An insect is generally considered resistant when it is able to withstand a dose of insecticide that would kill other individuals of the same species (Brown & Pal, 1971). Development of resistance in insect populations usually becomes apparent when insecticide treatments show poor results. There are several mechanisms by which insects can become resistant and often they exhibit more than one of these mechanisms at the same time.
- Penetration resistance: Decreased cuticular penetration has been considered a resistance mechanism since first described in the housefly (Forgash et al, 1962). This mechanism only confers low-levels of resistance, typically no higher than 5-fold, but usually does so to a wide range of insecticides and is often found in association with other mechanisms which intensifies their effects (Meng et al, 2023).
- Metabolic resistance: Having penetrated the cuticle, insecticides would then be subject to the detoxification systems that have evolved in both susceptible and resistant insects to protect them from the natural toxicants encountered in their environments (Philippou et al, 2010). These metabolic enzymes can render the toxicant less toxic or more easily excreted, or both. Resistant insects may possess a higher amount of the same enzymes than a susceptible or more efficient enzymes. Metabolic resistance usually confers a broad range resistance and is the most common mechanism reported.
- Altered target-site resistance: Having finally reached its site of action, the insecticide may find molecular modifications to the target protein resulting in insensitivity and thus resistance (Bass et al, 2011). This can lead to high levels of resistance although it is usually found in conjunction with metabolic resistance. Target-site resistance is the second most common mechanism reported.
Apart from these three main mechanisms, behavioural resistance is sometimes cited (Sanou et al, 2021). This is where insects appear to avoid the toxin, for example by moving to the underside of a sprayed leaf or by removing themselves away from a sprayed area entirely.
Having considered the mechanisms by which insects can become resistant to insecticides, how can this be reduced? It is important to consider an integrated pest management (IPM) approach (Horowitz, 2020). This can help to minimise resistance problems as well as minimise environmental concerns. IPM programmes will typically include both synthetic and biological insecticides and include consideration of beneficial arthropods (e.g. parasites and predators as well as pollinators). When possible, use of broad-spectrum insecticides should be minimised in favour of pest-specific insecticides. Much attention has been paid to rotation of insecticide classes to minimise increasing resistance. This is to avoid repeated application of insecticide(s) with the same mode of action, which can lead to increased resistance and/or cross-resistance. Instead, it is preferable to rotate insecticides with several modes of action to decrease the selection pressure on any given target protein. This should help to minimise target-site resistance, though it is questionable as to whether it helps minimise metabolic resistance.
Apart from these methods, other avenues include expanses of refugia which should help to preserve susceptible or less-resistant individuals which may have a fitness advantage over the resistant insects and so help to dilute the resistant gene pool within a population (Towles, 2021).
REFERENCES
- Bass C, Puinean AM, Andrews MC, et al. Mutation of a nicotinic acetylcholine receptor β subunit is associated with resistance to neonicotinoid insecticides in the aphid Myzus persicae. BMC Neurosci, 12, No. 51 (2011).
- Brown AWA & Pal R, Insecticide resistance in arthropods. Wld Hlth Org Monograph series No. 38 (1971)
- Forgash AJ, Cook BJ and Riley RC, Mechanisms of resistance in diazinon-selected multi-resistant Musca domestica. J Econ Entomol. 55, 544-551 (1962).
- Horowitz AR, Ghanim M, Roditakis E. et al. Insecticide resistance and its management in Bemisia tabaci species. J Pest Sci 93, 893–910 (2020).
- Meng LW, Yuan GR, Chen ML. et al. Cuticular competing endogenous RNAs regulate insecticide penetration and resistance in a major agricultural pest. BMC Biol 21, 187 (2023).
- Philippou D, Field LM and Moores GD, Metabolic enzyme(s) confer imidacloprid resistance in a clone of Myzus persicae (Sulzer) (Hemiptera: Aphididae) from Greece. Pest Manag. Sci. 66, 390, (2010).
- Sanou A, Nelli L, Guelbéogo WM et al. Insecticide resistance and behavioural adaptation as a response to long-lasting insecticidal net deployment in malaria vectors in the Cascades region of Burkina Faso. Sci Rep 11, 17569 (2021).
- Towles TB, Buntin GD, Catchot AL et al., Quantifying the Contribution of Seed Blended Refugia in Field Corn to Helicoverpa zea (Lepidoptera: Noctuidae) Populations, J Econ Ent, 114,1771-1778, (2021).