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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Bio-Rational Pesticides

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Pesticides vary in their toxicity and in their potential to cause undesirable ecological impacts. Pest control materials that are relatively non-toxic with few ecological side-effects are sometimes called ‘bio-rational' pesticides, although there is no official definition of this term. Some, but not all, biorationals  qualify for use on organic farms. The major categories of bio-rational pesticides include botanicals, microbials, minerals, and synthetic materials.

Botanicals are plant-derived materials such as rotenone, pyrethrum, sabadilla, ryania, etc. Nicotine products, although natural, are not considered bio-rational due to their high mammalian toxicity. Botanicals are generally short-lived in the environment, as they are broken down rapidly in the presence of light and air, thus they do not provide pest control for very long, perhaps a day or several. Most botanicals are broad spectrum, so they kill beneficial insects, too. They tend to be moderately toxic to people and wildlife; many are irritating to mucous membranes.

Some newer botanical insecticides have low mammalian toxicity. These include products made from extracts of Neem tree seeds, such as 'Azatin' and 'Align' which are labeled for many vegetable crops. Azadirachtin is the active ingredient. It works by inhibiting development of immature stages of many insects, and by deterring feeding by adults. Garlic and hot pepper-based materials are other low-toxicity botanicals used by some growers, although their efficacy is uncertain.

Microbial pesticides, formulated from micro-organisms or their by-products, tend to have advantages over the botanicals in that they are safer to use, and are more selective in what they kill, so beneficials are not harmed. Bt, or Bacillus thuringiensis, is the most widely-used  microbial insecticide. There are many different Bt products which contain various crystals made by a bacterium that are toxic to specific insect pests such as caterpillars ('Dipel', 'Xentari') and Colorado potato beetle larvae ('Novodor', M-Trak). To be effective, Bt must be applied to foliage when small larvae are actively feeding.

Some microbial insecticides are entomopathogenic fungi which can infect insects and kill them. Products containing live spores of Beauvaria bassiana are available, such as 'Mycotrol', which work best when applied at the onset of an infestation of soft-bodied insects such as whitleflies or aphids. It takes a week or more after spraying for the spores that came in contact with the pest to germinate, penetrate, and grow throughout the insect thus killing them.

Other microbials are available that work as fungicides, such as 'Mycostop', a soil drench derived from Streptomyces fungus, and 'Gliogard', derived from Gliocladium fungus. Both these products are labeled to control some root-rotting organisms that cause damping off and similar problems in greenhouse and seedling crops. Products containing Trichoderma are labeled for control of  pathogens in greenhouse soils and potting mixes ('Root Shield') as well as on seeds and seed pieces as a  planter box treatment ('T-22'). Foliar applications of this organism will soon be registered for control of powdery mildew, botrytis, etc.('Topshield').

Some biorational pesticides are minerals, such as sulfur for control of foliar diseases and in some cases, mites. Kaolin, or white clay-based products such as 'Surround' have been developed and will eventually be registered for crop use to suppress a variety of insects and foliar diseases.

Low-toxic synthetics include soaps, or fatty acids of potassium salts, which are formulated to be both insecticides ('M-Pede') and herbicides ('Scythe'). Soaps work by suffocating soft-bodied insects and by burning the leaves of weeds. Application directly onto exposed insects is critical to good control. Soaps can be phytotoxic to some crops, and harmful to some beneficials. Horticultural oils are petroleum-based but organically-allowed means of smothering scale and other insects, and have been used to suppress aphid feeding, too.

Potassium bicarbonate is a recently registered material ('Armicarb-100','Kaligreen') for prevention of powdery mildew and other diseases on crops like cucurbits, and ornamentals. Just like more conventional fungicides, it must be applied to healthy tissue in advance of infection in order to be effective.

Copper compounds are fungicides and bactericides available in different formulations, including Bordeaux mixture, tri-basic copper, copper hydroxide ('Kocide'), cupric oxide, copper sulfate, elemental sulfur, calcium polysulfide (lime sulfur) and copper/zinc. Although for most foliar diseases these are not as effective as other synthetic fungicides, with a good spray program and cultural practices that maximize leaf drying, they can provide effective prevention. Caution must be used because of potential phytotoxicity, especially with temperatures over 80 degrees.

The so-called "beneficial" insects include predators and parasitoids such as lady beetles and various wasps, as well as certain nematodes that are used for insect control. These are classified by regulatory agencies as biological controls, not as pesticides. Most suppliers of these organisms provide good information on how to best use them.

With bio-rational pesticides, as with all others, read the label carefully to be sure it applies to the crop and pest in question, and to learn the most effective means of application. Remember that labels may change from year to year as crops and pests are added or withdrawn, or new application procedures developed.

(Mention of brand names is for information purposes only; no endorsement is implied nor is discrimination intended against products not mentioned).

Published/last revised: May 1999
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