What is insecticide resistance?
According to Insecticide Resistance Action Committee (IRAC), 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 is known as insecticide resistance. Frequent use or overuse/misuse of an insecticide is one of the main reasons for the resistance development in various insect pests and the consequent evolution of insecticide-resistant pest populations. The resistance levels greatly depend on the selection pressure on the pest species affected by its biology, insecticide specificity, and the insecticide application strategy used.
The mechanism or way an insecticide targets an insect is called the mode of action (MoA). Insecticides belonging to a specific chemical group having a common target site are designated by the same number and letter by IRAC. Applicators should incorporate a sequence of diverse MoA classes into their pest management programs to avoid or delay the development of insecticide resistance. The common mode of action creates a significant risk of cross-resistance to all the compounds in the same sub-group. It is advised to use alternations, rotations, or sequences of different insecticide MoA classes to avoid resistance or cross-resistance selection.
Some recent resistance cases have been reported in different insect pests. About 75 Bt resistance cases have been reported in Helicoverpa armigera in USA to date. Similarly, acetamiprid resistance in melon aphid (Aphis gossypii) and spinosad resistance in field populations of melon fly (Zeugodacus cucurbitae) have been reported.
Insecticide resistance mechanisms
1. Metabolic resistance Metabolic resistance is the most common type of resistance, where an insect can get rid of or clear its body of a toxic compound faster than the other susceptible insects.
2. Target-site resistance Target-site resistance occurs when the insecticide cannot connect at the site of action in an insect, minimizing the insecticidal effect of that insecticide.
3. Penetration resistance Penetration resistance is when the insect’s cuticle creates blockage, eventually slowing down insecticide absorption into their bodies. Penetration resistance is usually present along with other forms of resistance and can aggravate insects’ other resistance mechanisms.
4. Behavioral resistance When an insect can sense or detect insecticide danger and avoid the treated areas, it is known as behavioral resistance. Insects can simply move away or stop feeding at the application area to avoid an insecticide’s toxic effects.
The most effective strategy to manage insecticide resistance is to adopt an integrated approach to prevent its occurrence in the first place.
Insecticide Resistance Management (IRM) components include:
1. Closely monitor pest population and natural enemies. Management tactics should be used when the population exceeds economic thresholds.
2. Integrate multiple control strategies. Incorporate softer chemistries, biological insecticides, beneficial insects (predators/parasites), cultural practices, transgenic plants (where allowed), crop rotation, pest-resistant crop varieties, and chemical attractants or deterrents.
3. Read the product label. Follow the insecticide usage guidelines as well as resistance management recommendations on the label.
4. Refuges. Some unsprayed population refuges can be left adjacent to treated areas to allow the survival of susceptible insect individuals that may outcompete the resistant insects by diluting the potential impacts of any resistance that may have developed previously. This strategy is mainly used in genetically modified crops such as corn and cotton.
5. Assess if the resistance is “true”. It is important to realize that resistance isn’t always the problem. Poor application techniques i.e., wrong calibration, improper tank mixtures, and failure to use proper adjuvants resulting in inadequate coverage may also result in insufficient control. Proper identification and biology to target the most vulnerable stage of insect pests play a critical role (e.g., for aphids with multiple overlapping generations, rotate the products from different insecticide MoA groups for each spray).
Resources:
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries. UC ANR Publication 3392. https://ipm.ucanr.edu/agriculture/floriculture-and-ornamental-nurseries/...
Resistance management for sustainable agriculture and improved public health. https://irac-online.org/about/resistance/management/
Benda N. and A. Dale. Managing Insecticide and Miticide Resistance in Florida Landscapes. https://edis.ifas.ufl.edu/publication/IN714
Arthropod Pesticide Resistance Database | Michigan State University. https://www.pesticideresistance.org/display.php?page=species&arId=572