toxicity - pesticides, herbicides and insecticides - Blackherbals.com

Continued from page 85 – Natural Pesticides from PlantsRodenticidesPlants produce a myriad of compounds that arepoisonous to mammals. Some of these, such asstrychnine (Fig. 3), are used in commercialrodenticides. The chronic poison warfarin and severalanalogues are coumarin derivatives, This chemistry ledto discovery of indanediones and 4-hydroxy-2H-1-benzopyran-2-ones as rodenticides.FACTORS INFLUENCING DEVELOPMENT OFNATURAL PESTICIDESDiscoveryThe secondary compounds of plants are a vastrepository of compounds with a wide range ofbiological activities. This diversity is largely the resultof coevolution of hundreds of thousands of plantspecies with each other and with an even greaternumber of species of microorganisms and animals.Thus, unlike compounds synthesized in the laboratory,secondary compounds from plants are virtuallyguaranteed to have biological activity and that activityis highly likely to function in protecting the producingplant from a pathogen, herbivore, or competitor. Thus,a knowledge of the pests to which the producing plantis resistant may provide useful leads in predicting whatpests may be controlled by compounds from aparticular species. This approach has led to thediscovery of several commercial pesticides such as thepyrethroid insecticides. Isolation and chemicalcharacterization of the active compounds from plantswith strong biological activities can be a major effortcompared to synthesizing a new synthetic compound.However, the assurance of biological activity andimprovement in methods of purification and structuralidentification is shifting the odds in favor of naturalcompounds.Considering the probability of plant secondaryproducts being involved in plant-pest interactions, thestrategy of randomly isolating, identifying, andbioassaying these compounds may also be an effectivemethod of pesticide discovery. Biologically activecompounds from plants will often have activity againstorganisms with which the producing plant does nothave to cope. Many secondary compounds described inthe natural product, pharmacological and chemicalecology literature have not been screened for pesticidalactivity. This is due, in part, to the very small amountsof these compounds that have been available forscreening.The discovery process for natural pesticides is morecomplicated than that for synthetic pesticides (Fig. 4).Traditionally, new pesticides have been discovered bysynthesis, bioassay, and evaluation If the compound issufficiently promising, quantitative structure-activityrelationship-based synthesis of analogues is used to optimizedesirable pesticidal properties. The discovery process withnatural compounds is complicated by several factors.First, the amount of purification initially conducted is avariable for which there is no general rule. Furthermore,secondary compounds are generally isolated in relativelysmall amounts compared to the amounts of synthesizedchemicals available for screening for pesticide activity.Therefore, bioassays requiring very small amounts ofmaterial will be helpful in screening natural products fromplants. A number of published methods for assaying smallamounts of compounds for pesticidal and biologicalactivities are available in the allelochemical and naturalproduct literature. At some point in the discovery process,structural identification is a requirement. This step can bequite difficult for some natural products. Finally, synthesisof the compound and analogues must be considered. This isgenerally much more difficult than identification. Despitethese difficulties, modern instrumental analysis andimproved methods are reducing the difficulty, cost and timeinvolved in each of the above steps.DevelopmentFew pesticides that are found to be highly efficacious intesting are ever brought to market. Many factors must beconsidered in the decision to develop and market a pesticide.An early consideration is the patentability of the compound.A patent search must be done for natural compounds as withany synthetic compound. Prior publication of the pesticidalproperties of a compound could cause patent problems.Compared to synthetic compounds, there is a plethora ofpublished information on the biological activity of naturalproducts. For this reason, patenting synthetic analogues withno mention of the natural source of the chemical familymight be safer than patenting the natural product in somesituations.The toxicological and environmental properties of thecompound must be considered. Simply because a compoundis a natural product does not insure that it is safe. The mosttoxic mammalian poisons known are natural products andmany of these are plant products. Introduction of levels oftoxic natural compounds into the environment that wouldnever be found in nature could cause adverse effects.However, evidence is strong that natural products generallyhave a much shorter half-life in the environment thansynthetic pesticides. In fact, the relatively shortenvironmental persistence of natural products may be aproblem, because most pesticides must have some residualactivity in order to be effective. As with pyrethroids,Continnued on page 8786-- Traditional African Clinic July 2013