The effects of Bacillus thuringiensis var. israelensis ( Bti ) on tropical stream food webs vary between two consecutive dry seasons

Bacillus thuringiensis var. israelensis (Bti) is widely used for the biological control of mosquitoes and blackflies. Although considered non‐toxic to most aquatic organisms, its application in streams can disrupt food webs by reducing the abundance of filter‐feeding macroinvertebrates. We evaluated the effects of Bti application on aquatic macroinvertebrate communities in eight Atlantic Forest streams in southeastern Brazil. Each stream included one reach with long‐term Bti application and one untreated reach. Sampling was conducted during the dry season of two consecutive years (2015 and 2016), which differed in rainfall intensity. A total of 7525 macroinvertebrate individuals were collected using a Surber sampler, 60% of which occurred in untreated reaches. Community composition varied among streams but not by treatment; however, Bti‐treated reaches showed lower abundance and richness across multiple trophic categories. Structural equation models revealed, in 2016, direct negative effects of Bti on collector‐filterers and indirect impacts on predators via reduced filter‐feeders. These results indicate that direct and cascading impacts of Bti on stream food webs differ between years, highlighting the importance of environmental context when assessing the impacts of larvicide programmes.