Qualitative examination of diatom diversity in black fly (Diptera: Simuliidae) gut contents across a mortality gradient after exposure to Bacillus thuringiensis subsp. israelensis de Barjac, 1978

While Bacillus thuringiensis subsp. israelensis de Barjac, 1978 (Bti) is generally effective in reducing Simulium Latreille, 1802 pest levels, impaired toxicity has been identified. Diatoms are typically abundant in Simulium larval gut contents and thought to affect feeding behaviour and/or Bti ingestion. We sought to determine diatom composition in larval Simulium gut contents and periphyton in relation to mortality subsequent in situ Bti application across three stream reaches. Generally, lower larval mortality after Bti exposure was associated with comparatively greater diatom diversity inside gut contents, corresponding with an inverse relationship between periphyton diatom diversity and larval mortality across our study reaches. High profile diatoms inside gut contents were positively correlated with mortality, suggesting microbial morphology may affect Bti efficacy. Diatoms inside live larval guts were most similar to periphyton which indicates that these larvae may have been feeding less on lower quality detrital material and/or exhibiting comparatively greater benthic feeding.