There are a number of mechanisms by which insects may become resistant to insecticides, including reduced insecticide penetration, metabolic de-activation and alteration of the insect’s target site, all of which reduce susceptibility to the insecticide’s mode of action. Over 30 insect species are known to have developed resistance mutations to pyrethroid insecticides, often with more than one mechanism of resistance and/or compensatory mutations that contribute to reduced insecticide efficacy. Target site resistance is implicated in the expression of knock down resistance (kdr) to pyrethroid compounds in the grain aphid Sitobion avenae (Fabricius). This is caused by a mutation (L1014F) on
the S. avenae sodium channel gene, which is associated with up to 40-fold resistance. We report bioassay data from grain aphids collected in winter barley fields in Ireland, which show a substantial, up to 5-fold variation in pyrethroid toxicity in field-collected, kdr-resistant aphid populations in Ireland. This stimulated further investigation to determine whether another, specifically metabolic mechanism of pesticide detoxification may underpin our observation of wide variation in pyrethroid toxicity. An additional bioassay incorporating exposure to the synergist piperonyl butoxide (PBO), known to inhibit the enzyme-based detoxification of pyrethroids was undertaken. This assay showed a significantly enhanced toxicity of the pyrethroid compound in some aphid populations using PBO, which strongly suggests that in addition to the known incidence of the kdr mutation (L1014F) in grain aphid populations in both Britain and Ireland, a second enzyme-based pyrethroid detoxification mechanism is present in some Irish field populations. In the light of this finding, continued over-reliance on pyrethroid insecticides is likely to further exacerbate difficulties in controlling both grain aphid and its transmission of Barley Yellow Dwarf Virus
(BYDV), by imposing strong selection pressure for additional pyrethroid resistance mechanisms. In the near absence of alternative pesticide chemistry, it will become essential that non-chemical options are explored as part of a wider Integrated Pest Management (IPM) strategy.