Arthropod resistance to synthetic and natural insecticides and acaricides

Resistance is defined by WHO (1992) as 'an inherited characteristic that imparts an increased tolerance to a pesticide, or group of pesticides, such that resistant individuals survive a concentration of the compound(s) that would normally be lethal to the species'. In practice, the operational criterion is usually taken as 20% or more survivors of individual arthropods tested to the normally used diagnostic concentration of the pesticide, using WHO test kits in the field.

The appearance of insecticide-resistant vector or pest insect population is practically inevitable provided that the particular insecticide is used above a certain level for a certain duration of time. The genes providing for susceptibility of the arthropod vector populations to particular groups of pesticides may be considered a non-renewable resource that should optimally be maintained by all people engaged in vector or pest control operations. Since there are only a very limited number of pesticides available for use in vector control programmes, these chemicals should be regarded as a valuable resource and protected accordingly (WHO 1992).

According to Miller (1988), the types and mechanisms for insecticide resistance can be grouped within four categories:

1 Behavioural resistance, where insect behaviour becomes modified so that the insect no longer comes into contact with the insecticide.

2 Penetration resistance, where the composition of the insect exoskeleton becomes modified in ways that inhibit insecticide penetration.

3 Site-insensitivity, where the chemical site of action for the insecticide becomes modified to have reduced sensitivity to the active form of the insecticide.

4 Metabolic resistance, where the metabolic pathways of the insect become modified in ways that detoxify the insecticide, or disallow metabolism of the applied compound into its toxic form. The most important mechanisms of metabolic resistance involve multifunction oxidases, glutathione-S-transferase, and esterases in the case of pyrethroids (which are almost all esters). In general, site-insensitivity or metabolic detoxification are the main resistance mechanisms, and physiological resistance can be seen as resulting from an interplay of these factors (Miller 1988).

Management of pesticide resistance: Based on the previous statements all efforts should be done to prevent or at least delay the appearance of insecticide resistance among

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important disease vector arthropods and other pest populations. To extend the time of effective use of pesticides involves a reliable system of surveillance, early detection, and accurate monitoring should be used (WHO 1992). This is even more obvious when considering that cross-resistance and multiple resistance, to chemicals to which an arthropod has never been exposed, will often evolve. Resistance management may be achieved by (WHO 1992; Roberts and Andre 1994):

• The use of non-chemical, rather than chemical control, if feasible.

• Agricultural pesticides can select for resistance in mosquitoes. In general, the resistance selected for is so broad that only by banning whole classes of chemicals could the insecticides against mosquitoes be fully safeguarded. For instance, to prevent resistance to malathion in malaria vectors breeding in rice field one would have to ban the agricultural use of all organophosphates and carbamates (Lines 1988).

• Agricultural use of pesticides should be drastically limited and taken into consideration regarding the promotion of development of resistant arthropods of public health importance. For instance, pyrethroid-impregnated bed-nets may become less effective because of the widespread use of pyrethroids in agriculture, especially in paddy fields where the malaria vector may breed.

• The use of mixtures of chemically unrelated pesticides. The basic assumption is that vectors resistant to pesticide A will be killed by pesticide B and therefore less likely to produce any resistant offspring. The absence so far among medical vectors of resistance to Bacillus thuringiensis israelensis is believed to be because it contains several toxic proteins.

• Application against a single life stage (adult females rather than both sexes or all life stages) of the target species.

• The use of pesticides of relatively low persistence (since pesticides which degrade rapidly are less likely, compared to more persistent chemicals, to produce pesticide resistance.

• Use of pesticides that produce resistance only within a single class of pesticides.

• Varying the dose or frequency of pesticide application.

• Application of pesticides in limited areas, with high levels of disease transmission, rather than area-wide; the former method will reduce the selection pressure.

• Limitation of pesticide application to periods of the year when the vector problem is important.

• Alternating or rotating among different (unrelated) pesticides. If two pesticides, A and B, are used sequentially the resistance level to A will decrease when B is used.

• Use of the most appropriate pesticide formulations.

• Use of synergists if feasible. These chemicals inhibit specific detoxification enzymes and thereby reduce the selective advantage of arthropods that have such enzymes.

• Integrated control, including biological control and environmental management should be used wherever it is appropriate. With regard to pesticide usage, environmental management, and biological control will reduce the selection pressure and prolong the effective duration of life of the pesticide used.

• The development of biological control agents which are resistant to certain pesticides used in agriculture and/or public health.

• Intensified research on identification of new pesticides with different metabolic pathways than the ones presently available.

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