Bacteria and mosquito abundance in microcosms enriched with organic matter and treated with a Bacillus thuringiensis...

Bacteria and mosquito abundance were studied in outdoor tubs unenriched and enriched with 0.04% rabbit pellets during the winter, 1999. The irrigation water used to fill the tubs contained a total bacterial count of 1.15-1.35 x 10(3) cells/ml. Adding rabbit pellets for enrichment yielded a total bacterial count of 5.50-7.63 x 10(5) cells/g. Bacterial densities in unenriched water were significantly lower than in enriched tubs on every sampling day. When bacterial densities in both enriched and unenriched regimens reached peak populations on day 3 post-flooding, their numbers in enriched water were 25-fold higher than the unenriched water. Under cool weather conditions, mosquito oviposition activity was low and larval development was very slow. Egg raft counts and larval densities in enriched water were nevertheless higher than those in unenriched water. After reaching peak populations on day 3 post-flooding, the natural decline in bacterial densities in the top portion of enriched water without mosquito larvae was lower compared with that in water with larvae. In water with larval present, the decline of bacterial levels in top water was greater than in bottom water on day 7 post-flooding. VectoBac G, a granular formulation of Bacillus thuringiensis subsp. israelensis caused a reduction in larval numbers of 80, 93, 73% at the rate of 5.5 lb/ac and a reduction of 94, 93, 86% at the rate of 10.6 lb/ac on days, 1, 3, 7 posttreatment, respectively. After treatment, the reductions of bacterial densities in untreated tubs were greater than treated tubs. These results indicate that mosquito larvae play an important role in the decline of bacterial populations by their feeding activity.