Experiment 1: pure glyphosate and glyphosate-based herbicides
The purpose of this experiment was to test the impact of pure glyphosate
and glyphosate-based herbicides (formulation), on mosquito life history
traits and susceptibility to malaria parasite infection. For this
purpose, one day old mosquito larvae were randomly assigned to five
experimental groups (120 larvae/ per group). Two groups corresponded to
2 different concentrations of pure glyphosate, two groups corresponded
to the same 2 concentrations of glyphosate but in formulation. The
remaining group was reared without glyphosate (control). Larvae were fed
daily with 0.5 mg of food, which is consistent with our standard colony
maintenance diet (1:1 Fish food and rabbit pellets). For pure glyphosate
solution, rearing solutions were prepared from a stock solution of 2
mg/l produced with solid glyphosate (96% purity; provided by
Sigma-Aldrich, Switzerland) dissolved in mineral water. From the stock
solution, 2 derived solutions were prepared, with the following
concentrations: 0.05mg/L and 0.1mg/L. For the glyphosate-based
herbicides, a formulated glyphosate solution (glyphosate concentration:
360g/L; Sintagro®) was diluted ten times with mineral water to obtain a
stock solution (glyphosate concentration: 3.60 g/L). From this stock
solution, 2 derived solutions were prepared to obtain two glyphosate
concentrations: 0.05mg/L and 0.1mg/L. The concentrations of glyphosate
used in our study (0.05 and 0.1mg/L) were chosen based on concentrations
observed in aquatic systems (Giesy et al., 2000) as well as in previous
studies on the impact of sub-lethal doses of pesticides on aquatic
invertebrates species (Baglan et al., 2018; Kibuthu et al., 2016; Riaz
et al., 2009).
Stock solutions were renewed every 6 days, to avoid any degradation
effect (Baglan et al., 2018). Rearing solution were replaced every 3
days to ensure that the larvae were exposed to the same concentration of
glyphosate during all their development.