Natural Pesticides
Most yard and garden pesticides available today are synthesized chemicals. Because some of these pesticides have been associated with health and environmental hazards, interest in alternatives has been increasing in recent years.
Microbes are one natural source of pest control. Many bacteria, fungi and other organisms cause diseases that kill or cripple insect pests. These are called microbial pesticides.
Combinations of natural products have also proved effective pest controls. For example, combining vegetable oils with an alkaline substance, such as potassium hydroxide, produces soaps that can be used to control mites and insects. These products are known as insecticidal soaps. Vegetable oils or, more commonly, refined petroleum oils, yield horticultural oils that can be highly effective for pest management.
Plants themselves have proven to be sources of some of the most potent pest-control products. Many plants produce a host of chemical defenses that they use to naturally resist attack from various pests. Some plants are especially rich in chemicals that can be extracted and used for insect control. These products are known as botanical insecticides or, simply, botanicals.
Pesticides derived from natural sources, like those that are manufactured from petrochemicals, have a wide range of effects. Most botanical pesticides, for example, do less ecological damage than synthetics because they break down rapidly when exposed to heat, light and water. Others are as acutely toxic (or sometimes more toxic) than common synthetic garden pesticides. (Acute toxicity is a measure of the damage they can do to you if they're ingested, inhaled or absorbed through the skin.) All pesticides—synthetic and natural—are regulated as pesticides by the Environmental Protection Agency and the states, and by law must be used strictly in accordance with all instructions on the product labels.
Microbial Pesticides
The microbe most commonly used for garden pest control is the bacterium Bacillus thuringiensis, better known as Bt. Different strains of Bt, which occurs naturally in soils around the world, produce toxins that affect different insects. For example, the "kurstaki" strain of Bt (Btk) kills caterpillars, such as gypsy moth, hornworms and cabbageworms. Leaf beetles, such as the Colorado potato beetle, are susceptible to the "tenebrionis" strain (Btt).
To be effective, Bt must be eaten by the pest. Susceptible insects stop eating soon after ingesting Bt, as it destroys the lining of their gut. Death often follows in a few days.
The primary advantage of Bt is its highly selective action. Most Bt products only kill caterpillars that eat it. This means that most beneficial insects are spared the adverse effects. Bt is considered quite safe to humans, and most products can be used right up to harvest.
Bt does have some limitations, however. Because it must be eaten, thorough coverage of the affected plant is critical. Bt also breaks down rapidly upon exposure to sunlight and water, rarely lasting more than a few days. What's more, Bt kills not only the larvae of pests, but also the larvae of butterflies. Know the insect you are spraying for and keep away from plants where butterfly larvae feed.
Another bacterium long used for insect control is Bacillus popilliae, which produces the "milky spore" disease of Japanese beetle grubs, a major lawn pest. This bacterium has become distributed widely throughout eastern North America, originally through government programs and subsequently by the insects themselves. Milky spore is sold through many garden catalogs.
Microbes may sometimes be used for indirect control of plant pests. Perhaps the best example is the product Clandosan, sold for control of nematodes. This product consists primarily of crab shells and related material that is mixed with the soil. It stimulates the growth of microbes that feed on chitin, the main component of crustacean shells. Nematodes also are covered with chitin and so are susceptible to the microbes as well.
Microbial pesticides are applied as sprays, dusts or granules, just as conventional pesticides are.
Horticultural Oils
Refined petroleum oils have long been used for managing insects and mites. Oils smother insects by plugging the orifices, called spiracles, through which they breathe. They may also be toxic to some insects and mites. Oil products developed for use in pest management are typically referred to as either horticultural oils or dormant oils. They are usually used as sprays, mixed with water in a 1 to 3 percent solution.
The primary problem with oils is that they can damage your plants if used improperly. The first oils to be used horticulturally were the "dormant oils," which could only be used safely on plants in a dormant state. However, our understanding of what makes oils useful as pesticides and what causes plant injury (phytotoxicity) has increased, and oil products that can be used safely on many plants, even when they've leafed out, are now available. Some plants, such as walnut, certain maples and cedar, do remain "oil shy" even to the most refined horticultural oils. Read the label for details on sensitive plants and on when not to spray (spraying under certain environmental conditions can cause injury).
Oils have remained a popular pest-management option because they are quite effective for many difficult problems. They're most commonly used as dormant sprays to control insects and mites that spend the winter on trees and shrubs. However, the refined oils now on the market are also useful for controlling whiteflies, young scales, mites and many other plant pests present during the growing season. Oils have also proven useful in managing some plant diseases.
Horticultural oils are considered quite safe to humans and other wildlife. Adverse effects on beneficial organisms are also minimal, particularly those of dormant season sprays, which are applied when most beneficial insects are not yet present in the garden. Like soaps, horticultural oils act strictly through contact action and have no residual effects, so thorough coverage is essential.
Insecticidal Soaps
Soaps have been used as insecticides for over 200 years, but recently their use has increased exponentially. This is largely because there is now a better understanding of which types of soaps make the most effective insecticides, yet do not damage plants.
Insecticidal soaps are applied as dilute sprays (1 to 3 percent concentration) and work primarily by damaging the cell membranes of insects and mites. A wide range of insects are sensitive to soaps—primarily small, soft-bodied species such as aphids, leafhoppers and spider mites. But some larger insects, such as Japanese beetles, are also susceptible. Effects are rapid, usually resulting in death of susceptible insects within a few minutes after exposure. Soaps are sometimes sold in mixtures with other insecticides, such as pyrethrins, to increase their effectiveness.
The selective action of soaps and their high degree of safety to humans are their major advantages. Generally, they have a minimal impact on beneficial species. (One significant exception is that soaps kill predator mites, often an important control of spider mites.) Most insecticidal soaps are registered for use on a wide range of vegetable and ornamental plants.
One of the main limitations of soaps is that they work strictly on contact and have no residual effects. This means that they must be applied directly on the target pests, and so good spray coverage is essential. Also, soaps are more sensitive to certain environmental conditions than other insecticides are. For example, the minerals in hard water react with soaps to reduce their activity. And soaps may be less effective if applied during periods when they dry very rapidly.
Although the insecticidal soaps have been developed with plant safety as a major consideration, some plants are sensitive to soaps and can be injured. Most of these are listed on the product labels under the section outlining hazards associated with use. Indeed, research has also identified soaps that are particularly injurious to plants, and these herbicidal soaps are now marketed as contact "weed killers."
Many household soaps and liquid dishwashing detergents can be used effectively as insecticides. These should be applied as dilute sprays. Their main disadvantage is that their effects on plants and insects have not been tested and there is a greater chance that they'll cause accidental injury to your plants.
Alcohol
Alcohol affects many types of insects, apparently by causing them to dry out and die. Although there are no commercial alcohol insecticides, alcohol is an ingredient in some insecticidal soaps and "ready-to-use" insecticides.
Alcohol is often used to control mealybugs on houseplants. Usually, it is applied directly onto the insects with a cotton swab in order to avoid injuring the plant. However, many plants tolerate alcohol well, and insects can be controlled with a spray of alcohol and water in equal parts. Try spraying the alcohol solution on a small part of the infested plant first to make sure it does no damage. If after a few days the plant shows no adverse effects, go ahead and spray the entire plant.
Botanical Insecticides
Pyrethrum
The most widely used of the botanical insecticides are extracts from the flowers of the pyrethrum daisy, Chrysanthemum cinerariifolium. Powdered pyrethrum flowers are rarely sold for pest control, but there are numerous products containing the extracted active ingredients, pyrethrins. Formulations sold for garden use often combine pyrethrins with other ingredients such as soap, diatomaceous earth or rotenone, another botanical insecticide.
Pyrethrins have some unusual insecticidal properties. Perhaps most striking is the rapid "knockdown" effect they have, which causes most flying insects to drop almost immediately upon exposure. Pyrethrins are also highly irritating to insects and can therefore be used as a "flushing agent" to disperse pests. They also rapidly degrade when exposed to light or moisture and so do not persist for long in the environment.
Most insects are highly susceptible to pyrethrins, so quite low concentrations are applied. At the same time, pyrethrins are quite non-toxic to most mammals, making them among the safest insecticides in use. The short persistence and low toxicity of pyrethrum-derived insecticides have enabled federal regulators to permit their use on a wide variety of crops, typically with little or no interval required between application and harvest. Pyrethrins also are among the few insecticides that are cleared for use around food handling and preparation areas.
In the past few decades, synthetic pyrethrins, or pyrethroids, have been developed. The pyrethroids have the basic chemistry of pyrethrins but are synthetically modified to improve persistence, insecticidal activity and other features. Few pyrethroids are available for yard and garden use, although they are used widely in commercial agriculture.
Rotenone
Rotenone is one of the oldest botanical insecticides. Records suggest that it was first used against insects in 1848. (For centuries before that it was used as a fish poison.) Most rotenone is derived from South American species of the genus Lonchocarpus. Rotenone is used most commonly as a dust prepared by grinding the plant roots or extracting the active ingredients and coating dust particles. Several rotenone/pyrethrins mixtures are marketed.
The Environmental Protection Agency has permitted use of rotenone on a wide variety of vegetables and small fruits. It is both a contact and stomach poison to insects. Rotenone is used primarily for control of various leaf-feeding caterpillars and beetles, such as cabbageworms and Colorado potato beetle. Some insects with sucking mouthparts, such as aphids and thrips, are also susceptible to rotenone. It is a relatively slow-acting insecticide, often requiring several days to actually kill susceptible insects, although they stop feeding shortly after exposure.
Gardeners should be aware of the fact that rotenone is the most acutely toxic of the widely available botanicals—more toxic than most common synthetic pesticides. It is moderately toxic to most mammals, and highly toxic to fish and aquatic life.
Ryania is the powdered extract from the roots and stems of the shrub Ryania speciosa, native to South America. It is sold primarily as a wettable powder. Ryania is also available in some combination formulations with pyrethrins and rotenone.
Ryania has shown promising insecticidal action against many insects. It is sold primarily for control of codling moth. Many caterpillars, leaf beetles and thrips also are susceptible to ryania extracts. Ryania affects these insects either on contact or when eaten. However, it has minimal effects on many beneficial insects, so it can be used with these and other biological controls. Ryania breaks down more slowly after application than other botanical insecticides. It is considered relatively non-toxic to mammals.
Sabadilla is an insecticide produced by grinding the seeds of the sabadilla plant, Schoenocaulon officinale. For several years sabadilla products were not available in the U.S. However, they are now sold by several mail-order suppliers, and garden centers have begun to carry sabadilla products as well.
Sabadilla is both a contact and stomach poison and has shown greatest promise against several of the "true bugs," such as squash bug, chinch bug, harlequin bug and stink bugs. It has proven effectiveness against leaf-feeding caterpillars, Mexican bean beetles and thrips. Use of sabadilla on certain vegetables, including squash, cucumbers, melons, beans, turnips, mustard, collards, cabbage, peanuts and potatoes, is permitted by the EPA..
The ground seeds of sabadilla sold for garden use are considered among the least toxic of the various botanicals. However, sabadilla dusts can be highly irritating to the respiratory tract, often provoking a violent sneezing reaction if inhaled. Be sure to wear a dust mask when applying it and, as with all pesticides, follow precautions listed on the product labels. In addition, several of the alkaloids in sabadilla can cause rapid depression of blood pressure in mammals.
Neem
The newest of the botanical insecticides are those derived from seeds of the neem tree, Azadirachta indica. Extracts from neem seeds and other parts of the tree have long been used for pharmaceutical purposes, for example in toothpaste, particularly in India. Recently, neem has received a great deal of attention because it is so safe to humans and has unusual properties against insects.
Sprays of neem applied to leaves often deter feeding. Furthermore, neem apparently affects the hormones many insects need to develop, killing them as they attempt to molt or emerge from eggs. Many leaf-chewing beetles and caterpillars can be controlled with neem insecticides. Aphids and most other sucking insects generally are less susceptible.
Because of its demonstrated safety, neem was recently exempted by the EPA from food-crop restrictions, enabling manufacturers to market it for use on any edible or ornamental plant.