Maize: the gift of the gods

Aphids on a potato leaf Spider mite (Tetranychus sp.) Cotton whitefly (Bemisia tabaci)How does resistancedevelop?All organisms of a particular specieshave certain genetically-determined characteristicsin common. However, all individualswithin a species differ slightly – inother words, no two organisms of the samespecies are identical. This is because theparticular characteristics of a species areexpressed differently – according to thegenetic variation present within thespecies. The variability within a populationreflects the potential for change to occurwith regard to particular characteristics. Itis an important determinant of how capablea population is of surviving when environmentalconditions shift, and representsa significant factor in the process of evolution.The ability to adapt is latent in the genepool of a species, and it begins to showwhen the selection pressure imposed by acertain factor in the environment increases.Consider the relevance of this to the use ofinsecticides: certain individuals within apest population may already possess genetically-determinedresistance to a particularactive substance – even before it has beenapplied. Individuals possessing genes forresistance might normally only be presentas a small proportion of the population, butwhen an insecticide is applied, the selectionpressure in favour of the resistancethey possess increases. Resistance willthen spread quickly within the population,particularly if the organism has a rapidturnover of generations and reproducesitself in large numbers.Examples of pests with a strong abilityto develop insecticide resistance areaphids, white flies, mites, and various lepidopteranspecies, including the generaSpodoptera, Heliothis and Plutella. Thesepests possess the biological characteristicsdescribed above: they produce several generationsa year, and show a very highreproduction rate. This allows geneticselection and adaptation to changes inexternal factors to happen particularlyquickly in these types of insect.The development of resistance is oftenassociated with reduced biological fitnessin the absence of the factor selecting forthat resistance; so if the insecticide is nolonger used, the frequency of resistancewithin the population usually declines. Butit may remain latent, such that it can reemergeif selection pressure increasesagain, should the insecticide enter into regularuse once more.Reduced activity, or indeed completeloss of activity of insecticides, can occur asthe result of a selection process acting overseveral generations of insect pest. IRACdefines resistance as a genetically caused“reduction in the sensitivity of a population,which is reflected in repeated failureof a product to achieve the expected levelof control when used according to the labelrecommendations for that pest species andwhere problems of product storage, applicationand unusual climatic or environmentalconditions can be eliminated.”Resistance can take various forms:Metabolic resistanceThe resistant pest can degrade the activesubstance before it has a chance to expressits toxicity.Target-ResistanceThe site of action of the active substance –i.e. the molecular target in the pest – haschanged so much that the active substanceis no longer effective.Resistance to penetrationThe resistant pest takes up the active substancemore slowly and/or in lower quantitiesthan the normal, sensitive pest.Behavioural resistanceThe resistant pest avoids contact with theactive substance. This leads to reduceduptake of the insecticide, and therefore,reduced activity.Quite often, resistance is not the resultof a single mechanism, but of the interactionbetween two or more mechanisms.Which factors favour thedevelopment and expressionof resistance?The ability to develop resistancedepends strongly on the biology of a particularspecies (generation frequency, reproductionrate, genetic variability). But pestbiology is only one factor influencingresistance development. Not using pesticidesaccording to the manufacturer’srecommendations can also encourage thedevelopment of resistance.Active substances generally bind to aparticular site (receptor) in the pest organism.Compounds in the same chemicalclass tend to be active at the same receptorsite, i.e. they have a common mode-ofaction.For example, chitin synthesis18 COURIER 1/05