Clothianidin-resistant Anopheles gambiae adult mosquitoes from Yaoundé, Cameroon, display reduced susceptibility to SumiShield® 50WG, a neonicotinoid formulation for indoor residual spraying

Abstract

Background: Chronic exposure of mosquito larvae to pesticide residues and cross-resistance mechanisms are major drivers of tolerance to insecticides used for vector control. This presents a concern for the efficacy of clothianidin, an agricultural neonicotinoid prequalified for Indoor Residual Spraying (IRS).

Methods: Using standard bioassays, we tested if reduced susceptibility to clothianidin can affect the efficacy of SumiShield® 50WG, one of four new IRS formulations containing clothianidin. We simultaneously monitored susceptibility to clothianidin and to SumiShield 50WG, testing adults of Anopheles gambiae, An. coluzzii and Culex sp sampled from urban, suburban and agricultural areas of Yaoundé, Cameroon.

Results: We found that in this geographic area, the level of susceptibility to the active ingredient predicted the efficacy of SumiShield 50WG. This formulation was very potent against populations that reached 100% mortality within 72 h of exposure to a discriminating concentration of clothianidin. By contrast, mortality leveled off at 75.4 ± 3.5% within 7 days of exposure to SumiShield 50WG in An. gambiae adults collected from a farm where the spraying of the two neonicotinoids acetamiprid and imidacloprid for crop protection is likely driving resistance to clothianidin.

Conclusions: Despite the relatively small geographic extend of the study, the findings suggest that cross-resistance may impact the efficacy of some new IRS formulations and that alternative compounds could be prioritized in areas where neonicotinoid resistance is emerging.

Keywords: Anopheles; Clothianidin; Malaria; Neonicotinoids; Resistance; SumiShield® 50WG.

Fig. 1

Map of the sampling locations where Anopheles and Culex mosquitoes were collected to evaluate their susceptibility to clothianidin and to SumiShield 50WG. The city of Yaoundé (brown areas) is surrounded by degraded forests

Fig. 2

Picture of a larval breeding site in Nkolondom (A) and samples of empty containers found in the farm indicating the use of formulations of imidacloprid and acetamiprid (B)

Fig. 3

Baseline susceptibility of laboratory strains and field populations to clothianidin monitored for 7 days using CDC bottle bioassays. A Mortality values of Anopheles and Culex female adults exposed to 150 µg/ml of clothianidin. B Gradient of susceptibility revealed in wild populations of An. gambiae. Error bars represent the standard error of the mean and (n) the number of individuals tested. * Fisher’s exact test (p < 0.05) indicates lower mortality in An. gambiae compared to An. coluzzii. ** Fisher’s exact test (p < 0.05) indicates lower mortality in An. gambiae from Nkolondom compared to any other conspecific population

Fig. 4

Knockdown values after 1 h exposure to 150 µg/ml of clothianidin in bottle bioassays. Knockdowns were compared between Anopheles and Culex species (A) and between field populations of An. gambiae (B). Standard errors of the mean are shown as vertical bars. * p < 0.05 (Fisher’s exact test)

Fig. 5

Synergistic effect of PBO. A standard test with 150 µg/ml of clothianidin (1X) and a synergist bioassay (PBO) showed a significant increase in mortality in the presence of PBO. Error bars represent the standard error of the mean. * p < 0.05 (Fisher’s exact test)

Fig. 6

Relationship between susceptibility to clothianidin as revealed by CDC bottle bioassays and efficacy of SumiShield 50WG evaluated with WHO tube tests. Any population susceptible to clothianidin (150 µg/ml) reached 100% mortality within 3 days of exposure to SumiShield 50WG. Error bars represent the standard error of the mean. * Fisher’s exact test (p < 0.05) indicates lower mortality in An. gambiae from Nkolondom compared to any other population tested with SumiShield 50WG