Using Botanical Pesticides in Your Organic Garden

Insect and disease killers that come from plant extracts are called botanical pesticides or botanicals. Although derived from natural sources, botanicals are not necessarily safer or less toxic to non-pest insects, humans, and animals than synthetically derived pesticides. In fact, most botanicals are broad-spectrum insecticides, which kill both good and bad bugs indiscriminately. Some botanicals cause allergic reactions in people, others are highly toxic to fish and animals, and some may even cause cancer. All pesticides — including botanicals — should be used only as a last resort after thoroughly reading the label on the package. The pesticides in this section are listed from least to most toxic to humans.

Hot pepper wax and powder: The chemical capsaicin causes the heat in hot peppers and it's the active ingredient in these useful botanical products. In low doses, such as found in ready-to-use sprays and dusts, hot pepper wax repels most common insect pests from vegetables and ornamental plants. It doesn't cause the fruit or vegetables to become spicy hot, but instead stays on the surface of the plant where it remains effective for up to three weeks. Stronger commercial formulations kill insects as well as repel them. Hot pepper wax is even reportedly effective in repelling rabbits and tree squirrels.

Neem: This pesticide is made from the seeds of the tropical neem tree, Azadirachta indica, and it comes in two forms — azadirachtin solution and neem oil. Unlike the other botanical insecticides in this section, neem does not poison insects outright. Instead, when insects eat the active ingredient, it interrupts their ability to develop and grow to their next life stage or lay eggs. It also deters insects from feeding and is effective against aphids, thrips, fungus gnats, caterpillars, beetles, leafminers, and others. Amazingly, plants can absorb neem so that any insects that feed on them may be killed or deterred from feeding.

It breaks down in the presence of sun and soil within a week or so. To discourage insects from eating your plants, spray neem before you see a large infestation. The product Safer BioNeem contains azadirachtin solution.

Neem oil, the other seed extract, also works against some plant leaf diseases, such as black spot on roses, powdery mildew, and rust diseases. Mix the syrupy solution with a soapy emulsifier to help it spread and stick to the plants. The neem oil products called Rose Defense and Fruit & Vegetable Defense (from Green Light) control insects, mites, and leaf diseases.

Pyrethrins: These insecticidal compounds occur naturally in the flowers of some species of chrysanthemum plants. The toxins penetrate the insects' nervous system, quickly causing paralysis. In high enough doses or in combination with other pesticides, the insects die. Powerful synthetic compounds that imitate the natural chrysanthemum compounds are called pyrethroids. Pyrethroids are not approved for use in organic farms and gardens. Also avoid any pyrethrins that list "piperonyl butoxoid" on the label. This additive is not approved for organic use.

Although relatively harmless to humans, pyrethrins are very highly toxic to fish and bees and moderately toxic to birds. It kills both beneficial and pest insects. To keep bees safe, spray pyrethrins in the evening after bees have returned to their hives for the night and avoid spraying blooming plants. The compound breaks down rapidly when exposed to sun and air and becomes less effective if stored for longer than one year. Many commercial products contain pyrethrins.

Ryania: This pesticide comes from the tropical Ryania speciosa plant. Although it controls fruit and codling moths, corn earworm, European corn borer, and citrus thrips, it is also moderately toxic to humans, fish, and birds. It is very toxic to dogs. Seek other botanical pesticides before considering ryania.

Sabadilla: Made from the seeds of a tropical plant, sabadilla is a powerful broad-spectrum insect killer. It's especially useful for controlling thrips, aphids, flea beetles, and tarnished plant bugs, but it also kills bees and other beneficial insects, and some people have severe allergic reactions to the chemical. Use it only as a last resort.