Mechanism and management of insecticide resistance
Author: Gugulotu Laxman
Division of Entomology, ICAR-Indian Agricultural Research Institute, New Delhi-110012
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A heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species (IRAC).

Mechanisms of Resistance

Agricultural insect pests use a variety of mechanisms to survive exposure to toxicants. Resistance can develop more easily when two or more of these mechanisms are used at the same time. The resistance mechanisms fall into the following general categories:

• Detoxification (enzymatic): Resistance through detoxification is most often found in insects. It is based on enzyme systems that insects have developed to detoxify naturally occurring toxins found in their host plants and in the blood ingested by blood feeding insects. These systems include esterases, cytochrome P450 mono-oxygenases, and glutathione S-transferases. Resistant insects may have elevated levels of a particular enzyme or altered forms of the enzyme that metabolize the pesticide at a much faster rate than the non-altered form.

• Reduced sensitivity of target-site: The site where the toxin usually binds in the insect becomes modified to reduce the insecticide's effects. This is the second most common mechanism of resistance

• Reduced penetration: Resistant insects may absorb the toxin more slowly than susceptible insects. Penetration resistance occurs when the insect's outer cuticle develops barriers which can slow absorption of the chemicals into their bodies. This can protect insects from a wide range of insecticides but this mechanism produces only low levels of resistance. Penetration resistance is frequently present along with other forms of resistance, and reduced penetration intensifies the effects of those other mechanisms.

• Sequestration: In insects (aphids, Culex mosquitoes, etc.) metabolic enzymes are significantly amplified (up to 15% of the total body protein) and bind to the insecticide but the insecticide is not metabolised, i.e. the insecticide is sequestered.
• Behavioural resistance: It refers to any modification in the organism's behaviour that helps to avoid the lethal effects of insecticides. This mechanism of resistance has been reported for several classes of insecticides, including organochlorines, organophosphates, carbamates and pyrethroids. Insects may simply stop feeding if they come across certain insecticides, or leave the area where spraying occurred

Factors favorable for rapid development of resistance

Biological factor:

1. Generation turnover: many generations per year

2. Population size is a major factor in the development of resistance. If the population is large, even if the percentage of resistant individuals is low, the number of survivors following a pesticide application could be a rather large.

3. Reproductive potential

4. Insects multiply by asexual reproduction

5. Pesticide metabolism: Increased metabolic degradation of a insecticide is one of the resistance mechanisms found in insects and mites

6. Number of target sites of the pesticide: Resistance develops more quickly when a pesticide has a single target site. If a pesticide has multiple target sites, the pest has to develop resistance at all of these sites.

7. Pest host range: Pests with a wide host range, infesting more crops, may have a higher risk of developing resistance than pests which are very crop-specific

Genetic factors:

1. Occurrence of resistance genes

2. Number of resistance mechanisms

3. Frequency of resistance genes

4. Dominance of resistance genes

5. Fitness of resistance individuals

6. Cross resistance

Operational factors:

1. Activity spectrum of the pesticide

2. Pesticide application rate

3. Presence of secondary pests

4. Pest control tactics

5. Coverage

6. Treatment frequency

Resistance management

1. Monitor insect population development in fields to determine if and when control measures are warranted.

2. Avoid broad spectrum and persistence insecticide sprays early in a crop cycle to encourage beneficial insects

3. Leave untreated population refuges where susceptible pests can survive and insert their genetics into populations

4. Use alternate insecticides with a different mode of action

5. Encouragement of biological controls

6. Judicious application of insecticides

7. Microencapsulation

8. Development of the newer molecules

9. Using insecticide mixture

10. Use of more dosages of insecticides to kill all the population including heterozygous individuals

11. Rotation of the insecticides

12. Use of synergists

13. Use of negatively correlated insecticides

14. Use of insect pheromones and insect hormones

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