DNA metabarcoding data MARC project

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Testing possible exposure routes of pesticides is essential for assessing the risks to pollinators. In Australia, the biorational pesticide Bacillus thuringiensis var. israelensis (Bti) is widely used in mosquito control programs and is applied aerially to mangrove and saltmarsh habitats to control nuisance dipteran species. Bti formulations contain both bacterial spores and toxins, allowing detection using DNA-based methods, while bee foraging behaviour can be quantified by DNA metabarcoding of sampled pollen. We combined these two approaches to investigate both the foraging behaviour of stingless bees and their potential exposure risk to Bti in coastal mosquito habitats dominated by Avicennia marina (grey mangrove; Acanthaceae). Sentinel hives of the native stingless bee Tetragonula carbonaria (Meliponini) were placed at two field sites subjected to routine aerial Bti treatments. Stingless bee foraging behaviour was assessed by ITS2 metabarcoding of pollen collected from returning foragers, and a Bti-specific qPCR assay was applied to the same pollen samples, hive stores and the bodies of foraging bees after pollen removal. ITS2 metabarcoding revealed that foragers predominantly visited A. marina flowers during the early exposure period, a period of peak flowering in A. marina, shifting to Schinus terebinthifolia (Brazilian peppertree; Anacardiaceae) and other non-mangrove species later in the study. Bti was detected in pollen loads from 27% of pollen samples and 4% of adults after pollen removal, identifying contaminated A. marina pollen as the primary exposure pathway in the studied environments. A single pollen pot in one of the exposed hives tested positive at the study’s end, suggesting potential accumulation of Bti in hive stores. Although detected Bti concentrations matched expected operational field concentrations and likely pose little risk to stingless bees, our findings highlight exposure mechanisms that could be significant for pesticides with greater toxicity to bees.