The Interaction of Resistance Factors

Some resistance factors provide only minor levels of resistance (two- to fourfold), so they would not be expected to add significantly to resistance levels. However, resistance factors normally do not occur alone. They frequently interact in a synergistic way so that the combined effect is greater than the sum of the individual effects. The most common interaction is that between penetration and metabolism. Interactions also occur between kdr and P450s, between nerve insensitivity and P450s, and between various other combinations of these factors. 

The combination of various resistance factors can act multiplicatively. For example, Yeoh et al. (1981) showed that a 519-fold resistance to malathion was observed in oriental houseflies having a 5-fold increase in carboxylesterase activity, a 10-fold increase in GST activity, and a 10-fold increase in AChE insensitivity (i.e., 5 x 10 x 10 = 500). Similarly, a 238-fold resistance to fenitrothion was found to have a 10-fold increase in GST activity and a 25- to 30-fold increase in AChE insensitivity (i.e., 10 x 25 = 250). 

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