Nature reported (17 Oct 2013) of a new study done on the causes of colony collapse. John Bryden and colleagues (6 Oct 2013) described a system of sub-lethal stresses exerted on bee colonies can cause collapse when the colony's threshold (breaking-point) is reached.
They maintain that no single factor is a direct cause of colony failure. Instead several stress factors act on bee colonies that are detrimental, but not lethal. These indirectly pile up and sooner-or-later the colony cannot maintain itself and results in collapse at unpredictable times. Examples of stress factors can be combined scenarios of habitat loss, pesticide use, pathogens and parasites. Worker bee memory, foraging capability, and mobility are effected when exposed to field concentrations of pesticides. Pathogens and parasites have also been shown to influence body temperature regulation and energy levels as well as impairing learning of infected bees. This does not result in bee deaths, but decreases the efficiency and viability of the colony as a whole.
To investigate their hypothesis they modelled sub-lethal stress (SLS) on several healthy and impaired bumble bee colonies belonging to the Bombus terrestris species. The bees were fed sucrose treated with sublethal concentrations of neonicotinoid pesticide (higher than that received in the field). All of the colonies' bee numbers expanded during initial growth stages, but later only untreated colonies kept expanding. Colony growth decreased in treated colonies, due to bees becoming behaviourally impaired and colony performance decreased. Subsequently, treated colonies became unsustainable and collapsed at random intervals.
The SLS model created from the data allowed the scientists to predict what would happen at field-level exposure to pesticides. Their SLS model showed variable outcomes (some colonies continued growing, whereas others failed) depending on the type and amount of additional stressors. They showed that non-lethal levels of pesticides alone would not cause colony collapse. This provides an explanation for why no single factor could be attributed to colony collapse in the past. Rather, multifactorial stress can cause colony failure as a result of the critical stress level being reached through the accumulation of sublethal factors.
Original Article:
J. Bryden, Gill R. J., Mitton R. A. A., Raine1 N. E., and JansenV. A. A. (2013) Chronic sublethal stress causes bee colony failure. EcologyLetters. doi: 10.1111/ele.12188.
This is an open article and is freely available to the public.
Nature Article: Why bee colonies collapse
Previous Post: Colony Collapse Disorder - 05 Jan 2012
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