Assessment of the Dominance Level of the R81T Target Resistance to Two Neonicotinoid Insecticides in Myzus persicae (Hemiptera: Aphididae).

Myzus persicae (Sulzer, 1776), a major crop pest worldwide, displays insecticide resistance to most molecules. The R81T substitution on the ß1 subunit of nicotinic receptors of acetylcholine (nAChR) confers target site resistance to neonicotinoids and is widespread in aphid populations colonizing peach tree orchards in Southern Europe. But the impact of this resistance in the field, as well as ways to optimize its management, depends largely on the dominance level of the R81T mutation. In this study, we measured by in vitro assays the response of R81T mutation to two neonicotinoids (imidacloprid and thiacloprid) in 23 M. persicae clones with different resistance genotypes in order to assess the dominance status of this allele. In this study, all homozygous clones for the R81T mutation (genotype 81(TT)) showed a much higher level of resistance to both active substances than other clones. The heterozygous clones 81(RT) displayed a slightly higher level of resistance than wild homozygous, though resistance phenotypes against both neonicotinoids in these two genotypes were overlapping. A great variation of resistance level was found within these two latter clones' categories. The dominance level of insecticide resistance (DLC) strongly suggested that the mutant allele 81T is semirecessive (the wild 81R allele being rather dominant) for both insecticide molecules under test. Mean DLC values were 0.316 for imidacloprid and 0.351 for thiacloprid. Cross-resistance was shown between imidacloprid and thiacloprid. This partial recessivity is valuable information to broaden the knowledge on neonicotinoid resistance, a prerequisite for devising adapted management strategies against insecticide-resistant populations of M. persicae.

Molecular features and toxicological properties of four common pesticides, acetamiprid, deltamethrin, chlorpyriphos and fipronil.

Structural features and selected physicochemical properties of four common pesticides: acetamiprid (neonicotinoid), chlorpyriphos (organophosphate insecticide), deltamethrin (pyrethroid) and fipronil (phenylpyrazole) have been investigated by Density Functional Theory quantum chemical calculations. The high flexible character of these insecticides is revealed by the numerous conformers obtained, located within a 20kJmol(-1) range in the gas phase. In line with this trend, a redistribution of the energetic minima is observed in water medium. Molecular electrostatic potential calculations provide a ranking of the potential interaction sites of the four insecticides. The theoretical studies reported in the present work are completed by comparative toxicological assays against three aphid strains. Thus, the same toxicity order for the two susceptible strains Myzus persicae 4106A and Acyrthosiphon pisum LSR1: acetamiprid>fipronil>deltamethrin>chlorpyriphos is revealed. In the resistant strain M. persicae 1300145, the toxicity order is modified: acetamiprid>fipronil>chlorpyriphos>deltamethrin. Interestingly, the strain 1300145 which is known to be resistant to neonicotinoids, is also less sensitive to deltamethrin, chlorpyriphos and fipronil.

Uncommon associations in target resistance among French populations of Myzus persicae from oilseed rape crops.

Within the framework of a molecular exploration of target resistance in populations of Myzus persicae on oilseed rapes in France, (1) the S431F mutation (coding gene ace2), although previously reckoned to be rare, revealed to be frequent, (2) M918L (phenotypically characterised) and L932F (both on para) were found for the first time in M. persicae, and (3) a linkage was revealed between M918L and S431F. While until recently populations developing on French oilseed rapes were dominated by genotypes possessing pyrethroid target resistance and esterase overproduction, to date a different type of dominating genotype, equipped with carbamate and pyrethroid target resistance, seems to be invading such fields.