For the Love of Bees and Beekeeping

April 2014

On Small-Cell Foundation

by Keith Delaplane

The worldwide spread of the Varroa mite has morphed beekeeping from one of the most chemical-averse agricultural industries to one of the most chemical-dependent. It is widely thought that the only practical control for mites is the use of toxic miticides inside the hive. It is equally agreed that this is a sorry state of affairs because it’s a fine line between killing one arthropod and sparing another when both live in the same space.

As a result, there has been an outpouring of creativity among scientists and beekeepers with the aim of controlling mites without pesticides. One of the most visible of these projects has been the use of small-cell foundation.

The idea behind small-cell foundation is the fact that mites can only reproduce in bee brood cells, and a few studies have shown that, if given a choice, Varroa mites prefer comparatively large brood cells. This preference seems to hold whether mites are given a choice of brood cells made by European bees (larger cell size) versus African bees (smaller cell size)1 or if mites are experimentally given a choice of three cell sizes: 4.8 mm, 5.2 mm, or 5.3 mm2. It is reasonable to assume that colony mite population growth would be correspondingly reduced in colonies with brood cells smaller than the mites’ natural range of choices.

These observations ultimately led to a commercially-available product, a small-cell foundation that measures 4.9 mm per cell compared to conventional foundations ranging around 5.2 mm to 5.4 mm. This product is available in bee supply catalogs and many beekeepers use it as part of an overall Varroa control strategy.

The trouble is, the practice has not held up to experimental challenge. I was part of a team that tested small-cell foundation as a means for reducing colony mite populations3, and I want to give an overview of our experience here.

We set up three independent studies with colonies with one of two brood cell types: small-cell (4.9 mm cell width) or conventional-cell (5.3 mm). In one study, ending colony bee population was significantly higher in small-cell colonies than conventional-cell. However, the main interest for small-cell foundation is its effects on mites, and on this count small-cell didn’t do so well. In fact, small-cell colonies were significantly higher for mite population in brood, percentage of mite population in brood, and mites per 100 adult bees. We were forced to conclude that small-cell foundation does not slow Varroa population growth. This conclusion is reinforced by the fact that the experiment was replicated independently three times with start dates ranging between spring and fall and test periods ranging from 12-40 weeks.

The work of Martin and Kryger4 seemed to support small-cell foundation when they observed that mortality of male offspring mites was increased under conditions which constrict the space between the bee pupa and male. However, these same authors pointed out that, “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.” We found some evidence to support this. In one of our trials we compared average body weight of bees reared in the two cell types and found that average bee weight was smaller in small-cell colonies.
These results were pretty convincing to me, but other authors have chimed in too. Small-cell foundation was shown to be ineffective in reducing mite population growth in independent tests in Florida5 and New Zealand6. In fact, I am unaware of any publicly-accessible peer-reviewed papers that directly support it.

If our results were convincing to ...