4.5 Pure glyphosate versus glyphosate-based herbicide
We also investigated whether a glyphosate-based herbicide was more harmful than glyphosate alone. Numerous studies have demonstrated that glyphosate formulations (glyphosate mixed with surfactants) are more toxic than pure glyphosate (reviewed in (Nagy et al., 2019)) but our study did not show any difference. However, the majority of these studies used Roundup® formulation which is described as the most cytotoxic herbicide (Mesnage et al., 2014; Nagy et al., 2019). The surfactants associated to glyphosate vary from one formulation to another. Here, we used the Sintagro® formulation and our results suggest that the surfactant included in this formulation (i.e. alkyl polyglucosid), which is different from that contained in Roundup® (i.e. polyoxyethyleneamine), had no effect, at the concentrations used in our study, on mosquito’s life history traits.
Conclusion
Our results show that exposure to field-realistic doses of glyphosate at larval stages, pure or in formulation, did not affect mosquito larval survival rate, adult size and female fecundity. One of our two experimental blocks showed, however, an effect of glyphosate on the development time and on the probability of female infection byPlasmodium parasite. However the effect on infection prevalence was lost when the larvae are subjected to a nutritional stress, probably due to a lower ingestion of glyphosate. Altogether, our study and those published recently, tend to suggest that mosquito larvae are more tolerant to glyphosate than many other invertebrate species. It has been recently shown that adult exposure to glyphosate perturbs the gut microbiota of honey bees and ultimately their susceptibility to parasite infection (Motta et al., 2018). It would be therefore relevant to also investigate the effect of glyphosate exposure at the adult stage in mosquito vector.