toxicity - pesticides, herbicides and insecticides - Blackherbals.com

African Traditional Herbal Research ClinicVolume 8, Issue 6 NEWSLETTER July 2013FEATURED ARTICLESNatural Pesticides from PlantsBy Wesley GibbingsJanuary 13, 2013INTRODUCTIONSeveral pressures have accelerated the search formore environmentally and toxicologically safe andmore selective and efficacious pesticides. Mostcommercially successful pesticides have beendiscovered by screening compounds synthesized inthe laboratory for pesticidal properties. The averagenumber of compounds that must be screened todiscover a commercially viable pesticide hasincreased dramatically, so that new discoverystrategies must be considered. Increased emphasis onreduced-tillage agriculture will make adequatecontrol of weeds more dependent on chemicalcontrol. New herbicides will be needed to fully meetthis challenge. The increasing incidence of pesticideresistance is also fueling the need for new pesticides.Furthermore, most synthetic chemicals that have beencommercialized as herbicides are halogenatedhydrocarbons with relatively long environmentalhalf-lives and more suspect toxicological propertiesthan most natural compounds. Thus, naturalcompounds have increasingly become the focus ofthose interested in discovery of pesticides.Tens of thousands of secondary products of plantshave been identified and there are estimates thathundreds of thousands of these compounds exist.There is growing evidence that most of thesecompounds are involved in the interaction of plantswith other species-primarily the defense of the plantfrom plant pests. Thus, these secondary compoundsrepresent a large reservoir of chemical structures withbiological activity. This resource is largely untappedfor use as pesticides. This review will provide anoverview of those compounds from plants that havebeen utilized for pest control, examples of somecompounds with pesticidal potential, and a discussionof considerations in development of natural plantcompounds for pesticidal use.PLANT-DERIVED COMPOUNDS WITHPESTICIDAL POTENTIALHerbicidesInhibition of plant growth and production ofphytotoxic symptoms by certain plants and theirresidues is a well established phenomenon. Insearching for potential herbicides from plants,screening of compounds known to function in plantplantinteractions is a logical strategy. All plantsproduce secondary compounds that are phytotoxic tosome degree. However, in only a relatively few caseshas it been established that particular compoundsprovide the producing species a competitive advantageover other species that are less tolerant to thecompound. Only a few of these allelochemicals havebeen actively pursued as herbicides and, in thesecases, the natural compound has been modified. Aderivative of the terpenoid allelochemical 1,8-cineole(Fig. 1), with the common name of cinmethylin (Fig.1), is being commercially developed. Toxaphenereg.,a mixture of chlorinated camphene derivatives, wassold as a herbicide and an insecticide, but wasremoved from the market in 1982 by the EPA. Othervery weakly phytotoxic compounds from plants suchas benzoic acids can be made much more herbicidallyactive by halogen substitutions. Several benzoic acidderivatives such as dicamba (3,6-dichloro-2-methoxybenzoic acid) are widely used as herbicides.A few highly phytotoxic plant-produced compoundshave been discovered. However, none have beendeveloped as herbicides. The sesquiterpenoid lactone,artemisinin from Artemisia annua L., was found toinhibit plant growth as well as the commercialherbicide cinmethylin. Other compounds, such as 2,4-dihydroxy-1,4-benzoxazin-3-one are as active as plantgrowth inhibitors as many herbicides. Plants producemany photodynamic compounds, such as hypericin(Fig. 1), that are strongly phytotoxic, provided theyContinued on page 8483-- Traditional African Clinic July 2013