How does resistance develop?
Certain individuals within a pest population may already possess genetically-determined resistance to a particular active substance – even before it has been applied. Individuals possessing genes for resistance might normally only be present as a small proportion of the population, but when an insecticide is applied, the selection pressure in favour of the resistance they possess increases. Resistance will then spread quickly within the population,particularly if the organism has a rapid turnover of generations and reproduces itself in large numbers.
Examples of pests with a strong ability to develop insecticide resistance are aphids, white flies, mites, and various lepidopteran species, including the genera Spodoptera, Heliothis and Plutella. These pests possess the biological characteristics described above: they produce several generations a year, and show a very high reproduction rate. This allows genetic selection and adaptation to changes in external factors to happen particularly quickly in these types of insect.
The development of resistance is often associated with reduced biological fitness in the absence of the factor selecting for that resistance; so if the insecticide is no longer used, the frequency of resistance within the population usually declines. But it may remain latent, such that it can reemerge if selection pressure increases again, should the insecticide enter into regular use once more.
Reduced activity, or indeed complete loss of activity of insecticides, can occur as the result of a selection process acting over several generations of insect pest.
Resistance can take various forms:
Metabolic resistance -The resistant pest can degrade the active substance before it has a chance to express its toxicity.
Target-Resistance-The site of action of the active substance –i.e. the molecular target in the pest – has changed so much that the active substance is no longer effective.
Resistance to penetration-The resistant pest takes up the active substance more slowly and/or in lower quantities than the normal, sensitive pest.
Behavioural resistance-The resistant pest avoids contact with the active substance. This leads to reduced uptake of the insecticide, and therefore,reduced activity.
Quite often, resistance is not the result of a single mechanism, but of the interaction between two or more mechanisms.
Which factors favour the development and expression of resistance?
The ability to develop resistance depends strongly on the biology of a particularspecies (generation frequency, reproduction rate, genetic variability). But pest biology is only one factor influencing resistance development. Not using pesticides according to the manufacturer’s recommendations can also encourage the development of resistance.
Wherever a pest species develops resistance to a particular active substance, it is usually also resistant to other active substances with the same mode of action. This means that crops can no longer be protected against pest attack using products based on this particular chemical class.
As many pests are not restricted to a single crop, and because crop protection agents may be used in different crops against the same pests, predictions of the risk of resistance developing should consider the entire spectrum of crops attacked by a particular pest within a region – this determines how often and how intensively a pest species is exposed to a particular active substance.
Isolated pest populations – for example those found in association with large-scale glasshouse growing – represent a particularly strong risk of resistance development.
How can the risk of resistance development be reduced?
The activity of a product can only be guaranteed when:
1. the product is only applied at the recommended rate, with the right timing, and with appropriate application equipment;
2. no more than the maximum permissible number of applications is made;
3. beneficial insects are protected, according to the spirit of integrated agriculture;
4. agronomic activities (removal of harvest residues, avoiding monocultures and short rotations) are performed as part of an ’Integrated Crop Management’approach.
Treating with reduced application rates can jeopardize the chances of successful control, especially where infestation pressure is high; it also increases the risk of resistance development. The recommended application rates should also be kept to,even when products are mixed. In fact,combining products or alternating with active substances from different chemical classes can bring about a higher level of activity, completing the spectrum of action, and reducing the risk of resistance development. Detailed information about the correct use of a crop protection agent is provided in the instructions for use that accompany every packaged product.
Resistance development in pests is a complex process. Nevertheless, it is important to remember that resistance can spread rapidly within a pest population. Therefore,all of the agronomic measures available under Good Agricultural Practice should be put to use, and the available crop protection agents should be used responsibly,in the spirit of sustainable agriculture.
(Compiled and written by Harsh saxena)