Laboratory evaluation of pyriproxyfen and spinosad, alone and in combination, against Aedes aegypti larvae

In this study, the efficacy 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 concentration-mortality 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, LC50 and LC95 were 1.1 x 10(-4) (1.0 x 10(-4)-1.1 x 10(-4)) and 3.2 x 10(-4) (2.9 x 10(-4)-3.6 x 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 x 10(-4) mg/liter). Spinosad activity was -500 times lower than that of pyriproxyfen against the Bora strain, with LC50 and LC95 values estimated at 0.055 (0.047-0.064) and 0.20 (0.15-0.27) mg/liter, respectively. A binary mixture of pyriproxyfen and spinosad was realized at the ratio 1:500 by considering the values of the LC50 obtained for each product. The LC50 and LC95 of the mixture were 0.019 (0.016 - 0.022) and 0.050 (0.040 - 0.065) mg/liter, respectively. The mixture combined both the larvicidal activity of spinosad and the juvenoid action of pyriproxyfen. From the LC70 to LC99 a significant 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 five 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.