When an insecticide is being chosen for a control programme, the vector must be tested against various strengths of the insecticide to determine the discriminatory dose (this is when 99.9% mortality of the sample occurs). These tests need to be repeated from time to time during the course of the programme to determine whether the vector remains sensitive. If there are technical reasons why this cannot be done, then resistance will probably only be noticed by an increase in number of insects or cases of the disease. This might, however, indicate deficiencies in the spraying programme and these should first be ruled out. Correct application of insecticide can be measured as mentioned above, while a simple field test for suspected resistance can be performed by placing a few of the insects in a glass jar held against the sprayed surface for a minute. If they are not all killed, then resistance should be suspected and entomological assistance obtained.
Resistance may be partial or complete. If partial, then increasing the concentration of insecticide may be sufficient to control the vector. Unfortunately, complete resistance is soon likely to develop. Resistance is a genetic character and resistant strains are selected out under pressure of insecticides. Initially resistance to one insecticide occurred, but subsequently cross-resistance has developed making several insecticides ineffective. Some species now have multiple resistance. Biological control or trying a completely different strategy may be effective.
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