Honey bees are major pollinators of crops with high economic value. Thus, bees are considered to be the most important nontarget organisms exposed to adverse effects of plant protection product use. The side effects of pesticides are one of the major factors often linked to colony losses. Fewer studies have researched acute poisoning incidents in comparison to the study of the sublethal effects of pesticides. Here, we compared pesticides in dead/dying bees from suspected poisoning incidents and the suspected crop source according to government protocols. Additionally, we analyzed live bees and bee bread collected from the brood comb to determine recent in-hive contamination. We used sites with no reports of poisoning for reference. Our analysis confirmed that not all of the suspected poisonings correlated with the suspected crop. The most important pesticides related to the poisoning incidents were highly toxic chlorpyrifos, deltamethrin, cypermethrin and imidacloprid and slightly toxic prochloraz and thiacloprid. Importantly, poisoning was associated with pesticide cocktail application. Almost all poisoning incidents were investigated in relation to rapeseed. Some sites were found to be heavily contaminated with several pesticides, including a reference site. However, other sites were moderately contaminated despite agricultural use, including rapeseed cultivation sites, which can influence the extent of pesticide use, including tank mixes and other factors. We suggest that the analysis of pesticides in bee bread and in bees from the brood comb is a useful addition to dead bee and suspected crop analysis in poisoning incidents to inform the extent of recent in-hive contamination.

ALS Limited ALS Czech Republic Na Harfe 336 9 Prague 9 Vysocany CZ 190 00 Czechia

Bee Research Institute at Dol Maslovice Dol 94 Libcice nad Vltavou CZ 252 66 Czechia

Crop Research Institute Drnovska 507 73 Prague 6 Ruzyne CZ 161 06 Czechia

Crop Research Institute Drnovska 507 73 Prague 6 Ruzyne CZ 161 06 Czechia; Czech University of Life Sciences Faculty of Agrobiology Food and Natural Resources Department of Plant Protection Prague 6 Suchdol CZ 165 21 Czechia

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