It is important to remember that the following "least toxic" pesticides are pesticides and should be treated as such. Many botanically-derived insecticides have gained favor in recent years, due in part to the perception that, because they originate from plant material, they are more safe or "natural." These pesticides are often used for growing crops organically, according to guidelines set forth by certification programs. However, it is important to be aware that they are pesticides, and that they fall under the same state and federal regulations as synthetic pesticides. Pesticides may only be used as indicated on the label. It is important to realize that produce grown using these less toxic alternatives is not "pesticide free." These pesticides also leave residues, can be disruptive to natural enemies and may be toxic to humans. The same precautions should be taken in the use of "less toxic pesticides" as with synthetic pesticides. Relying heavily on the use of any pesticide is an un-ecological practice.

Pyrethrins:

Pyrethrins are botanical insecticides made from the dried flower heads of a relative of the common chrysanthemum. Pyrethrins affect the insect on contact, creating disturbances in the nervous system which eventually result in convulsions and death. Low doses, however, often cause temporary paralysis from which the insect may recover. Pyrethrins are only slightly toxic to man and not hazardous to birds and other wildlife. They have almost no residual activity, breaking down rapidly from exposure to sunlight, air or moisture. Pyrethrins are effective as broad-spectrum insecticides to control pests such as aphids, whiteflies, stinkbugs and mites.

Neem:

Neem is a botanical insecticide derived from a tree native to the Middle East, where it has been used for centuries to control insects. One of the most desirable properties of Neem is its low degree of toxicityÜit is considered almost nontoxic to humans and animals, and is completely biodegradable. It is used as an ingredient in toothpaste, cosmetics, pharmaceuticals and other products. Neem products can be used to manage pests on vegetables, fruit, ornamentals, and lawns and can be found at many home garden centers. Neem has been advertised as effective or moderately effective for more than 200 pest insect species and some of the plant diseases, including certain mildews and rusts. Effectiveness, however, is variable and test results have been inconclusive in many cases. Because the products are relatively new, it is not yet clear how effectively the products control each of these pests. Generally, chewing insects are affected more than sucking insects. Insects that undergo complete metamorphosis are also generally affected more than those which do not undergo metamorphosis. Neem often works more slowly than other pesticides, and effectiveness is reduced in cooler climates. Neem does not persist in the environment and should be reapplied after rain. Neem has little effect when applied directly on insects, except in the oil formulations; most insects are affected only after consuming foliage that has been treated. Neem is most effective as a foliar spray applied periodically to new flushes of growth. On some species of plants Neem also works as a systemic pesticide, absorbed into the plant and carried throughout the tissues, ingested by insects when they feed on the plant. This may make it effective against certain foliage-feeders that cannot be reached with spray applications, such as leafminers and thrips.

Bacillus Thuringiensis:

At present, Bt is the only "microbial insecticide" in widespread use. A naturally occurring bacterial disease of insects, Bt is an insecticide with unusual properties which make it useful for pest control in certain situations. Bt insecticides are most commonly used against some leaf- and needle-feeding caterpillars. Strains have recently been produced which affect certain fly larvae, such as mosquitoes, as well as the larvae of leaf beetles. Insecticidal activity is specific to each Bt strain. Bt is considered safe to humans and wildlife. Some formulations can be used on food crops.

Properties: Unlike typical nerve-poison insecticides, Bt acts by producing proteins (delta-endotoxin, the "toxic crystal") which react with the cells of the gut lining of susceptible insects. These Bt proteins paralyze the digestive system, and the infected insect stops feeding within hours. Bt-affected insects generally die from starvation, which can take several days.

Disadvantages:
- Bt is susceptible to degradation by sunlight; most formulations persist on foliage less than a week following application. Some of the newer strains developed for leaf beetle control become ineffective within 24 hours.

- The highly specific activity of Bt insecticides might limit their use on crops where problems with several pests occur, including nonsusceptible insects (aphids, grasshoppers, etc.).

- As strictly a stomach poison insecticide, Bt must be eaten to be effective, and application coverage must be thorough. This further limits its usefulness against pests that are susceptible to Bt but which rarely have an opportunity to eat it in field use, such as codling moth or corn earworm which tunnel into plants.

- Bt-based products tend to have a shorter shelf life than other insecticides. Manufacturers generally indicate reduced effectiveness after two to three years of storage. Liquid formulations are more perishable than dry formulations. Shelf life is greatest when storage conditions are cool, dry and out of direct sunlight.

Horticultural Oil:

Various oils have been used for centuries to control insect and mite pests. Oils remain an important tool to manage certain pest problems (e.g., scales, aphids, mites) on fruit trees, shade trees and woody ornamental plants. Several recently developed oils extend this usefulness to flowers, vegetables and other herbaceous plants. Oils also can control some plant diseases, such as powdery mildew. Horticultural oils block the air holes (spiracles) through which insects breathe, causing death by asphyxiation. In some cases, oils also may act as poisons, interacting with the fatty acids of the insect and interfering with normal metabolism.

Oils pose few risks to people or to most desirable species, including beneficial natural enemies of insect pests. However, a mask or respirator should be worn when applying horticultural oils. Oils quickly dissipate through evaporation, leaving little residue. Oils also are easy to apply with existing spray equipment and can be mixed with many other pesticides to extend their performance.

The main limitation of spray oils is their potential to cause plant injury (phytotoxicity) in some situations. Oils also can stain some surfaces, particularly dark-colored house paints. Essentially all commercially available horticultural oils are refined petroleum products. Impurities in the oil that are associated with plant injury, such as aromatic compounds and compounds containing sulfur, nitrogen or oxygen are removed. Filtration, distillation and dewaxing complete the production of the finished base oil.

Vegetable oils also can be used as insecticides, although the type of oil can greatly affect its activity. Cottonseed oil is generally considered the most insecticidal of the vegetable oils. Soybean oil, the most commonly available vegetable oil used in cooking, has also provided fair to good control.

Insecticidal Soap:

Soaps can be used to control a wide range of plant pests. Small, soft-bodied arthropods such as aphids, mealybugs, psyllids and spider mites are most susceptible to soaps. In most cases, control results from disruption of the cell membranes of the insect. Soaps and detergents may also remove the protective waxes that cover the insect, causing death through excess loss of water. Insecticidal soaps act strictly as contact insecticides, with no residual effect. To be effective, sprays must be applied directly to, and thoroughly cover, the insect. Insecticidal soaps are considered selective insecticides because of their minimal adverse effects on other organisms. Lady beetles, green lacewings, pollinating bees and most other beneficial insects are not susceptible to soap sprays, for example. Predatory mites, however, which are often important in the control of spider mites, are vulnerable to insecticidal soaps.

One of the most serious potential drawbacks to the use of soap-detergent sprays is their phytotoxicity. Certain plants are sensitive to these sprays and may be seriously injured. The risk of plant damage is greater with homemade preparations of household soaps or detergents. When in doubt, test soap-detergent sprays for phytotoxicity problems on a small area a day or two before an extensive area is treated. Plant injury can be reduced by using sprays that are diluted more than the 2 to 3 percent suggested on label instructions. To reduce leaf injury, wash plants within a couple of hours after the application. Limiting the number of soap applications can also be important, as leaf damage can accumulate with repeated exposure.

Adapted from the Cornell Cooperative Extension and the Colorado State University Extension, 1999