Abstract
Monitoring pesticide resistance is essential for effective and sustainable agricultural practices. Bioassays are the basis for pesticide-resistance testing, but devising a reliable and reproducible method can be challenging because these tests are carried out on living organisms. Here, we investigated five critical parameters and how they affected the evaluation of resistance to the organophosphate phosmet or the pyrethroid lambda-cyhalothrin using a tarsal-contact protocol on Drosophila suzukii, a worldwide invasive pest. Three of the parameters were related to insect biology: (i) sex, (ii) age of the imago (adult stage) and (iii) genetic diversity of the tested population. The two remaining parameters were linked to the experimental setup: (iv) the number of individuals tested per dose and (v) the duration of exposure to the active ingredient. Results showed that response to insecticide differed depending on sex, males being twice as susceptible to phosmet as females. Age principally affected young females' susceptibility to phosmet, because 0-24 hour-old flies were twice as susceptible as 24-48 hour-old and 72-96 hour-old females. Genetic diversity had no observable effect on resistance levels. The precision and accuracy of the median lethal dose (LD50) were greatly affected by the number of individuals tested per dose with a threshold effect. Finally, optimal duration of exposure to the active ingredient was 24 h, as we found an underestimation of mortality when assessed between 1 and 5 h after exposure to lambda-cyhalothrin. None of the main known point mutations on the para sodium channel gene associated with a knockdown effect were observed. Our study demonstrates the importance of calibrating the various parameters of a bioassay to develop a reliable method. It also provides a valuable and transferable protocol for monitoring D. suzukii resistance worldwide.
Highlights
Bioassays are considered as the gold standard for pesticide resistance testing
With the growing ambition to employ sustainable practices of pest control, monitoring pesticide resistance is becoming the key to implementing efficient, integrated pest management strategies
The estimates of the LD50 values were very similar for the three age classes with 15.2 mg/l, 19.5 mg/l and 16.4 mg/l, from youngest to oldest
Summary
Bioassays are considered as the gold standard for pesticide resistance testing. They can detect new resistances and can assess resistance in an integrative manner, whether the underlying mechanism is known or unknown [1]. As in most experiments involving living material, the output of pesticide resistance bioassays can be affected by many parameters, depending on the type of bioassay used and the study species. The standard procedure consists in exposing insects to a range of doses of the active substance of interest and in counting the number of dead individuals for each dose. Results of bioassays may be affected by experimental conditions and design [3] and by the biological characteristics of the insect species [4,5,6,7,8]. It is necessary to develop, or at least to adapt, a standard experimental protocol to obtain reliable and repeatable results that will allow comparisons of pesticide susceptibility over space and time
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