Bumble bees are important pollinators whose populations have declined over recent years, raising widespread concern. One conspicuous threat to bumble bees is their unintended exposure to trace residues of systemic neonicotinoid pesticides, such as imidacloprid, which are ingested when bees forage on the nectar and pollen of treated crops. However, the demographic consequences for bumble bees of exposure to dietary neonicotinoids have yet to be fully established. To determine whether environmentally realistic levels of imidacloprid are capable of making a demographic impact on bumble bees, we exposed queenless microcolonies of worker bumble bees, Bombus terrestris, to a range of dosages of dietary imidacloprid between zero and 125 μg L(-1) and examined the effects on ovary development and fecundity. Microcolonies showed a dose-dependent decline in fecundity, with environmentally realistic dosages in the range of 1 μg L(-1) capable of reducing brood production by one third. In contrast, ovary development was unimpaired by dietary imidacloprid except at the highest dosage. Imidacloprid reduced feeding on both syrup and pollen but, after controlling statistically for dosage, microcolonies that consumed more syrup and pollen produced more brood. We therefore speculate that the detrimental effects of imidacloprid on fecundity emerge principally from nutrient limitation imposed by the failure of individuals to feed. Our findings raise concern about the impact of neonicotinoids on wild bumble bee populations. However, we recognize that to fully evaluate impacts on wild colonies it will be necessary to establish the effect of dietary neonicotinoids on the fecundity of bumble bee queens.

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19Neonicotinoid pesticides are currently implicated in the decline of wild bee populations. 20Bumble bees, Bombus spp., are important wild pollinators that are detrimentally affected by 21 ingestion of neonicotinoid residues. To date, imidacloprid has been the major focus of study 22 into the effects of neonicotinoids on bumble bee health, but wild populations are increasingly 23 exposed to alternative neonicotinoids such as thiamethoxam. To investigate whether 24 environmentally realistic levels of thiamethoxam affect bumble bee performance over a 25 realistic exposure period, we exposed queenless microcolonies of Bombus terrestris L. 26workers to a wide range of dosages up to 98 µg kg -1 in dietary syrup for 17 days. Results 27showed that bumble bee workers survived fewer days when presented with syrup dosed at 98 28 µg thiamethoxam kg -1 , while production of brood (eggs and larvae) and consumption of 29 syrup and pollen in microcolonies were significantly reduced by thiamethoxam only at the 30 two highest concentrations (39, 98 µg kg -1 ). In contrast, we found no detectable effect of 31 thiamethoxam at levels typically found in the nectars of treated crops (between 1 and 11 µg 32 kg -1 ). By comparison with published data, we demonstrate that during an exposure to field-33 realistic concentrations lasting approximately two weeks, brood production in worker bumble 34 bees is more sensitive to imidacloprid than thiamethoxam. We speculate that differential 35 sensitivity arises because imidacloprid produces a stronger repression of feeding in bumble 36 bees than thiamethoxam, which imposes a greater nutrient limitation on production of brood.

Currently, there is concern about declining bee populations and some blame the residues of neonicotinoid pesticides in the nectar and pollen of treated crops. Bumble bees are important wild pollinators that are widely exposed to dietary neonicotinoids by foraging in agricultural environments. In the laboratory, we tested the effect of a pulsed exposure (14 days ‘on dose’ followed by 14 days ‘off dose’) to a common neonicotinoid, imidacloprid, on the amount of brood (number of eggs and larvae) produced by Bombus terrestris L. bumble bees in small, standardised experimental colonies (a queen and four adult workers). During the initial ‘on dose’ period we observed a dose-dependent repression of brood production in colonies, with productivity decreasing as dosage increased up to 98 µg kg−1 dietary imidacloprid. During the following ‘off dose’ period, colonies showed a dose-dependent recuperation such that total brood production during the 28-day pulsed exposure was not correlated with imidacloprid up to 98 µg kg−1. Our findings raise further concern about the threat to wild bumble bees from neonicotinoids, but they also indicate some resilience to a pulsed exposure, such as that arising from the transient bloom of a treated mass-flowering crop.

In the original publication of the article, the conversion of imidacloprid concentrations from lg L -1 into parts per billion (ppb) has been miscalculated in certain sections of the text. The authors would like to correct this error and provide the corrected conversions in the altered text below (correct ppb values given in italics).• …exposure to imidacloprid at an environmentally realistic level of 1 lg L -1 (=0.8 ppb)… • …oocytes were smaller in bees from microcolonies exposed to imidacloprid at 125 lg L -1 (98 ppb)… • …syrup dosed with imidacloprid at 1 ppb (1.28 lg L -1 )…• …the strongly detrimental effects of imidacloprid on fecundity at dosages of 39 ppb (50 lg L -1 )… • Except at relatively high dosages (i.e. above 98 ppb)… • …the detrimental effects of imidacloprid on bumble bee fecundity at dosages below 98 ppb… • …dietary imidacloprid at levels up to approximately 39 ppb… • …bumble bee workers feeding on syrup at the highest dosage, 98 ppb… • …microcolonies feeding on syrups at dosages of 16 ppb (20 lg L -1 )… • …microcolonies exposed to an intermediate dosage of imidacloprid, 39 ppb… • …imidacloprid at 39 ppb may have disrupted social interactions.The online version of the original article can be found under