Organic pesticides are often promoted as more environmentally friendly than synthetic pesticides. In fact, some organics can be quite toxic to mammals or harmful to beneficial organisms. Like synthetics, organics must be handled with care and applied according to label directions. That may mean wearing gloves, goggles and a dust mask, and keeping pets and humans away from treated areas.
Here are some tips on popular organic products:
Beneficial Insects. The most commonly-sold predatory and parasitic insects include ladybugs and the tiny, nonstinging Trichogramma wasp. Because predatory and parasitic insects attack specific prey, know what pests are present before buying a particular batch of beneficial insects. Avoid using insecticides where beneficials have been released. Once beneficial insects have eaten their prey, they will abandon a site in search of more food.
Diatomaceous Earth. This talc-like powder is actually microscopic shards milled from the shells of fossilized, sea-dwelling, one-celled plants called diatoms.
The shards abrade soft-bodied insects, causing them to lose fluids and literally dry up. Diatomaceous earth works best on dry days because humid weather helps damaged insects retain fluids. The product works on contact, and an effective application needs to be thorough,. Insects controlled by diatomaceous earth may take up to 12 hours to die.
Prolonged exposure to diatomaceous earth will irritate lungs and other tissues of people or pets. Because it also kills honeybees, avoid applying the product to crops in flower.
Insecticidal Soap. This liquid contains fatty acids derived from plants or animals. An effective application must hit the underside of leaves where pests often gather. Avoid applying insecticidal soap on hot, sunny days because it may burn leaves.
Insecticidal soap kills many types of insects, including some beneficial species, on contact.
Neem. Extracts of the neem, or Indian lilac tree, possess a range of insecticidal properties and also have antibacterial, antiviral and antifungal properties. Neem extracts have been shown to prevent insect feeding and oviposition, and to act as a growth regulator. Several important greenhouse pests, including the greenhouse and sweetpotato whitefly, Western flower trhips and mealybug, are included on the label of the most widely-available formulation, Margosan 0.
Pyrethrum. Derived from the flowers of Chrysanthemum cinerariaefolium, pyrethrum is sometimes synergized, or made more effective, by piperonyl butoxide (PBO). The long residual synthetic pyrethroids are based on the chemistry of pyrethrum.
Pyrethrum is a contact insecticide and must be applied directly to the insect to be effective. Pyrethrum rapidly paralyzes pests. Insects still moving after treatment have received a less-than-lethal dose.
Because the pyrethrum mammalian toxicity is very low, it can be applied to food crops close to harvest. Some individuals with ragweed pollen allergies have suffered allergy attacks after exposure to pyrethrum dust. Pyrethrum has a high contact toxicity for common beneficials, such as Encarsia formosa, predaceous mites and Aphidlete aphidimyza. There is only a three-to 10-day residual effect.
Rotenone. Rotenone, made from the roots of tropical plants, is probably the most widely-available botanical insecticide. Many formulations are synergized by the addition of PBO. Rotenone lasts a week or less after application because it is rapidly degraded by sunlight.
Rotenone has the lowest DL50 value of the commonly-used botanicals, making it the most toxic. Like most synthetic pesticides, Rotenone acts on the nervous system. Small doses may be irritating to mucous membranes. This product is highly toxic to fish and other aquatic life. It has been commonly used as a fish poison.
Ryania. Ryania is derived from the roots and stems of Ryania speciosa, which grows in Trinidad. It is more persistent and more selective than rotenone and pyrethrum. It is generally not harmful to parasites and predators, but there is some indication of toxicity to certain predatory mites.
Sabadilla Dust. This insecticide comes from the crushed seeds of a tropical lily. The dust may be mixed with water and sprayed, but clogging of nozzles has been reported.
Sabadilla dust has a powerful short-term effect. It has a short residual, breaking down quickly in the enrionment. Sabidilla dust is believed to have little effect on most predators and parasitoids. It was found to be highly toxic to Typhlodromus pyri, a predatory mite active in some apple orchards. Because sabadilla dust kills honeybees, it should not be applied to crops in flower.
It has an even lower mammalian toxicity than pyrethrum. The dust can irritate the mucous membranes of people and pets and cause sneezing fits.
Spray Oils. Mineral oils, dormant oils and horticultural spray oils are terms used to describe various types of spray oils. Although most registered oils are petroleum distillates, research has been done on cottonseed, coconut, corn, palm, peanut, safflower, soybean and sunflower oils.
Spray oils kill insects and mites and the eggs of both on contact. Mechanisms of toxicity include suffocation by blockage of the respiratory openings, which reduces the abailability of oxygen and prevents exchange of gases for metabolic processes; and penetration of the air passages, muscles and nerves, which impairs physiological processes. Many plants are senstivie to oil sprays. Read and follow all label directions.
Oils, with very high oral and dermal LD50s, are relatively nontoxic compared to synthetic pesticides. Narrow-range oils are similar to the mineral oils used in most skin lotions and baby oils and to pharmaceutical-grade oils taken internally as purgatives. However, care should be used and label safety precautions should be observed.
Even though most organic insecticides are less toxic than most synthetic insecticides, they should be used with respect. The label should be read and followed. Precautions should be taken during mixing, application and storage of these products.