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  • Small cell study

    https://link.springer.com/article/10...5f_z5B1l8IrKLg
    Discussion

    Our data support the conclusion that smaller worker brood cell size can significantly contribute to the overall reduced reproductive success of V. destructor mites in susceptible colonies and may contribute to lower infestation rates; however, this effect was not significant in surviving colonies known to possess cell recapping as a mite-surviving trait (Oddie et al. 2018). The effect of cell size may be masked by these traits in surviving colonies, or else it is not present. Regardless, a small cell size does not appear to work in tandem with mite resistance traits favored by natural selection in the surviving Norwegian population.

    Cell size differences were comparable between surviving and susceptible test colonies and there was an observable distinction between large and small cell size foundation; however, foundation size did not yield significant differences in mite fecundity among surviving or susceptible bees. This is likely due to the fact that some frames built up from small cell foundation actually yielded larger average cell sizes, as was also found by Taylor et al. (2008). The comb-builders in some colonies likely follow the foundation base more accurately than others, making actual cell size a much more reliable measure than foundation base.

    When actual cell size was examined in depth, it was found that susceptible colonies did benefit from the small cell size: Cells of smaller average diameter showed slightly lower mite infestation rates as well as a significant increase in delayed reproduction, non-reproduction, and male offspring absence, all factors contributing to a lower mite reproductive success. No effect was found on mite infertility; however, the rates of infertility were very low in general within this study. In contrast, there was no significant effect of cell size in surviving colonies for any measured parameter; therefore, a smaller cell size does not appear to work together with other mechanisms reducing mite reproduction used by naturally-surviving populations (Locke et al. 2012; Oddie et al. 2017). The observation that the infestation rate was affected in the susceptible receiver colonies, but not in the surviving ones, may point to an effect put upon infested cells by the surviving phenotype, though at this point, it can only be speculated as to how this has occurred. If the adult-mediated trait does remove infested cells, it is possible the process is not affected by the size of the cell. The results for susceptible colonies at least align with those found in several previous studies: Message and Gonçalves (1995) uncovered a difference in V. destructor infestation rate and number of female mite deutonymphs between cells of 4.5–4.6 mm and those of 4.9–5.1 mm. Maggi et al. (2010) and Piccirillo and De Jong (2003) found differences in the rate of mite infestation between large and small cell sizes. Size ranges in both studies were comparable to the sizes used in this study. Maggi et al. (2010) also investigated mite reproductive rate, but found no significant effect while our dataset uncovered a small but significant reduction. A study in New Zealand (Taylor et al. 2008) did not find a significant influence of cell size on mite reproductive success but found an increase in infestation rate on smaller cell sizes. Berry et al. (2009) found similar results, overall mite populations were higher in colonies reared on the smaller cell size when they compared entire colonies given small or large comb sizes (4.9 ± 0.08 and 5.3 ± 0.04 respectively). These varying results provide evidence that there are parameters that have yet to be considered regarding V. destructor mite population dynamics in relation to the size of brood cells. It is possible that the different methods and environments of each study are contributing to the mixed results. Changing parameters within a colony such as cell size may change the behavior of the bees to the point where they create unmeasured differences between themselves and a control population. To give an example of this, the study performed by Piccirillo and De Jong (2003) only used bees accustomed to a small cell size for their trials and found higher infestation rates on the larger cell size comb. Taylor et al. (2008) used bees accustomed to the large cell size and found higher infestation rates on smaller comb, and both studies provided their bees with pre-built comb from another source for at least one of the treatment groups. Ellis et al. (2009) used bees accustomed to each cell size; they were kept on for the experiment and found no significant differences in overall mite population. It should then be said that the cell size on which the bees were reared as well as the origin of the comb used should be taken into consideration when analyzing highly variable parameters such as overall colony mite infestation rate and V. destructor foundress fecundity. Our study also introduced frames built out by other colonies, yet only two frames were given and kept within the colonies, and only for a single brood cycle; this may not have introduced enough change to elicit a response from the bees.

    Our study delved into V. destructor mite reproductive parameters in depth, investigating not only viable offspring number, but also proportion of delayed reproduction, non-reproduction, infertility, and male presence, all valuable parameters when considering mite reproductive success (Locke et al. 2012). The fact that we found significant patterns in all of these, save infertility, indicates that cell size does not affect just one parameter largely, but may have small, combined effects in each differing parameter that create an additive difference. Olszewski et al. (2014) found that rearing mite-susceptible bees on foundation size of 4.93 mm (compared to 5.56 mm) increased hygienic behavior performed, though this study could find no significant link to this elevated behavior and the number of immature mites found on the bottom boards. Olszewski et al. (2014) did suggest that small cell size could synergize with hygienic behavior in some populations but not in others depending on the level of hygienic behavior and the bees' adaptability to a smaller cell size. Within-colony variation in our study was large, making it difficult to isolate strong effects, but even between only 10 colonies, a distinct pattern was observed. This study, however, was not long term and could not take into account the effect of small cell size on the overall population dynamics of V. destructor in the test colonies, so the ability of small cell size to help control V. destructor cannot be reported here. To find more robust evidence that small cell size affects mite populations in a practical way, colonies would need to be bred and reared on small cell size and compared year-round with those bees from the same genetic background reared on large cells. Measuring cell diameter for each dissected cell individually instead of taking an average on a frame may also allow for a higher resolution of the collected information. Overall, there are many factors to consider when examining the effect of small cell size on V. destructor mite population dynamics. Our study finds evidence that at least in the Nordic ranges of domestic beekeeping, a smaller cell size seems to help reduce the reproductive success of V. destructor, but this effect does not seem entirely relevant for bees already known to survive the parasite by means of natural selection. Indeed, A. mellifera populations in temperate European regions naturally display a larger brood cell size compared to African subspecies. This confers an advantage on the African honeybees in terms of flight abilities solely on the grounds of morphometric dimensions due to a better engine to aircraft mass ratio (Hepburn et al. 1999). Nevertheless, natural selection has favored larger cell sizes in the temperate regions. This suggests that surviving colder temperatures may be involved. In colder climates, many species adhere to Bergmann’s rule (Bergmann 1847), displaying a trend of larger body sizes at higher latitudes (Cushman et al. 1993; Olson et al. 2009), as this is a better adaptation to tolerate low temperatures. Small cell size and its potential to reduce the sizes of worker bees (McMullan and Brown 2006) may then prove a detriment in the long run to populations in higher latitudes, and though it may produce an effect on V. destructor, the overall reduction in competitive ability may render the effect negligible regarding colony survival. Small cell size, though potentially useful in aiding the management of parasites, may not be the key factor in achieving treatment-free, mite-surviving bees in temperate climates.
    Nehawka, Nebraska. My website: bushfarms.com/bees.htm en espanol: bushfarms.com/es_bees.htm auf deutsche: bushfarms.com/de_bees.htm em portugues: bushfarms.com/pt_bees.htm My book: ThePracticalBeekeeper.com
    -----"Everything works if you let it."--James "Big Boy" Medlin-----
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