Natural pesticides from plants.
Duke, S.O.

In Janick, J. and J.E. Simon (eds.) Advances in New Crops, pp. 511-517. Timber Press, Portland, OR. 1990

Reviewer's note: Sustainable agriculture maximizes the use of on farm biological controls. When off-farm materials are required, the inputs chosen must be effective, environmentally and toxicologically safe, economical, and when possible, derived from natural sources. Secondary products of plants, especially those involved in the defense of the plant from pests, are a largely untapped reservoir of natural compounds with pesticidal properties. While these materials may be equally toxic as their synthetic counterparts, their value (and one of their disadvantages) comes largely in their lack of residual activity. This chapter (with 25 references) is a review of plant-derived pesticides and the factors influencing their discovery and development.

Herbicides

Plant-derived herbicides are usually far less active than commercial herbicides. Highly phytotoxic plant compounds are not common because they could be lethal to the plant unless it developed adaptive mechanisms. While some highly phytotoxic plant chemicals have been identified, none have been developed as herbicides. Some plant compounds, such as hypericin, induce a toxic reaction in the presence of light, i.e. they are "photodynamic." Photodynamic compounds are not likely to be used as herbicides due to their toxicity to all living organisms. However, plants can be stimulated to generate their own photodynamic compounds. The contact "laser" herbicide is being developed for this purpose. This relatively safe combination of compounds causes weeds to produce phytotoxic levels of photodynamic porphyrin compounds. leading to self-destruction of the weeds.

Insecticides

Plant products are well known for their use as insecticides, insect repellents, and insect antifeedants. Several Chrysanthemum species produce pyrethroids, which have been used in Asian countries for centuries; pyrethrum is currently being used by many organic farmers and gardeners. Numerous synthetic pyrethroids have been developed; these are more active and have longer residual activity than the natural forms.

Several natural alkaloid substances have also been used commercially. These include nicotine, derived from Nicotiana species, and ryanodine, from the tropical shrub, Ryania speciosa. Carbamate insecticides were designed from physostigmine, an alkaloid in Physostigma venenosum. The use of alkaloid-based insecticides is very limited due to their high cost, toxicity to mammals, and limited efficacy.

Several plant phenolic compounds have insecticidal properties and have been correlated with host plant resistance to insects. Rotenone (derived from the roots of three genera: Derris, Lonchocarpus, and Tephrosia) was used commercially as an insecticide in the 1930s and is used by some organic gardeners today; no other phenolic compound has been used commercially as an insecticide.

Fungicides

Plants rely primarily on the production of secondary compounds for defense against pathogens; these chemicals are called phytoalexins. There is evidence that some synthetic fungicides act by inducing the production of phytoalexins in plants. Numerous phytoalexins have been identified and many have proven successful in tests against fungal infection of crop plants. Although several fungicidal and bactericidal compounds have been identified in plants, antimicrobial pesticides have not been developed to any significant extent.

Discovery and Development of Natural Pesticides

Because plants have evolved with natural mechanisms for protection, secondary compounds from plants are very likely to have activity against pests. But identifying these natural biocides is a difficult task-more difficult than identifying synthetic pesticides. The process is complicated by several factors, including the following:

• There are no guidelines regarding the amount of initial purification required for compounds.
  
• Only small amounts of secondary plant compounds are capable of being isolated. Thus, bioassays that use very small amounts of the compound are required.
  
• Structural identification of the compound can be very difficult for natural products.

Determining which pests may be controlled by compounds from a particular plant species can be made easier by identifying the pests to which the producing plant is resistant. Pyrethroid insecticides were discovered in this manner. While this process can be a major effort, some form of biological activity is likely; modern techniques for identification and purification are "shifting the odds in favor of natural compounds."

Development of highly efficacious natural pesticides can be as difficult as their discovery. Patentability of the natural compound can be an early obstacle, since prior publication of its pesticidal properties may cause patent problems. Toxicological and environmental studies are also required; natural pesticides often degrade rapidly in the environment, but many are highly toxic to mammals.

A major challenge is to find cost-effective means of producing natural pesticides. Various alternative methods of production are discussed in this article. For example, tissue or cell culture can be used, but genetic stability in the production of secondary products has been a problem. Furthermore, cells that perform the desired function of producing potentially autotoxic compounds may be selected against, unless specialized techniques are employed to remove or limit production of the toxins. Genetic engineering and biotechnology are also discussed as means to develop plant-derived secondary products.

For more information write to: USDA ARS, Southern Weed Science Laboratory, P.O. Box 350, Stoneville, MS 38776.

(CI-PEST.048) Contributed by Chuck Ingels