Read the passage below and asnswer the following questions
Chemical pesticides lose their role in sustainable agriculture if the pests evolve resistance. The evolution of pesticide resistance is simply natural selection in action. It is almost certain to occur when vast numbers of a genetically variable population- are killed. One, or a few individuals may he unusually resistant (perhaps because they possess an enzyme That can detoxify the pesticide). If the pesticide is applied repeatedly, each 'successive generation of the pest will contain a larger proportion of resistant, individuals. Pests typically have a high intrinsic rate of reproduction, and so a few individuals in one generation may give rise to hundreds or thousands in the next, and resistance spreads very rapidly in a population.
This problem was often ignored in the past, even though the first case of DDT  resistance was reported as early as 1946. There is exponential increase in the numbers of invertebrates that have evolved resistance and in the number of pesticides against which resistance has evolved. Resistance has been recorded in every Family of arthropod pests (including dipterans such as mosquitoes and house flies, as well as beetles, moths, wasps, fleas, lice and mites) as well as in weeds and plant pathogens. Take the Alabama leaf-worm, a moth pest of cotton, as an example. It has developed resistance in one or more regions of the world to aldrin, DDT, dieldrin, endrin, lindane and toxaphene.
If chemical pesticides brought nothing but problems, if their use was intrinsically and acutely unsustainable then they would already have fallen out of widespread use. This has not happened. Instead, their rate of production has increased rapidly. The ratio of cost to benefit for the individual agricultural producer has remained in favour of pesticide' use. -fin the USA, insecticides have been estimated to benefit the agricultural products to the tune of around $5 for every $1 spent.
Moreover, in many poorer countries, the prospect of imminent mass starvation, or of an epidemic disease, are so frightening that the social and health costs of using pesticides have to be ignored. In general the use of pesticides is justified by objective measures such as ‘lives saved’, ‘economic efficiency of food production’ and ‘total food produced’. In these very fundamental senses, their use may be described as sustainable. In practice, sustainability depends on continually developing new pesticides that keep at least one step ahead of the pests pesticides that are less persistent, biodegradable and more accurately targeted at the pests.