3.2 Experiment 2: glyphosate and nutritional stress
Glyphosate exposure (0.05mg/L) and food treatments had no significant effect on larval mortality (model 7: χ²1 = 2.76, p = 0.097, χ²1 = 0.01, p = 0.904, respectively). Variation in larvae development time was explained by food treatment, glyphosate exposure and mosquito sex (model 8: F= 366.08, p < 0.0001, F= 8.123, p = 0.004, F= 655.21, p < 0.0001, respectively). While nutritional stress impacted negatively the development time (Figure 1A ), larvae exposed to glyphosate developed faster than unexposed larvae (Figure 1B ). As expected males had a shorter development time than females (male: 9.9 ± 0.1 day, female: 13.4 ± 0.2).
Food treatments and sex, but not glyphosate exposure, impacted mosquito size (model 9: F= 39.20, p < 0.0001, F= 384.85, p < 0.0001, F= 2.06, p = 0.153, respectively). Males were smaller than females and adults from larvae reared under standard diet condition were bigger than adults from larvae reared under nutritional stress (mean ± s.e. optimal feeding condition = 0.30 ± 0.003, food limitation = 0.28 ± 0.003).
The amount of blood ingested by females as well as the number of laid eggs did not vary between treatments (blood meal size: model 10: glyphosate exposure = χ²1= 1.27, p = 0.259; food treatments = χ²1= 1.29, p = 0.256; eggs number: model 11: glyphosate exposure χ²1= 0.01, p = 0.99; food treatment = χ²1= 3.49, p = 0.066). A positive relationship was observed between blood meal size and the number of laid eggs (model 11: χ²1= 5.70, p = 0.017).
Significant interaction of glyphosate exposure and food treatments was observed regarding the probability of females to be infected by malaria parasites (model 12: χ²1 = 7.67, p = 0.006,Figure 2A ). In the absence of glyphosate, nutritional stress tended to decrease the infection prevalence (infection prevalence = nutritional stress: 0.80, standard diet: 0.95; contrast analysis: χ²1 = 2.74, p = 0.097). However, in the presence of glyphosate, the infection prevalence observed in females from larvae reared with standard diet was roughly a third lower than that of females from the nutritional stress treatment (infection prevalence = nutritional stress: 0.95, standard diet condition: 0.66; contrast analysis: χ²1 = 5.18, p = 0.022). It is also interesting to note that when larvae are reared without nutritional stress, the infection prevalence observed in mosquitoes exposed to glyphosate was significantly lower than in unexposed mosquitoes (contrast analysis: χ²1 = 5.37, p = 0.020, Figure 2A ). Infection prevalence tended to but was not significantly impacted by blood meal size (model 12: χ²1 = 3.13, p = 0.076).
The intensity of the infection was not impacted by glyphosate exposure, food treatments or by the interaction between the two factors (model 13: χ²1 = 0.56, p = 0.455, χ²1 = 0.98, p = 0.321, χ²1 = 0.08, p = 0.782, respectively,Figure 2B ), but a positive relationship was observed between blood meal size and oocyst burden (model 13: χ²1= 13.97, p = 0.0001).
Discussion
In this study we assessed the consequences of larval exposure to pure glyphosate or glyphosate-based herbicide on Culex pipiensmosquito life history traits and susceptibility to avian malaria parasite infection. While we did not observe significant effect of glyphosate on mosquito life history traits, we found that this compound reduced the prevalence of Plasmodium parasite infection under standard diet. Interestingly this effect was lost when the larvae were subjected to nutritional stress.