Biphasic concentration-dependent interaction between imidacloprid and dietary phytochemicals in honey bees (Apis mellifera)

BACKGROUND: The presence of the neonicotinoid imidacloprid in nectar, honey, pollen, beebread and beeswax has been implicated in declines worldwide in the health of the western honey bee Apis mellifera. Certain phytochemicals, including quercetin and p-coumaric acid, are ubiquitous in the honey bee diet and are known to upregulate cytochrome P450 genes encoding enzymes that detoxify insecticides. Thus, the possibility exists that these dietary phytochemicals interact with ingested imidacloprid to ameliorate toxicity by enhancing its detoxification. APPROACH: Quercetin and p-coumaric acid were incorporated in a phytochemical-free artificial diet individually and together along with imidacloprid at a range of field-realistic concentrations. In acute toxicity bioassays, honey bee 24- and 48- hour imidacloprid LC(50) values were determined in the presence of the phytochemicals. Additionally, chronic toxicity bioassays were conducted using varying concentrations of imidacloprid in diets with the phytochemicals to test impacts of phytochemicals on longevity. RESULTS: In acute toxicity bioassays, the phytochemicals had no effect on imidacloprid LC(50) values. In chronic toxicity longevity bioassays, phytochemicals enhanced honey bee survival at low imidacloprid concentrations (15 and 45 ppb) but had a negative effect at higher concentrations (105 ppb and 135 ppb). p-Coumaric acid alone increased honey bee longevity at concentrations of 15 and 45 ppb imidacloprid (hazard ratio (HR): 0.83 and 0.70, respectively). Quercetin alone and in combination with p-coumaric acid similarly enhanced longevity at 45 ppb imidacloprid (HR:0.81 and HR:0.77, respectively). However, p-coumaric acid in combination with 105 ppb imidacloprid and quercetin in combination with 135 ppb imidacloprid increased honey bee HR by approximately 30% (HR:1.33 and HR:1.30, respectively). CONCLUSIONS: The biphasic concentration-dependent response of honey bees to imidacloprid in the presence of two ubiquitous dietary phytochemicals indicates that there are limits to the protective effects of the natural diet of honey bees against neonicotinoids based on their own inherent toxicity.