Effects of neonicotinoid imidacloprid exposure on bumble bee (Hymenoptera: Apidae) queen survival and nest initiation.

Neonicotinoids are highly toxic to insects and may systemically translocate to nectar and pollen of plants where foraging bees may become exposed. Exposure to neonicotinoids can induce detrimental sublethal effects on individual and colonies of bees and may have long-term impacts, such as impaired foraging, reduced longevity, and reduced brood care or production. Less well-studied are the potential effects on queen bumble bees that may become exposed while foraging in the spring during colony initiation. This study assessed queen survival and nest founding in caged bumble bees [Bombus impatiens (Cresson) (Hymenoptera: Apidae)] after chronic (18-d) dietary exposure of imidacloprid in syrup (1, 5, 10, and 25 ppb) and pollen (0.3, 1.7, 3.3, and 8.3 ppb), paired respectively. Here we show some mortality in queens exposed at all doses even as low as 1 ppb, and, compared with untreated queens, significantly reduced survival of treated queens at the two highest doses. Queens that survived initial imidacloprid exposure commenced nest initiation; however, they exhibited dose-dependent delay in egg-laying and emergence of worker brood. Furthermore, imidacloprid treatment affected other parameters such as nest and queen weight. This study is the first to show direct impacts of imidacloprid at field-relevant levels on individual B. impatiens queen survival and nest founding, indicating that bumble bee queens are particularly sensitive to neonicotinoids when directly exposed. This study also helps focus pesticide risk mitigation efforts and highlights the importance of reducing exposure rates in the early spring when bumble bee queens, and other wild bees are foraging and initiating nests.