Laboratory Evaluation of Pyriproxyfen and Spinosad, Alone and in Combination, Against Aedes aegypti Larvae

Abstract

In this study, the efÞcacy of pyriproxyfen and spinosad, alone and in combination, was evaluated against the dengue vector Aedes aegypti (L.). Larval bioassays were carried out on susceptible mosquito larvae to determine the concentrationÐmortality responses of mosquitoes exposed to each insecticide alone and in mixture. Synergism between pyriproxyfen and spinosad was determined by the calculation of a combination index (CI) by using the isobologram method. For pyriproxyfen, LC 50 and LC 95 were 1.1 10 4 (1.0 10 4 Ð1.1 10 4) and 3.2 10 4 (2.9 10 4 Ð3.6 10 4) mg/liter, respectively. Pyriproxyfen acted at very low concentrations by inhibiting the adult emergence of Ae. aegypti (97% inhibition rates at 3.3 10 4 mg/liter). Spinosad activity was 500 times lower than that of pyriproxyfen against the Bora strain, with LC 50 and LC 95 values estimated at 0.055 (0.047Ð 0.064) and 0.20 (0.15Ð 0.27) mg/liter, respectively. A binary mixture of pyriproxyfen and spinosad was realized at the ratio 1:500 by considering the values of the LC 50 obtained for each product. The LC 50 and LC 95 of the mixture were 0.019 (0.016 Ð 0.022) and 0.050 (0.040 Ð 0.065) mg/liter, respectively. The mixture combined both the larvicidal activity of spinosad and the juvenoid action of pyriproxyfen. From the LC 70 to LC 99 , a signiÞcant synergism effect was observed between the two insecticides (CI ranged from 0.74 to 0.31). This strong synergism observed at high concentrations allows a reduction by Þve and nine-fold of pyriproxyfen and spinosad amounts to kill almost 100% mosquitoes. Combination of pyriproxyfen and spinosad may then represent a promising strategy to improve mosquito control in situations with insecticide-resistant Aedes dengue vectors.