Organic gardeners maintain plant health and practice cultural controls to minimize pest problems. However, even the most carefully maintained garden occasionally experiences a pest outbreak that threatens your yield. Organic pesticides break down quickly for the most part, making them safer as a whole than many of their longer-lasting synthetic counterparts. When applying any product to your plants -- especially edibles -- read the label carefully and follow all instructions. (See References 1, page 445-446)
Insecticidal soap works on contact with small, soft-bodied insects like spider mites, scale insects, whiteflies, aphids and mealybugs. Soaps formulated for insect control generally contain fewer extras than household soaps, which may include lotions and other chemicals that can harm your plants. Insecticidal soaps must come in direct contact with the pest in order to work, so focus on the undersides of leaves and other places where pests hide. The residues rapidly disintegrate, making insecticidal soap safe for vegetable plants. (See References 1, page 457-460)
Oil-based insecticides come from petroleum and botanical sources (see References 1, page 457). Botanical oils carry no risk for toxicity to humans, pets or other nontarget organisms in the environment, making them a good choice for your vegetable garden. Botanical oils are concentrated from plants -- like mint oil, which controls aphids, and jojoba oil to kill whiteflies (see References 2). Neem, a popular organic pesticide, comes from the seeds of a tropical tree and controls some insects on plants; it does not, however, harm humans or nontarget organisms (see References 3).
Several pesticides of mineral origin control fungal diseases on plants. One teaspoon of baking soda mixed in a quart of water with a drop of soap stops spores from growing and may stop early growth. As a cooking ingredient, baking soda is safe for food plants, although "Rodale's Ultimate Encyclopedia of Organic Gardening" notes that a stronger solution can do damage to vegetation. Sulfur and copper are minerals with a long history of use to control fungal diseases. Because they are moderately and highly toxic to humans, respectively, avoid their use on edibles whenever possible and follow all label precautions if you must use them. (See References 1, page 475)
Pyrethrins come from chrysanthemum flowers and cause paralysis in insects. Although low in toxicity to humans, pyrethrins pose a significant risk to fish and other insects in the ecosystem. When purchasing pyrethrins, avoid pyrethroids, which are synthesized chemicals made to resemble pyrethrins (see References 4). The label should clearly state that the product comes from plants and is acceptable for organic agriculture.
- "Rodale's Ultimate Encyclopedia of Organic Gardening"; Fern Marshall Bradley et al., editors; 2009
- U.S. Environmental Protection Agency; Plant Oils Fact Sheet; February 2011
- U.S. Environmental Protection Agency; Cold Pressed Neem Oil (025006) Fact Sheet; February 2011
- National Pesticide Information Center; Pyrethrins & Pyrethroids; December 1998