Patterns of interactions between mechanisms of insecticide resistance

Insecticide resistance is an ever growing problem worldwide and is present in many insect pest populations. Insects can employ more than one resistance mechanism (multigenic resistance) when attempting to survive in an insecticide treated environment resulting from the application of multiple insecticide classes or to survive high doses of a single compound. Interactions occurring between resistance mechanisms can maintain multigenic resistance dictating the level of resistance which ultimately alters the effectiveness of a chemical control strategy. Interactions can be of an additive or epistatic type. Epistasis is the non-additive (synergistic or antagonistic) interaction between differing loci which contribute to a phenotype. Given that a limited number of mechanisms are responsible for resistance, it may be possible to observe patterns and establish rules for the type of interaction that results from a given scenario. Reviewing the available literature from different insects reveals the following patterns: multiple mechanisms in the homozygous genotypic condition interact synergistically, while multiple mechanisms in the heterozygous condition interact additively, and in scenarios where one mechanism is homozygous and another is heterozygous the interaction is of the synergistic type. Possible factors that drive these general patterns, observed exceptions, and future research needs are discussed.