Problems of insecticide resistance

Summary

It is now thirty years since the first of the organochlorine insecticides, DDT, was introduced to control insect vectors of human diseases, and twenty-eight years since the development of resistance to DDT was suspected in the housefly, Musca domestica, and the common house mosquito, Culex pipiens. During that period resistance has developed to some insecticide group or other in one hundred and eight species of arthropods of public health importance. Although the past six years have seen only six species added to that list, these recent years have been characterized by some dramatic developments. Not only resistant populations have spread over even wider geographical areas but some species become, in the same area, simultaneously resistant to almost all available insecticides while many other species are only susceptible to compounds that many developing countries cannot afford to purchase for their public health programmes.

The paper discusses the development of resistance in anopheline, culicine and Aedes aegypti mosquitos, as well as the Culex vectors of Japanese encephalitis, and lice, fleas, blackflies, triatoma, houseflies, ticks and cockroaches, and its impact on the control of the diseases they transmit.

It could be said that, although serious, the situation is not out of hand as most of the pests and vectors are susceptible to one or several insecticides over most of their range. However, it is obvious that many vectors are already resistant to the most common and the cheapest and/or safest insecticides over extended areas where no organized campaigns were carried out against them. The resistance is very often the side result of campaigns carried out against other vectors, or most commonly of the widespread use of agricultural pesticides. The matter would soon become much more serious if further organized control programmes were carried out, thus exerting an enormous selection pressure over the vector populations over large areas.

On the other hand, agricultural pesticides are only moderately used in most developing tropical countries, but the amount being applied is steadily increasing. The variety of the chemicals used is very great as the problem of insecticide resistance in agricultural pest control is as serious, if not more serious, than in public health. It can be anticipated that this trend will continue as the food and cash crop production must be increased at all costs to face the growing demand of the world population. The development of new insecticides is becoming increasingly expensive and manufacturers are becoming more and more reluctant to invest in this area when faced with the prospect that a newly developed compound may lose its efficacy through resistance after a short period.

A substantial degree of cross-resistance to the so-called “third generation of pesticides”, the juvenile hormone mimics (insect growth regulators), chemosterilants and even biological agents has been discovered in insect vectors of disease. Evidence is now available of A. aegypti and M. domestica becoming slightly tolerant to chemosterilants, C. tarsalis to growth regulators and houseflies to growth regulators and Bacillus thuringiensis. Thus, the resistance problem will become more and more difficult in the future.