Bees Under Stress: A Study of PFOS Exposure and Protected Cropping Stress Effects on the Western Honey Bee (Apis mellifera)

Abstract

<p>Bees provide pollination services to managed and wild ecosystems, thereby contributing to a $711 B (US) global food crop industry and the preservation of biodiversity. Bees, however, face numerous stressors including chemical and physical threats, such as exposure to environmental contaminants, and intensive agriculture. This thesis examines two emerging concerns for honey bee viability, one chemical threat, perfluorooctane sulfonate (PFOS), and one emerging physical threat, protected cropping systems. Perfluorooctane sulfonate is a persistent organic pollutant that accumulates and biomagnifies up the food chain, is toxic to bees and has been detected in various environmental media in almost every region of the globe. Bee exposure routes to PFOS are unclear and bee response to chronic sublethal PFOS concentrations, has not been confirmed. Intensive agriculture, specifically protective netting, and the monocultures contained in the protected environment, pose a threat to bees and are reported to cause declines in colony size and resource storage, but few replicated studies exist.</p> <p>Firstly, individual bee response to PFOS was studied in a Y-maze system using unspiked and PFOS spiked sugar syrup at a range of concentrations detected in water at contaminated sites. Bee response regarding choice behaviour and drink duration was recorded, whereby determining avoidance or consumption of PFOS spiked syrup. Bees displayed significant avoidance of the PFOS spiked syrup at concentrations of 61 and 103 µg L^-1, when given a choice with unspiked syrup, compared with the control. When the choice of consuming unspiked syrup was removed, the bees drank PFOS spiked syrup at all the PFOS concentrations tested, and avoidance was not evident. The consumption of PFOS spiked syrup, with no significant avoidance, at concentrations that are frequently reported in PFOS contaminated water, and are known to be detrimental to bee health, has serious implications for the viability of bee colonies in contaminated regions. </p> <p>Secondly, the impact of sublethal chronic PFOS exposure on a range of behavioural, biological and biomolecular markers was assessed to provide new lines of evidence regarding bee response at a colony and cellular level. Using a novel cage system, whole bee colonies were exposed chronically to sublethal PFOS concentrations of 5 to 20 µg L^-1 PFOS in sugar syrup for 25 days. Although the colonies appeared healthy, produced brood and stored resources, significant life stage development, behavioural and intergenerational effects were observed even at the lowest PFOS concentration of 5 µg L^-1. Fewer and smaller next generation bees were produced, and reduced honey production occurred in colonies exposed to PFOS at all exposure concentrations, compared with the control. Perfluorooctane sulfonate accumulated in adult and newly emerged bee tissue with a bioaccumulation factor of 1.72 and 0.69 respectively, transferred to honey and was eliminated from the bee body, at all PFOS exposure concentrations, indicating that accumulation and subsequent magnification did occur. The protein signature was differentially expressed in the 10 (<i>n</i> = 2) and 20 µg L^-1 (<i>n</i> = 15) exposure treatments, with the greatest impact on proteins associated with cellular processes and function, hormone and pheromone production, fertility, pupal development, and metabolic and central nervous system function.</p> <p>To understand the influence of protected cropping, specifically protective netting, on honey bee colony viability, a field experiment was conducted on blueberry farms in Northern New South Wales, Australia, for a period of 12 weeks. Bee access to outside resources, resource storage and colony size, both outside of and under anti-hail and anti-bird netting was studied. Anti-hail and anti-bird netting adversely impacted honey bee access to outside resources by inhibiting bee movement through the nets. Hives under net cover displayed reduced pollen storage and hive weight when compared to those outside of net cover, evident within four weeks of placement under net cover.</p> <p>This research demonstrates that individual honey bees will consume PFOS contaminated sugar syrup at concentrations that significantly exceed values considered toxic. This behaviour has implications for bee resilience and adaption to a changing climate and the associated impact on floral resources. At a colony level, chronic sublethal PFOS exposure detrimentally impacts a range of intergenerational and biochemical responses, essential for colony survival and accumulates in bee tissue (including adults produced during the experiment), is partially eliminated in faeces and transfers to honey. At a cellular level, chronic sublethal PFOS exposure alters proteins in the honey bee head identified as essential for honey bee development and function. This research also shows that the protected cropping environment, in particular antihail and anti-bird netting compromises honey bee colony viability. Bees take more time to exit netting and hence have less time to forage and this is impacting brood metrics.</p> <p>Perfluorooctane sulfonate guidelines for invertebrate pollinators do not currently exist, nor do guidelines for the protection of pollinators in protected cropping systems. Guideline development is hindered by insufficient data regarding the routes of exposure to and impact of PFOS, and protective covers on honey bee health. Guidelines are urgently needed to protect the health and viability of the pollinators upon which agriculture relies, and consumers of bee products, thereby supporting human health, the economic stability of agricultural industries, and food security.</p>