Environmental factors can influence the effectiveness of microbial insecticides based on spores and crystal proteins of Bacilus thuringiensis israelensis (Bti) and B. sphaericus (Bs) in mosquito control programs. The impact of water quality and sunlight on the activity and persistence of commercially available microbial larvicidal formulations based on Bti and Bs was investigated within irrigation fields in the Wrocław area. Bioassays were conducted in field- and semi-field conditions using Ochlerotatus caspius [Pallas] larvae. The survey on the persistence of Bti and Bs strains after the application of microbial insecticides based on Bti and Bs was carried out by isolating both bacilli strains from the sediment found in sewage channels. The results showed different activity patterns in larval mortality, but no significant differences between dosages after both biocides application. It was observed that natural sunlight decreases the effectiveness of Bti and Bs. The modeling of the Oc. caspius larvae survival data after Bti application showed a substantial difference between the proportion of mosquito larvae mortality and survival rate in sunlit and shaded conditions. In sunlit conditions, the activity of Bs was also reduced, but the mortality effect demonstrated significant interaction between time and exposure habitats. The presence of bacilli in sewage channels showed their occurrence in sediment. However, the great number of Bti and Bs strains among the total spore-forming bacilli in environmental samples was insignificant, ranging from 3.5% to 6.1%.