Effects of larval habitat substrate on pyriproxyfen efficacy against Aedes albopictus (Diptera: Culicidae)

Pyriproxyfen, a juvenile hormone analog insect growth regulator (IGR), is a recommended insecticide for the control of container-inhabiting mosquitoes. The effects of eight container substrates (three plastics, wood, tire rubber, clay pot, concrete, and glass) were studied on reduction of bio-efficacy of pyriproxyfen for Aedes albopictus (Skuse) in the laboratory. Insect growth regulator-bioassay of third instars showed significant increase of LC50 with some substrates compared with control probably because of pyriproxyfen adsorption to the substrate. The LC50 for the control substrate (glass mug) was 0.029 microg/liter. In comparison to the control, LC50 increased 150.1 times with tire rubber (4.354 microg/liter) and 8-10 times with wood and high-density polyethylene plastic containers. Slight increases of LC50 were detected for clay pots, polypropylene, and polystyrene plastics (1.9-2.7 times). Pyriproxyfen efficacy with concrete substrate was higher than with glass containers, which might be caused by high pH. The pH of water with wood (5.6) and concrete (9.8) substrates differed from the other substrates tested (6.3-7.1). There was no pupal mortality at pH 5-11 without pyriproxyfen; however, interaction of pH (5-11) with pyriproxyfen (0.05 microg/liter) showed additive effects at both low and high pH values. Different substrates have variable impacts on pyriproxyfen efficacy, which might be associated with the failure of larval control strategies for the container mosquitoes under certain field conditions.