Abstract
The ectoparasitic mite Varroa destructor is a key threat for European honeybee subspecies (Apis mellifera) globally. However, some A. mellifera populations are known to survive mite infestations by means of natural selection (naturally surviving), likely due to reduced mite reproductive success. The effect of small brood cell size on mite reproductive success has not been investigated in these surviving populations and we have little knowledge of its relationship with mite-surviving traits. Here we tested the impact of smaller worker brood cell size on mite reproductive success in susceptible and naturally surviving colonies in Norway. The data show that mite reproductive success was significantly reduced in smaller cells in susceptible colonies (higher rates of non-reproduction, delayed reproduction, and male absence), but not in the surviving colonies. The results support the claim that smaller cell size can have an impact on V. destructor reproductive success, but this seems not to work in tandem with mite-surviving mechanisms favored by natural selection.
Highlights
There is consensus that the ectoparasitic mite Varroa destructor is at present the most impactful global influence on the health of managed honeybees (Apis mellifera ) of European origin (Dahle 2010; Genersch et al 2010; Neumann and Carreck 2010; Rosenkranz et al 2010; VanEngelsdorp et al 2011)
A smaller cell size has been found in the surviving African and Africanized A. mellifera populations; as a mechanical method of controlling V. destructor managed by European beekeepers, evidence has been mixed, with studies supporting (De Jong and Morse 1988; De Ruijter and Calis 1988; Message and Gonçalves 1995; Piccirillo and De Jong 2003; Maggi et al 2010) and refuting the effect (Ellis et al 2009; Taylor et al 2008; Berry et al 2009)
Aside from this ambiguity, interactions with other surviving mechanisms have not yet been thoroughly investigated: Small cell size can increase honeybee responsiveness to certain hygienic tests, such as the example provided by Olszewski et al (2014); bees on small cell size tended to remove pin-killed brood faster than bees on a larger cell size Though known surviving African/ized populations have a smaller cell size, the role small cell size plays in populations of naturally-surviving European honeybees is currently not known
Summary
There is consensus that the ectoparasitic mite Varroa destructor is at present the most impactful global influence on the health of managed honeybees (Apis mellifera ) of European origin (Dahle 2010; Genersch et al 2010; Neumann and Carreck 2010; Rosenkranz et al 2010; VanEngelsdorp et al 2011). A smaller cell size has been found in the surviving African and Africanized A. mellifera populations; as a mechanical method of controlling V. destructor managed by European beekeepers, evidence has been mixed, with studies supporting (De Jong and Morse 1988; De Ruijter and Calis 1988; Message and Gonçalves 1995; Piccirillo and De Jong 2003; Maggi et al 2010) and refuting the effect (Ellis et al 2009; Taylor et al 2008; Berry et al 2009). Given that small cell size may play a significant role in reducing mite reproduction, we expect that manipulating the cell size will yield effects on mite reproductive success in both surviving and susceptible colonies and may act synergistically with the adult-mediated trait present in surviving colonies
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