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PREVENTION AND MANAGEMENT OF INSECTICIDE RESISTANCE

Thursday, 23rd of February 2012

• PREVENTION AND MANAGEMENT OF INSECTICIDE RESISTANCE

Excerpts from ‘Different approaches to resistance management’

Full text is at

http://www.irac-online.org/wp-content/uploads/2009/09/VM-Layout-v2.6_LR.pdf

4.1 Approaches to resistance management

Resistance management can be attempted using insecticide-based approaches in conjunction with other non-insecticidal vector-control methods (integrated vector and pest management; see also chapters 5.2 and 10.3). In practice, many integrated control programmes work well in experimental trials, but become inoperable when scaled-up into longterm control programmes. Operationally, the simplest form of resistance management is likely to be insecticide-based, and this could take several forms. 

4.1.1 Rotation

Rotational strategies are based on the rotation over time of two or preferably more insecticide classes with different modes of action.

This approach assumes that if resistance to each insecticide is rare then multiple resistance will be extremely rare. Hence, any resistance developed to the first insecticide will decline over time when the second insecticide class is introduced. The time frame for rotation needs to be sufficiently short for resistance to still revert rapidly after it has been selected for.

Although with most vector-borne disease-control programmes annual rotation is practical, the rotation of several classes of insecticides (with different modes of action) within a growing season is practised in many agricultural cropping systems.

 

4.1.2 Mixtures

The use of mixtures to avoid the development of antibiotic (and plant-pathogen) resistance is common. Once again, the theory is that if resistance to each of the two insecticide compounds within a mixture is rare then multiple resistance to both will be extremely rare. This approach is unlikely to be successful if resistance to one of the insecticides used is already present at a detectable level. The use of tank mixes is a relatively easy resistance-management tactic to implement and can have other benefits in terms of an improved spectrum of activity, particularly in agricultural systems. However, for mixtures to work well in practice both insecticides need to be used at their full operational target dose, and the efficacy and persistence of the two insecticides should be broadly similar. Tank mixing of products is rarely adopted in vector-control programmes on grounds of cost, safety and the limited number of recommended compounds. However, this should not preclude further investigations of the use of mixtures as a means of managing resistance in vector populations in future. 

4.1.3 Fine-scale mosaic

Spatially separated applications of different compounds against the same insect constitute a “mosaic” approach to resistance management.

Fine-scale mosaics can be achieved in vector-control programmes, for example, by using two insecticides in different houses within the same village. This creates the potential for insects within a single generation to come into contact with both insecticides, and would reduce the rate of resistance selection – provided that multiple resistance within the vector population was extremely rare. If such a fine-scale mosaic is to be used, careful records of which insecticide was used in each house are essential.

Research is currently under way looking at mosquito nets treated with two insecticides with differing modes of action. This achieves a similar mosaic effect to treating houses with different compounds but on a much finer scale.

 

For resistance-management purposes, each insecticide product (X) will belong to one chemical group (e.g. OP´s). A given insect population may contain individuals naturally resistant to X (and to other OPgroup insecticides) and these individuals will become the dominant type if such insecticides are used repeatedly. Eventually these resistant insects may not be controlled by X (or any other OP insecticides), and local experts and commercial distributors should be consulted for local resistance management recommendations. Although the classification scheme shown in Table 1 is based on mode of action, resistance in insects and mites to insecticides and acaricides, respectively, can also result from enhanced metabolism, reduced penetration or behavioural changes as outlined in chapter 3. These are not linked to any site-of action classification, but are specific to chemical classes and sometimes even to individual chemicals. Despite this, alternation of compounds from different chemical classes remains a viable management technique, and to delay insecticide resistance: 

• avoid exclusive repeated use of insecticides from the same chemical subgroup;

 

• integrate other control methods (chemical, cultural, biological) into insect-control programmes.

 

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4.4 Summary points

 • Successful resistance management depends upon reducing the selection pressure exerted by a particular mode of action or chemistry on a population. 

• Selection pressure can be reduced through a number of strategies, including rotation, the use of insecticide mixtures, and mosaic applications.

 

• The IRAC mode of action classification scheme is an up-to-date and accurate guide which may be used in formulating resistancemanagement guidelines.