Correct computation of resistance ratio of an arthropod pest population from bioassay reliable and useful in insecticide and acaricide resistance management

Abstract

The resistance ratio (RR) or resistance factor (RF) measures the level of resistance of a test population or strain of arthropod pest and is useful for monitoring and detection of resistance to insecticides and acaricides for resistance management. Two research teams working on insecticide resistance in the diamondback moth (DBM), Plutella xylostella (L.) in Ghana calculated the RR of a DBM population from bioassay by dividing the susceptibility of the DBM population by the recommended field concentration, a method which is erroneous and undermines insecticide and acaricide resistance management. The correct computation of RR is by dividing the susceptibility of the test population or strain of arthropod pest by the susceptibility of a fully susceptible strain of the same species. From the correct calculation of RR, it is shown that the RR of a test population is greatly underestimated when it is calculated with the recommended field concentration. Thus, calculating the RR with the recommended field concentration does not permit the detection of resistance in its early stage of development because even when the method detects an insignificant decrease in susceptibility, there is already a significant accumulation of resistance genes in the population which renders insecticides and acaricides ineffective. When the RR is calculated correctly with a fully susceptible strain, the true resistance level of the test population is known and resistance research results are reliable and useful in the management of insecticide resistance development in arthropod pests.