it smothers.
Mineral (or elemental) pesticides are those composed of mined and/or refined materials. The sulfur naturally present in garlic is also available in pure form for use as a fungicide, as is elemental copper. Both can be toxic at high levels, but are rarely used extensively enough to cause immediate problems. Arsenic and mercury are far more toxic, and were once widely used, though rarely now. Diatomaceous earth is mined from ancient marine crustacean deposits and forms a gritty dust that gets into the joints of the exoskeleton, or shell, of hard-bodied insects, wears holes in it, and thus causes them to die of dehydration.
Synthetic pesticides are those that are manmade. Soap is a relatively benign example; liquid soap sprays, properly formulated, can break the skin of many insect species and so kill them by dehydration. Most of the chemical pesticides are synthetic materials, and among them are different kinds, with different levels of toxicity and effectiveness. The first generation of synthetics, developed primarily as a result of Allied chemical warfare research during World War II, were the organochlorines, or chlorinated hydrocarbons. The most famous (or infamous) of these is DDT; but others, some even more toxic, include Chlordane, Aldrin, Endrin, and Dieldrin. These are all nerve poisons. In addition to their effectiveness at killing not only insect pests but other wildlife, they are responsible for numerous human deaths and illnesses. They persist both in the environment and in the tissues of plants and animals (including humans), so that exposure to these extremely toxic chemicals is cumulative over time.
A second group of pesticides, developed by the Germans for their own nerve gases during World War II are the organophosphates. As problems began to develop with DDT and the others, these new compounds found widespread use both on farms and in the garden. Look on the hardware store shelf: you’ll see two widely used organophosphate pesticides: Diazinon and Malathion. While they seem not to be as toxic to humans as the organochlorine pesticides, organophosphates are very toxic to bees and other beneficial insects, so their use interferes with the natural balance of the garden.
The third category of synthetic pesticides that interest us here is the carbamates, developed in the late 1940s. Their method of action and toxicity are similar to organophosphates. The major carbamate insecticide used in home gardens is carbaryl, sold under the trade name Sevin. Aldicarb, or Temik, a chemically related farm insecticide, was responsible for a highly publicized mass poisoning some years ago after it was illegally applied to a crop of watermelons sold to a supermarket chain.