How varying parameters impact insecticide resistance bioassay: An example on the worldwide invasive pest Drosophila suzukii

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.

Comparison of the variability in mortality data generated by CDC bottle bioassay, WHO tube test, and topical application bioassay using Aedes aegypti mosquitoes

“…No formal guidelines exist for the topical application bioassay, though if used to establish dose–response curves, the total number of mosquitoes used is larger than either CDC bottle bioassay or WHO tube test, as multiple doses need to be tested unless intensity assays are performed, which require two or three times the number of mosquitoes. If the topical application bioassay is used to determine resistance with a diagnostic dose [ 14 ], the optimal number of mosquitoes exposed will need to be formally established, similarly to how this was performed for bioassays with Drosophila suzukii [ 8 ].…”

“…These findings are crucial, especially since the CDC bottle bioassay is frequently used in the field, and its use may increase with newer insecticides and to determine resistance intensity. Our results imply that care should be taken in interpreting data coming from such assays, particularly when small samples have been used [ 8 ]. While both CDC bottle bioassays and WHO tube tests may be effective as an easy and crude method to diagnose highly resistant populations, topical application bioassays would perhaps be more suitable than current intensity tests for assessment and comparison of levels of resistance, particularly for low-level and moderate-resistance populations.…”

“…Such false positives could similarly occur due to a lack of sensitivity of the assay to accurately detect mortality lower than 90 or 97%, too small sample sizes, inaccuracy of preparation of the assay (exposing mosquitoes to too small dosages), and inaccuracy of assessing mortality (for instance, identifying mosquitoes with erratic flight as alive, whereas it follows the definition for “dead” [ 5 , 7 ]). Therefore, it is important to keep the bioassay preparation standardized, mortality assessment as objective as possible, and choose assays with the highest level of accuracy [ 8 ]. Since obtaining sufficient mosquito numbers is a problem in many areas, a bioassay with a low level of variation is preferable as this reduces the number of mosquitoes needed to be confident in the classification of a resistant versus susceptible population.…”

Drosophila suzukii population dynamics and control efficiency of mineral dusts with a focus on grape protection

“…The differences between these studies might be explained by the variation of experimental conditions; indeed, Blouquy et al. (2021) demonstrated that sex, age, duration of the experiment and the number of individuals tested per dose can have an effect on the outcome of an experiment. Oviposition was previously reported to be independent of fruit skin thickness, °Brix and grape variety, but increased when penetration force decreased (Shrader et al., 2019).…”

Collection and Processing of Behavioural Data of the Olive Fruit Fly, Bactrocera oleae, When Exposed to Olive Twigs Treated with Different Commercial Products

“…Additionally, it is known that the populations of B. oleae may exhibit genetic diversity with consequently different levels of susceptibility to pesticides, which requires alternative control methods [4][5][6]. This phenomenon is indeed very common in worldwide distributed herbivore species, with genetic variability among populations [7][8][9]. Directive 2009/128/EC promotes integrated pest management (IPM) strategies or organic farming control methods for pest control.…”