Main Objectives
• Rotation (alternation) of modes of action {MoA) is the most effective insecticide resistance management {IRM) strategy.
• Insecticides should be used at the full label dose.
• Mixtures of insecticides should include both mixture components at the doses used when applied solo - a 'double dose' mixture.
Mechanistic modelling studies {AHDB cross-commodity project RD-2012-3780) have shown that that these strategies may be counter-productive and speed up the development of resistance.
For most plausible scenarios, relevant to a wide range of pest species on arable and horticultural crops, we have shown that:
• Mixtures are likely to result in longer effective lives for insecticide MoA than alternation.
• Resistance is minimised by using the lowest dose at which robust control can be obtained.
• Adjusting the dose of mixture components to achieve the same combined efficacy as a solo product is a better IRM strategy. Such mixtures are more likely to be approved by regulators than 'double dose' mixtures.
These conclusions mean that the guidance for insecticides would no longer conflict with the guidance for fungicide resistance management. However, such substantial changes to longstanding guidance require a high level of proof. The industry is more likely to be convinced of the need for change if there is experimental evidence in a pest species of major cross-commodity importance. This project will provide such evidence, to prolong the effective life of existing chemistry and new active substances currently in the pipeline.
Myzus persicae, the most important vector of virus yellows, is resistant to several MoA in the UK (including pyrethroids), so managing future evolution of resistance is vital. This work will be crucial for the continued control of virus yellows vectors, as part of an integrated control strategy with improved beet varieties with yellows tolerance.
Effective resistance management strategies identified from the modelling work developed as part of AHDB cross-commodity project RD-2012-3780 will be validated in replicated, randomised 'cage' trials. This method overcomes issues of unwanted movement between plots which have, for example, affected resistance management field trials artificially inoculated with fungal pathogens. The modelling results showed the factors that had the largest impact on which IRM strategy was the most effective were the rate of immigration of less resistant individuals into the insect population being treated, and the mortality (i.e. efficacy) achieved by a given dose of insecticide.
Starting populations of M. persicae, consisting of a specific size and ratio of susceptible and resistant genotypes (each conferring resistance to a different MoA), would be exposed to the following IRM strategies:
1. Alternation of two insecticides with different MoA applied at their label dose.
2. Mixtures of two insecticides with different MoA applied at their label dose.
3. Mixtures of two insecticides with different MoA applied at a reduced dose that together results in the same mortality as a single label dose.
The effect of no and high immigration would also be assessed for each of these IRM strategies. The proportion of resistant and susceptible genotypes would be assessed throughout the experiment using molecular methods to determine the effect of the IRM strategy on the populations of resistant and susceptible genotypes. M. persicae would be used as the test pest species, as it is important in a wide range of agricultural and horticultural crops, has a short generation time (so multiple generations of resistance evolution can occur in a season) and resistant genotypes to a number of insecticide MoA are available. The results would be compared with model predictions.
