Report on a field test of aerial application of VectoBac G® small granules against floodwater mosquito larvae in Sweden

Floodwater mosquito control in Sweden has been conducted since 2002, applying the larvicide VectoBac G® large granules by helicopter. The dosage has been gradually reduced from 15 kg/ha to 11 kg/ha, but a lower dosage than that was deemed too risky to achieve sufficient larval mortality in areas with patchy small water bodies. The small granulate variant of VectoBac G® consists of small even-sized granules with high bulk density. The number of granules per unit area is about 7.5 times higher than with the large granules which should allow for lower dosage with similar efficacy. We evaluated VectoBac G® small granules against floodwater mosquito larvae with 8 and 11 kg per hectare in a field test in four swamps and four wet meadows at Lake Färnebofjärden, Sweden, during a flood in May 2023. Dosage was randomly allocated to an equal number of each wetland type. In each study area, mosquito larvae were counted before and 24 hours after VectoBac G® small granules application. The mean number of mosquito larvae differed significantly before and after aerial application of small granules and total reduction of mosquito larval abundance was 92.6% with a dosage of 11 kg/ha and 98.5% with a dosage of 8 kg/ha. In the study areas with 8 kg/ha, larval abundance reduction was similar in wet meadows (98.8%) and swamps (98.1%). With 11 kg/ha, larval abundance reduction differed markedly between wet meadows (99.6%) and swamps (86.3%), due to one sample site in a swamp where a high number of alive larvae were found after application. This sample could be considered an outlier but it could also reflect natural conditions. We feel confident with the results and the usage of VectoBac G® small granules with a dosage of 8 kg/ha has been successfully implemented in regular treatments.