© INRA/DIB-AGIB/EDP Sciences, 2010

1. INTRODUCTION

The mite Varroa destructor Anderson and Trueman is a natural ectoparasite of the eastern honey bee Apis cerana F, but now parasitizes the western honey bee Apis mellifera L. throughout much of its modern range. Mite reproduction is limited to the brood cells of its host bee, and it is clear in free-choice studies that Varroa preferentially enter comparatively large brood cells. When Message and Gonçalves (1995) compared brood reared in small worker cells produced by Africanized bees with brood reared in large cells produced by European bees, they found a 2-fold increase in mite infestation rates in the larger cells. When Piccirillo and De Jong (2003) compared Varroa infestation rates in three types of brood comb with different cell sizes (inner width), 4.84 mm, 5.16 mm, or 5.27 mm, they found that percentage of cells infested was significantly higher in the largest cells compared to the other two groups.

These kinds of observations have led to an interest among beekeepers in down-sizing comb foundations as a cultural control against Varroa. In North America, the resulting “small-cell” foundation measures 4.9 mm per cell (Dadant & Sons, Hamilton, IL, USA) compared to that of conventional foundation measuring between 5.2 mm and 5.4 mm. These numbers are derived by measuring the width of 10 cells in a straight line, inclusive of wall widths. In this study we challenged a null hypothesis of no difference in Varroa and bee population metrics between bee colonies housed on combs of small-cell or conventional-cell foundation.

2. MATERIALS AND METHODS

In three independent experimental replicates, we compared biometrics of Varroa mite and honey bee populations in bee colonies housed on one of two brood cell types: small-cell or conventional-cell. In spring 2006, foundation of both types was drawn during natural nectar flows prior to set up of the experiment. Small-cell foundation was drawn out by colonies containing honey bees which had themselves been reared in small-cell combs. Conventional foundation was similarly drawn out by colonies whose bees were derived from conventional combs. Once combs were drawn we determined realized cell width (walls inclusive) by counting the number of cells in 10 cm linear (n = 60 samples each cell type). Cell width from small-cell combs was 4.9 ± 0.08 mm and from conventional-5.3 ± 0.04 mm. In August 2006, bees were collected from a variety of existing colonies (irrespective of rearing history) and combined in large cages to achieve a homogeneous mixture of bees and Varroa mites. Twenty screened packages were made up, each containing ca. 2.0 kg (15966) bees. Packages were transported to a test apiary in Oconee County, Georgia, USA (33° 50′ N, 83° 26′ W) where each was used to stock one of 20 single-story deep Langstroth hives. Ten of the hives each contained ten frames of drawn small-cell comb, and the other ten contained drawn conventional-cell comb. One alcohol sample of ca. 300 bees was collected from each package to derive starting mite: adult bee ratios and, by extrapolation, beginning mite populations (colonies were broodless so all mites were phoretic on adults). Queens from a single commercial source were introduced into colonies. All colonies received sugar syrup and pollen patties as needed. Colonies were removed from the experiment if they died or their queens failed.

In March 2007 a second experiment of twenty colonies was established in the same manner as before with the following differences: each package contained ca. 1.45 kg (11612) bees, and colonies were established on foundation instead of drawn comb. A third experiment was set up in April 2008, each colony with 1.36 kg (10886) bees and started on drawn comb of the appropriate experimental type stored from the previous year; honey was removed from combs to remove variation in beginning food stores.

In June 2007 (for colonies started in August 2006 and March 2007) and in August 2008 (for colonies started in April 2008) we collected the following ending parameters: daily mite count on bottom board sticky sheet (72-h exposure), average mites per adult bee recovered from alcohol samples (ca. 100–300 bees), mites per 100 cells of capped brood, and brood area (cm². A measure of ending bee population was made by summing the proportions of whole deep frames covered by bees (after Skinner et al., 2001) then converting frames of adult bees to bee populations with the regression model of Burgett and Burikam (1985). Brood area (cm² was converted to cells of brood after determining average cell density as 3.93 per cm² for conventional-cells and 4.63 for small-cell. From cells of brood we calculated the number of cells sealed by applying the multiplier of 0.53 derived by Delaplane (1999). From mites on adult bees and mites in brood we could derive ending mite populations and percentage of mite population in brood – a positive indicator of the fecundity of a mite population (Harbo and Harris, 1999). Finally, for the August 2006 colonies we sampled adult bees in October 2006 for average body weight.

The duration of time between experiment start date and collection of ending Varroa population metrics was ca. 40 weeks for August 2006 colonies, 12 weeks for March 2007 colonies, and 16 weeks for April 2008 colonies. A field test of no more than 9–10 weeks is adequate to accurately appraise Varroa population change (Harbo, 1996).

An initial analysis was run as a randomized block analysis of variance recognizing the three experiment start dates as blocks and using the interaction of treatment and block as test term (Proc GLM, SAS 2002–2003). There was an interaction between treatment and block for ending colony bee population, so for this variable the analysis was performed separately for each start date and residual error used as test term. Differences were accepted at the α ≤ 0.05 level and where necessary means separated by Tukey’s test.

3. RESULTS

Significant effects of cell size were detected for ending mites in brood (F = 38.3; df = 1,2; P = 0.0252), percentage of mite population in brood cells (F = 57.4; df = 1,2; P = 0.0170) and ending mites per 100 adult bees (F = 23.8; df = 1,2; P = 0.0396). The ending number of mites in brood, percentage of mite population in brood, and mites per 100 adult bees were significantly higher in small-cell colonies (Tab. I). There was a significant interaction between start date and treatment for ending colony bee population (F = 5.14; df = 2,33; P = 0.0114) which is explained by the fact that populations tended to be higher in small-cell colonies except for the April 2008 start date. The advantage for small-cell colonies was significant for the August 2006 start date (F = 11.8; df = 1,4; P = 0.0264) (Tab. II).

We failed to detect significant effects of cell size on cm² brood, cells of brood, mites per 24 h sticky sheet, mites per 100 brood cells, and colony mite populations (Tab. I).

4. DISCUSSION

Although a significant and favorable trend for small-cell colonies was indicated for ending bee populations for the August 2006 start date (Tab. II), the chief interest in small-cell technology resides in its potential as a non-chemical limiter of Varroa population growth. By this criterion, the present results are not encouraging. The ending number of mites in brood, percentage of mite population in brood, and mites per 100 adult bees were significantly higher in small-cell colonies (Tab. I). Moreover, with all remaining ending Varroa population metrics, mean trends were unfavorable for small cell (Tab. I). We conclude that small-cell comb technology does not impede Varroa population growth. This null conclusion is reinforced by the facts that: (1) the experiment was replicated independently three times with start dates varying between spring and fall and test periods ranging from 12–40 weeks, (2) there were no interactions between start date and treatment for ending Varroa metrics, showing that responses were consistent across experiments, (3) the question of Varroa population growth was examined holistically with six dependent variables, and finally (4) the bar for performance should be high before a candidate technology is recommended for field use. It is worth noting that Varroa densities in this study (3.3–5.1 mites per 100 bees, Tab. I) were not within the action threshold of ca. 13 mites per 100 bees shown for the region by Delaplane and Hood (1999).

Interest in small-cell foundation has been fueled in part by observations of Martin and Kryger (2002) that conditions which constrict the space between the host pupa and male protonymph mite promote male mite mortality. However, as these authors point out, “reducing cell sizes as a mite control method will probably fail to be effective since the bees are likely to respond by rearing correspondingly smaller bees”. The present study supports this deduction directly, and its premise indirectly: average bee live weight in October was numerically smaller in small-cell colonies than conventional (Tab. I).

Acknowledgments

Technical assistance was provided by Dan Harris, Cody Sorensen, Eleanor Spicer, and Nicholas Weaver.

References

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