Toxicity persistence of Bacillus thuringiensis var. israelensis (Bti) was evaluated in laboratory and field trials to develop a new protocol for Aedes albopictus monitoring. In the laboratory, the residual toxicity of 5 Bti formulations was compared using the percentage mortality at days 4, 8, 12, and 16 of Culex pipiens larvae exposed for 24 h to Bti treatments. The results showed a good performance of all tested formulations (>97% mortality at day 14, for all the formulations), but only Vectobac™ 12AS at the concentration of 1 ml/liter showed an efficacy of 100% for 2 wk.
A field study was designed to test the effect of Bti on the ovitrap check interval or influence of ovipositional response of gravid Ae. albopictus females. Three different ovitrap treatments were used: ovitraps with tap water checked weekly; ovitraps with tap water checked every 2 wk; ovitraps with Bti (Vectobac 12AS, dose of 1 ml/liter) checked every 2 wk. Our study demonstrated that in the ovitrap, the toxic action of a 1% solution of Bti was maintained for at least 14 days with mortality of 100% and that rainfall did not seem to negatively influence the residual action of Bti. Therefore the probability that the larvae may complete the developmental cycle in ovitraps with Bti seems to be very low. The oviposition activity index showed that Bti enhances the oviposition rate of Ae. albopictus by 17.4%.