Effects of Chronic Neonicotinoid Exposure on Saskatchewan Honey Bees

Abstract

Saskatchewan leads Canada in field crop production, growing approximately fifty percent of Canada’s canola and wheat crop each year [1]. As well, Saskatchewan is home to a vibrant beekeeping industry which supports Saskatchewan agriculture through pollination services. Each summer, Saskatchewan beekeepers produce much of their honey crop during the canola bloom, which provides an excellent source of nectar and pollen for honey bees. However, the majority of Saskatchewan canola is grown from seed treated with a neonicotinoid insecticide, most commonly thiamethoxam [2], resulting in chronic neonicotinoid exposure of honey bees foraging on canola. Saskatchewan honey has been shown to contain some of the highest residues of thiamethoxam reported worldwide, at mean concentrations in honey of 17.2 ng/g [3]. Discrepancy in the scientific literature regarding the effects of neonicotinoid residues on non-target insects, such as honey bees, has fostered government re-evaluation of neonicotinoid use in Canada. There is a lack of field and laboratory data regarding the effects of chronic exposure of Saskatchewan honey bees to the high levels of neonicotinoid residues found in this province. To address this gap, the objectives of this thesis were to investigate the effects of chronic neonicotinoid exposure on the health and survival of Saskatchewan honey bees, including nucleus colonies, overwintering colonies, adult honey bee workers and worker honey bee brood. We found that chronic exposure of nucleus colonies to 20 ng/g neonicotinoids significantly decreased colony weight gain (honey production) by 30% and decreased adult bee cluster size by 21%. Chronic exposure to 100 ng/g of the neonicotinoid thiamethoxam significantly decreased overwinter survival of strong fall colonies by 55%, with 20 ng/g thiamethoxam resulting in a statistically nonsignificant, 15% decrease in overwinter survival. Chronic exposure to 40 ng/g thiamethoxam significantly decreased survival of summer adult workers in the laboratory but had no effect on their hyopharyngeal gland development. Compared to adult workers, worker brood required thiamethoxam concentrations which were 23 times higher before significant decreases in survival were observed. Co-exposure of worker adults and brood to neonicotinoids and fungicides was not shown to have additive negative effects on mortality; however, co-exposure of worker brood to fungicides and field-unrealistic doses of thiamethoxam was shown to predispose worker larvae to the bacterial brood disease European foulbrood. In light of these findings, chronic environmental exposure of honey bees to neonicotinoids should be maintained at levels below 20 ng/g to ensure Saskatchewan honey bees and Saskatchewan agriculture continue to thrive.