Pest resistance to pesticides

Ever since chemicals have been used to keep pests such as insects, weeds, fungi and worms from ruining crops, the pests have fought back by developing resistance to the chemicals. A similar problem has long plagued efforts by public health officials to eradicate disease-carrying pests such as mosquitoes and rodents. Pests are the major cause of post-harvest losses and current storage systems and technology dictate that control of these pests is based largely on their resistance to pesticides and thus is a matter of considerable concern: there is no chemical that can immediately replace malathion, currently the most widely used material. Those that may be cleared for international use in the near future are unlikely to have similar spectra of activity, economy and other attributes. Inevitably resistance will impose a considerable economic burden on the industries concerned - a burden which will increase as resistances spread and intensify and infestation levels tend towards those that existed before malathion was introduced.

Since the World Health Organization began to use DDT against Anopheles mosquitoes, the carriers of malaria, the lives of millions of people in malaria-infested parts of the world have been saved by eliminating mosquitoes, and similar successes were reported for other pests. The future seemed bright for both the farmers and the pesticide industry until the first warning came in 1947, when houseflies began to develop resistance to DDT. Insects with genes giving them resistance to DDT survived the massive sprayings in fields, gardens and homes, and these insects reproduced offspring that were also resistant. As more species of insects evolved into DDT-resistant populations, other chemicals were tried, such as organophosphates and carbamates, but many insects developed resistance to these as well.

The number of pesticide-resistant insect pest species worldwide has increased, with many such insects being resistant to even the newest chemicals. The variety and severity of pest infestations increase, thus increasing the threat to agriculture in the areas concerned. There is increasing concern throughout the world over the problems of pesticide resistance in pests which affect stored grain. Resistant strains of these pests have been shown to be moving actively in world trade and there is a serious threat to the effective use of chemicals in maintaining stored foodstuffs in a sound and insect-free condition. Insecticide resistance is now so widespread in certain species that it has become essential that resistant strains are used in the evaluation of candidate alternative materials. Currently, more than 200 species are known to be resistant to insecticides. The number of pesticide-resistant insects and mites has doubled in 12 years. The extent to which resistance can be met by increased dosages is severely limited by the level of residues internationally acceptable in stored foodstuffs. Countermeasures to resistance will thus necessitate the use of alternative insecticides and possibly fumigants; but because of stringent requirements with respect to chemical residues, there are comparatively few materials that can be used for the control of grain pests. These few materials, however, after repetitive use have led in many instances to the development of resistance.

Stored grain insects are unique in that most of the major species are cosmopolitan and readily move about in domestic and international trade. Hence, resistant strains are also moving throughout the world, reaching countries where resistance had not been suspected. Fumigation has long been regarded as a basic method of controlling stored product insects and one which would be of material assistance in delaying the development of resistance to the unrelated residual pesticides. The method itself, by virtue of the low variability in response of individual insects to the commonly-used materials such as methyl bromide, is also usually considered to be less prone to resistance development than the normal methods with residual pesticides. It is disturbing then to note the increasing prevalence of resistance to fumigants and the resultant weakening of one of the most powerful tools available in stored product pest control for delaying or preventing development of resistance. The emergence of resistance to fumigants under practical conditions is a matter for particular concern. With major world dependence on fumigation both as a routine disinfestation treatment and as a means of combating insecticide-resistant strains, the occurrences reported, although as yet limited in number and often at marginal resistance levels, are of considerable significance and pose a real threat to a continued ability to store grain safely.

Only about 1% of the applied pesticide actually reaches the target pest. The other 99% enters the ecosystem. The ecological impact of pesticides has been well documented and the list includes the near extinction of the peregrine falcon, osprey, and bald eagle, as well as many other creatures. A less well-known but far-reaching effect of pesticides is on bee populations. As early as 1944, there was evidence that pesticides were seriously affecting honeybee and wild bee populations.