Abstract
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.
Highlights
There is concern about declines in bee populations [1,2] and some implicate neonicotinoid pesticides as culprits [3,4]
We found a dose-dependent decrease in brood production up to 98 ppb imidacloprid and so we extended our experiment to create a pulsed exposure, feeding bees for an additional 14 days on an imidacloprid-free diet, because a scenario such as this may be relevant to wild bumble bee colonies
Bayesian Hierarchical Models (BHM) procedures were implemented in WinBUGS v1.4.3 [39], while all other statistical analyses were conducted in R v3.0.0 [40]. In both pulsed and continuous exposure experiments, B. terrestris queens in experimental colonies began producing eggs after approximately two days and some brood progressed to a larval stage within the 14-day periods
Summary
There is concern about declines in bee populations [1,2] and some implicate neonicotinoid pesticides as culprits [3,4]. Whether neonicotinoids are a principal cause of bee declines is unclear [13,14], but in regions where they are not banned [4] bees are certainly exposed to them on a massive spatial scale by foraging from treated agricultural crops. Other bee-attractive crops such as sunflower and alfalfa are often protected with neonicotinoids [18,21], and so the exposure of bees to these pesticides is widespread. To understand whether a widespread exposure to neonicotinoids is capable of causing bee populations to decline, we must understand their demographic toxicity, which occurs when a toxic agent detrimentally affects the birth and death rates of the exposed species [22]
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