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Scientific background

Today’s agricultural production is under the influence of global competition, environmental regulations and public scrutiny. To maintain competitiveness, large and intensive production systems are employed, which has increased pest pressure on animals and crops. Even though crop protection practices are moving to more integrated approaches, pesticides are and will continue to be a vital component of crop protection measures. Preserving the efficacy of pesticides is therefore vital to secure food production, but pesticide resistance increasingly compromises pest control.


In recent years the number of pesticide resistant biotypes has increased significantly worldwide and pesticide resistance is regarded a major threat to sustain effective pest control. Figuratively speaking agrochemical companies and farmers are locked in an “arms race” that they seem to be losing, in particular with respect to weeds due to a declining rate of discovery of new herbicide modes of action (Rüegg et al. 2007, Hollomon 2012). More diverse cropping systems have delayed the development of pesticide resistance in Denmark in comparison to some of our neighboring countries, but in recent years a significant increase in pesticide resistance has been reported. The situation is further exacerbated by the fact that many old compounds have and will be withdrawn due to stricter EU and national regulation. Furthermore the number of pesticides with new modes of action has been decreasing within the last 20 years. Recently the Danish government proposed a new pesticide taxation scheme that will add to this development because compounds with some of the less resistant-prone modes of action will become considerably more expensive.  Safeguarding the existing arsenal of pesticides against resistance is more important than ever to secure crop production and the earning capacity of Danish agriculture.          


More intelligent use of pesticides making them “evolution-proof” can be achieved by targeting specific life stages or ensuring refuge of susceptible individuals, as well as rotating pesticides with different modes of action or negative cross-resistance (Thrall et al. 2011, Read et al. 2009). Most of the resistance management methods are expecting major gene or target-site resistance not to be associated with a fitness cost while minor gene or non-target resistance is. Theoretically this ensures reversion of non-target resistance, when not using pesticides, while target-site resistance biotypes will not disappear from the population but the empirical data on this aspect are few. It is not unlikely that minor genes were selected during the early stages of selection for major gene resistance (Neve et al. 2009, Gressel 2010) and that this selection of minor genes was associated with a loss in fitness that was offset during the process of selection (Anderson 2003). A better knowledge of genetic variation in pesticide susceptibility and fitness costs during the early stages of selection is required and should be incorporated in resistance management models (Neve et al. 2009).   


Resistance research traditionally focuses on genetic and physiological interactions at the organism level. The dynamics of resistance at the population level are much more difficult to study experimentally. Recent developments in functional genomics, and the availability of genome sequences of many animals, plants and pathogens, as well as advances in bioinformatics, provide promising tools for rapid characterization of pest organisms at population level. We will take advantage of these new technologies to unravel pesticide resistance mechanisms as well as determine the fitness cost of resistance (Ellegren & Sheldon 2008), which is an important parameter in management strategies (Hendry et al. 2011).


Molecular studies of insecticide, herbicide and fungicide resistance have revealed polymorphisms in the major targets of conventional pesticides. These studies led to the paradigm that a single nucleotide replacement in a major resistance gene can confer resistance and identical replacements can occur across a wide range of species. However, in some cases the molecular basis of resistance is more complex as it involves metabolic detoxification by minor resistance genes e.g. cytochrome P450 monooxygenases (P450s), delayed activation or alterations in uptake. In the latter case cross-resistance thus extends from one group of active compounds to another. The similarities in resistance mechanisms between insect pests, weeds and fungi suggest that a trans-organismal study of resistance development could be beneficial for the development of new evolution-proof pest management tools.