Plants growing in nature, in order to survive, produce a host of chemical-defensive compounds (often called allelochemicals) that are used to ward off attack by potential herbivores (plant-feeding insects and mites). These compounds may be directly harmful to herbivores or modify (that is, slow down) their development, thus increasing their susceptibility to natural enemies such as parasitic wasps (= parasitoids) and/or predators. Humans have made use of these naturally derived compounds for many years, and a number of botanical insecticides have been formulated for use by professionals and homeowners. These botanical insecticides are processed in one of three ways: (1) preparations of the crude plant material, which are ground up into a dust or powder, (2) extracts obtained from plant resins formulated into liquid concentrations, and (3) isolation of the pure chemicals obtained from plants by extraction or distillation.
There is a common misconception that natural, or botanical, insecticides are always safer than synthetically derived insecticides. Nothing could be further from the truth because a number of registered botanicals are toxic to fish, beneficial insects and mites, and mammals. This is based on the LD50, which is a term used to describe the lethal dose required to kill 50% of the test animals (mainly mice and rabbits), expressed as milligrams (mg) of toxicant per kilogram (kg) of body weight. The lower the LD50, the more toxic the compound is to humans. In fact, several botanical insecticides have a lower LD50 than the synthetically derived insecticides carbaryl (Sevin) and malathion. Although naturally occurring insect toxins are extracted from plants, “natural” does not necessarily imply “safe” or “non-toxic.” Bottom line: Natural compounds derived from plants are not inherently less toxic to humans than synthetically derived compounds.
Many botanical insecticides, particularly those containing pyrethrin, are formulated with a “synergist,” which is a compound that increases the effective toxicity of botanical insecticides. This prevents insects from “coming back to life” by inhibiting detoxification enzymes such as mixed-function oxidases (MFOs) or by deactivating enzymes. This blocks the ability of insects and mites to break down the toxin. The most commonly used synergist is piperonyl butoxide, or PBO.
In most cases, botanical insecticides are less toxic to humans than synthetically revied insecticides. There are a number of advantages and disadvantages when using botanical insecticdes to manage plant-feeding insects and mites in landscapes and gardenns. The botanical insecticides that have primarily been or are commercially available include pyrethrin/pyrethrum, rotenone, sabadilla, ryania, nicotine, citrus oil extracts, and neem.