The effect of coexistence between larvae of Anopheles gambiae and Culex quinquefasciatus on larvicidal efficacy of Bacillus thuringiensis var. israelensis

Background: The efficacy of Bacillus thuringiensis var. israelensis (Bti) is affected by several factors including the species of the mosquito. Mosquito larvae of different species are found to coexist in larval breeding habitats. This study evaluated whether the coexistence between Anopheles gambiae and Culex quinquefasciatus affect the larvicidal activity of Bti. Methods: Two parallel larval bioassay experiments were conducted to test A. gambiae sensu stricto (s.s) and C. quinquefasciatus larvae susceptibility to Bti. They were followed by three parallel bioassays in which A. gambiae s.s and C. quinquefasciatus larvae were mixed in different proportions such that the earlier species contributed three quarters, half and a quarter of the larvae in each testing cup respectively. In each bioassay, six Bti concentrations were tested in four replicates and repeated on three different days. Larvae mortality was scored 24 hours after application of Bti and subjected to Probit analysis. Results: C. quinquefasciatus was significantly more susceptible to Bti than A. gambiae s.s at both lethal concentration values (LC50 and LC95). In coexisting scenario, LC50 of Bti was significantly lower when the proportion of C. quinquefasciatus exceeded 50%. No significant variation in susceptibility to Bti was observed at LC95 in any proportion of coexistence between the two species. Conclusion: The findings show that larvae of C. quinquefasciatus were significantly more susceptible to Bti than those of A. gambiae s.s. Moreover, when larvae of the two species coexisted, there was a general trend of increase in sensitivity to Bti with higher proportion of C. quinquefasciatus. Although this increase in sensitivity of coexisting larvae to Bti is worth noting, our findings suggest that it will not impact larval control where A. gambiae s.s and C. quinquefasciatus coexist.