Understanding how insects resist insecticides is key to slowing down the development of resistance, which reduces the products’ effectiveness. Scientists from the unit ‘Characterisation and monitoring of phenomena in the development of resistance’ (CASPER) at ANSES’s Lyon laboratory, which is under contract to INRAE, have conducted a study into the neonicotinoid resistance mechanisms of green peach aphids. The study, which was published in the journal Pest Management Science in July 2024, focused on the neonicotinoid thiacloprid. Although this family of products is now banned in France, studying aphid resistance to neonicotinoids is still very useful for gaining a better understanding of insect resistance to plant protection products in general.

Two ways to resist insecticides

“We already knew from previous studies that the resistance of these aphids to insecticides is due to two mechanisms, but we didn't know what contribution each mechanism made to the total resistance observed”, explains Claire Mottet, a scientist in the CASPER unit and first author of the study.

One of these mechanisms is called target resistance. This type of resistance is specific to the insecticide used. Neonicotinoids act by attaching themselves to a receptor in the insect's nervous system. In insects with target resistance, a genetic mutation has altered the configuration of this receptor and the molecule is unable to bind to it, rendering the insecticide ineffective.

The second mechanism is metabolic resistance. This involves an overproduction of detoxification enzymes that enable the aphids to break down the insecticide. The production of these enzymes depends on the presence and expression of several genes.

To determine the respective contributions of these two mechanisms to aphid resistance, the scientists measured the efficacy of thiacloprid on aphid lines with and without these resistance mechanisms. Some aphids had no resistance genes, others only metabolic resistance genes, with differing numbers of copies, and still others carried both metabolic and target resistance genes. The team also used a product capable of blocking metabolic resistance to deduce the effect of target resistance.

Up to 240 times greater resistance due to the simultaneous action of the two mechanisms

Aphids carrying both mechanisms have a very high level of resistance. The median lethal concentration (LC50), i.e. the concentration needed to kill half the aphids, is up to 240 times higher for aphids carrying both resistance mechanisms than for those carrying no resistance.

The scientists discovered that the two resistance mechanisms act in synergy: in insects carrying both mechanisms, resistance exceeds what might be expected if the effect of the two types of resistance were simply added together, suggesting an interaction between the two resistance mechanisms.

“We also observed that metabolic resistance contributed to a significant proportion of total resistance. We didn't expect this result because in other insects target resistance is usually considered more important than metabolic resistance”, commented the scientist.  Moreover, metabolic resistance increases with the expression of the corresponding resistance genes, but only up to a certain threshold; beyond an expression of these genes five times greater than that of susceptible aphids, the capacity for resistance no longer seems to increase.

“A better understanding of insect resistance is important for adapting and adjusting control strategies. In the case of our study, the interaction between the two mechanisms seems too complex for any immediate application, but our results show that it is possible to gain a better understanding of the nature of the resistance observed in aphids by ascertaining the relative importance of each of the different resistance mechanisms involved”, concludes Claire Mottet.