Jim Miller makes the point:

But don’t commercial pesticides add an additional risk? Not necessarily. And to understand that, you need to know a fascinating fact about plants. Plants do not (with exceptions such as fruits) like to be eaten. And so they have evolved, over the years, many defenses against animals that would eat them. In particular, they have invented, if you will, their own chemical pesticides, such as nicotine.

Producing these pesticides is expensive for the plant; it takes away energy that the plant could be putting into stems, leaves, and seeds, so the plants vary the amounts of pesticides they produce depending on the level of attack from animals, especially insects. Plants grown “organically” often have higher levels of insect damage, which suggests that that they also have much higher levels of pesticides, natural pesticides. And those pesticides are not, unlike most commercial pesticides, on the surface, where they can be washed off.

Plant breeders have tried to make plants more pest resistant by traditional breeding techniques. This 1989 article by Bruce N. Ames, Chairman of the Dept. of Biochemistry, Univ. of Calif. at Berkeley, points out the problem:

All plants produce their own natural pesticides to protect themselves against fungi, insects, and predators such as man. Tens of thousands of these natural pesticides have been discovered,and every species of plant contains its own set of toxins, usually a few dozen. When plants are stressed or damaged, such as during a pest attack, they increase their natural pesticide levels manyfold, occasionally to levels that are acutely toxic to humans. Only a tiny percentage of these natural pesticides has been tested in animal cancer tests, but of those that have been tested, the percentage that turns out to be carcinogenic is about as high as for man-made pesticides (about 30 percent). The same appears to be true for natural teratogens (agents that cause birth defects). It is highly probable that almost every plant product in the supermarket contains natural carcinogens and teratogens. The pesticides that we are eating are 99.99 percent all natural (we eat 10,000 times more natural than man-made pesticides), are relatively new to the modern diet, because most of our plant foods were brought to Europe within the last 500 years from the Americas, Africa, and Asia (and vice versa). In response to the environmentalist campaign about tiny traces of man-made pesticides, plant breeders are active in developing
varieties that are naturally pest resistant. However, the primary way plant breeders are able to increase natural resistance to pests is to breed plants with increased levels of natural pesticides. It should be no surprise, then, that a newly introduced variety of insect-resistant potato had to be withdrawn from the market, due to acute toxicity to humans caused be much higher levels of the teratogens solanine and chaconine than are normally present in potatoes.

Similarly, a new variety of insect-resistant celery recently introduced in the United States had to be withdrawn after it caused widespread outbreaks of dermatitis due to a concentration of carcinogens at 9,000 ppb rather than the usual 900 ppb. Many more such cases are likely to crop up–they are undetected as yet due to lack of immediate observable effects–because there is a fundamental trade-off between nature’s pesticides and man-made pesticides.

Worse, organic foods require more space for cultivation. More farm land means less space for nature preserves. So, skip the expensive organic sections at your local supermarket and head for the regular aisles. Better to wash off man-made pesticides than ingest an excess of natural pesticides.

Note that genetically engineered plants are a different story.