The European honey bee (Apis mellifera) is a highly valuable, semi-free-ranging managed agricultural species. Since 2006, when high annual colony losses started gaining attention, articles have proliferated in the popular press raising alarm over the perceived decline of bees. Suggested causes include pesticides, genetically modified crops, habitat fragmentation, and introduced viruses and parasites. A formal causal analysis process using a weight-of-evidence approach and specified causal criteria was employed to evaluate the relationship of candidate causes to reduced overwinter survival of commercial colonies in the U.S. The causal criteria used to evaluate the strength of evidence for each candidate cause were: evidence of preceding causation, spatial and temporal co-occurrence, stressor-response relationship, interaction, and alteration. Relevant evidence was scored based on the degree of confidence in the supporting weight of evidence. Evidence scores ranged from �convincingly weakens� to �convincingly supports�. Candidate causes were categorized as being probable, possible, unlikely or indeterminate. Varroa mites plus viruses was judged to be a �probable cause� of the reduced survival of commercial honey bee colonies. Neonicotinoid pesticides were judged to be �unlikely� as the sole or a primary cause of reduced survival, although they could possibly be a contributing factor in some cases. The available data indicate that there may be effects to individual honeybees housed under laboratory conditions or exposed to unrealistically high concentrations of these insecticides. However, under field conditions and exposure levels, reduced overwintering survival as a direct result of neonicotinoid exposure has not been documented. We conclude that crop-applied pesticides in general, and neonicotinoids in particular, are not a major risk factor for honeybee colonies, given the current approved uses and beekeeping practices. This study illustrates the utility of the causal analysis methodology in providing a systematic, transparent, and objective approach to identifying environmental stressors.

Causal analysis of observed declines in managed honey bees (<i>Apis mellifera</i>) with a focus on pesticides

248th National Meeting (2014)

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Causal analysis of observed declines in managed honey bees (Apis mellifera) with a focus on pesticides