1. Key points

• Hives that are closely spaced, arranged in rows, and highly similar promote bee drifting.
• This drifting can facilitate the transfer of varroa mites between colonies.
• Swarming sometimes temporarily reduces infestation, but this advantage can be lost in a dense apiary.
• Late summer remains the critical period: varroa directly threatens the winter bees.
• At the apiary level, the challenge is to limit drifting, robbing, and re-infestation, without replacing the varroa management concept.

2. What the study shows

Fig.: Map of the study site showing the location of the 24 colonies studied: 12 hives in the apiary (indicated by the row of white squares) and 12 hives in and around the field (indicated by black squares surrounded by circles).

This study experimentally tests a very concrete question: does concentrating colonies in the same apiary facilitate the transmission of varroa between colonies?

Research question. Seeley and Smith sought to determine whether grouping colonies in an apiary increases their vulnerability to Varroa destructor. Their hypothesis is straightforward: when hives are close together, similar in appearance, and oriented in the same direction, return errors by bees — drifting — can facilitate the passage of mites and viruses from one colony to another.

Method. The authors installed 24 colonies near Ithaca, New York. Twelve colonies were placed in a densely packed apiary in a row, with approximately 1 m between hives. Twelve other colonies were distributed across the same environment but spaced 21 to 73 m apart, with a mean distance to the nearest neighbour of approximately 34 m. Colonies were constituted in matched pairs with comparable initial varroa infestation levels, then left without varroa treatment for the two years of the trial (Seeley & Smith, 2015).

To measure drone drifting, the authors used two lineages producing drones of different colours. They then observed whether the drones returned to the hive corresponding to their origin or to a different hive. Varroa pressure was monitored by natural mite drop on a greased varroa insert board and, in 2012, also by the icing sugar method on 300 bees.

Results. In the dispersed group, drones almost always returned to the hive corresponding to their origin. In the densely packed group, the result was very different: approximately one third of the drones observed did not match the type expected in the colony being visited. In other words, the dense arrangement caused substantial drone drifting.

During the second season, seven out of eleven colonies swarmed in each of the two groups. After swarming, colonies that had swarmed showed much lower varroa pressure than colonies that had not swarmed. At the end of June, non-swarming colonies had an average of 18.5 varroa mites per 300 bees, compared with 2.9 mites per 300 bees in swarming colonies. At the end of July, the difference remained clear: 24.6 versus 4.0 mites per 300 bees.

The decisive result appeared in late summer. In the dispersed group, colonies that had swarmed and then successfully requeened maintained a low infestation level. In the densely packed group, the two comparable colonies experienced a sharp increase in infestation. By the end of August, they averaged 11.5 mites per 300 bees, compared with 1.6 in the swarming colonies of the dispersed group.

Interpretation. The study suggests that swarming can temporarily reduce the varroa population of a colony, but that this effect may be negated in a dense apiary. The authors propose two probable mechanisms: drifting of mite-carrying bees or drones, and robbing of weakened colonies. By late summer, heavily infested colonies showed signs of poor health, notably bees with deformed wings. All heavily infested colonies died over winter, while lightly infested colonies survived.

3. Critical appraisal

The result is compelling, but it should not be turned into too simple a rule: the study demonstrates a plausible mechanism within a specific experimental design.

The main strength of the study is its comparative design. Both groups of colonies were installed in the same environment, with matched starting colonies and monitoring over two seasons. The comparison between densely packed and dispersed hives therefore rests not merely on field observation but on an experimental setup.

The study also links several levels that are relevant for the beekeeper: drifting behaviour, varroa dynamics, swarming impulse, health status in late summer, and winter survival. It serves as a reminder that varroa pressure is not solely a matter for an individual colony: it also depends on exchanges between colonies.

The limitations are, however, significant. The number of colonies remains small: 24 at the outset, then 22 after the loss of two colonies weakened by chalkbrood. In the second year, only two colonies in the densely packed group had both swarmed, successfully requeened, and could be compared with the five comparable colonies in the dispersed group. The signal is strong, but the sample size remains small.

The context also differs markedly from a Swiss or European apiary managed according to current recommendations. The colonies received no varroa treatment for two years. This choice is useful for observing the natural dynamics of the parasite, but it does not correspond to responsible varroa management in Switzerland. The study should therefore not be read as an invitation to leave colonies untreated.

A further point of caution: the study does not establish exactly how mites passed from one colony to another. Drone drifting is well documented in this experimental setup, but worker drifting was not measured directly. Robbing is also plausible, especially during a dearth, but it was not quantified colony by colony. The authors therefore remain cautious about the precise mechanism.

Finally, natural swarming cannot be transformed into a straightforward recommendation. Yes, in this study it reduces varroa pressure in the short term. But it also involves swarm loss, the risk of requeening failure, reduced production, and less controlled management. In practice, brood breaks used against varroa must be planned as controlled biotechnical measures, not as a laissez-faire approach.

4. What related studies show

Related studies confirm the general finding, but with an important nuance: apiary density matters primarily when it promotes drifting, robbing, or the entry of foreign bees.

The results of Seeley and Smith are not isolated. A more recent experimental study compared very dense apiaries — with closely spaced, similar, aligned hives — with less dense apiaries arranged in a circle and made more visually distinct. The less dense apiaries showed less drifting, lower varroa levels, better honey production, and better winter survival (Dynes et al., 2019). This is the most direct support for the idea that the physical organisation of an apiary can alter parasite dynamics.

Another study tested the effect of inter-colony distance by placing hives 0, 10, or 100 m apart. The most distant colonies had on average fewer varroa mites than closely spaced colonies, supporting the idea of proximity-dependent transmission. These distances are not, however, directly applicable to all Swiss or European production apiaries (Nolan & Delaplane, 2016).

The drifting mechanism is also well documented. Classic work on hive arrangement shows that homogeneous rows, entrances oriented in the same direction, and a lack of visual landmarks increase return errors, while irregular arrangements, different colours, and varied orientations reduce them (Jay, 1965, 1966a, 1966b, 1968; Pfeiffer & Crailsheim, 1998). More recently, Forfert et al. showed that inter-hive distance and position strongly structured drifting in a dense apiary, and that heavily infested colonies could accept more foreign bees (Forfert et al., 2015). This point is important: a heavily infested colony can become not only diseased itself but also more involved in undesirable exchanges between colonies.

Autumn re-infestation is a second related mechanism. Frey and Rosenkranz measured markedly stronger autumn mite invasions in high-colony-density environments than in less dense ones (Frey & Rosenkranz, 2014). Other work shows that weakened or collapsing colonies can act as varroa sources for neighbouring colonies, particularly through robbing or visits by foreign bees (Peck & Seeley, 2019; Kulhanek et al., 2021). This directly supports the interpretation proposed by Seeley and Smith: in a dense apiary, the temporary advantage of a colony that has reduced its infestation can be cancelled out by incoming mites from other colonies.

Studies on swarming and brood breaks provide a useful complement but should not be overinterpreted. Fries et al. showed that natural swarming could temporarily reduce varroa levels, primarily through the departure of a proportion of mite-carrying bees and through interruption of mite reproduction during a period without capped brood (Fries et al., 2003). More recent work on induced brood breaks confirms that the temporary absence of brood can disrupt varroa reproduction (Gabel et al., 2023). These results do not, however, make natural swarming a recommended control method in a managed apiary: the effect may be partial, temporary, and rapidly cancelled by re-infestation.

The relationship between late-summer varroa, viruses, and winter survival is also well supported. Work by Dainat and van Dooremalen shows that varroa infestation pressure and Deformed Wing Virus reduce the lifespan of winter bees and increase the risk of winter losses (Dainat et al., 2011; van Dooremalen et al., 2012). This gives the study by Seeley and Smith its full practical significance: what matters is not merely the presence of varroa mites, but the infestation level at the point when the colony is producing the bees destined to survive the winter.

The convergence is not, however, absolute. A recent Swiss study indicates that mite immigration can account for a substantial proportion of the mites present in late summer, but that this immigration is not always explained simply by local colony density within a given radius (Guichard et al., 2024). In other words, it would be too simplistic to state: "the more hives, the more varroa mites." Density creates a risk, but that risk also depends on the treatment schedule, the presence of weakened colonies, robbing, nectar availability, drifting, and the practices of neighbouring beekeepers.

In summary, related studies reinforce the interpretation of Seeley and Smith: dense, homogeneous, and row-arranged apiaries promote drifting and can facilitate the spread of varroa. The evidence is strongest for drifting and re-infestation at the apiary or immediate neighbourhood scale. It is more nuanced at the landscape scale, where management practices and coordination between beekeepers can carry as much weight — or more — than colony density alone.

5. Practical implications for the apiary

For practice, the study is a reminder that an apiary is not merely a collection of independent colonies: the health of one hive can influence that of its neighbours.

• Limit drifting where possible. Where the site allows, avoid long rows of identical, closely spaced hives all oriented in the same direction. Different colours, visual landmarks, slightly varied orientations, less regular groupings, or more generous spacing can all help.
• Think about varroa at the apiary level. A heavily infested, weakened, or collapsing colony is not merely an individual problem: it can become a source of re-infestation for neighbouring colonies. Failing colonies must be assessed promptly, united if that is warranted, or destroyed in accordance with health regulations.
• Pay particular attention to late summer. After the honey harvest and during the preparation of the winter bees, a late rise in infestation must be taken seriously, even in a colony that appeared strong a few weeks earlier. Monitoring by natural mite drop, icing sugar, or alcohol wash remains indispensable depending on the context.
• Reduce the risk of robbing. During a dearth, reduce hive entrances if necessary, work cleanly, avoid prolonged manipulations, do not leave honey accessible, and feed without triggering competition between colonies.
• Use brood breaks as a controlled tool, not natural swarming. Swarming illustrates the biological principle, but remains too unpredictable to serve as a varroa management tool. Planned brood breaks do not replace the varroa management concept: they must be integrated into a coherent control strategy, with authorised treatments and infestation monitoring.

Read the original study

Seeley, T. D., & Smith, M. L. (2015). Crowding honeybee colonies in apiaries can increase their vulnerability to the deadly ectoparasite Varroa destructor. Apidologie, 46, 716–727. DOI: 10.1007/s13592-015-0361-2.

Further reading on ApiSavoir

• Practical Guide 1.1: Varroa management concept
• Practical Guide: 1.5.1 Natural mite drop count
• Practical Guide: 1.5.2 Icing sugar method
• Practical Guide: 4.8.3 Robbing
• Varroa: brood break
• Practical Guide: 4.9 Choice of apiary site

Bibliography

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Dynes, T. L., Berry, J. A., Delaplane, K. S., Brosi, B. J., & de Roode, J. C. (2019). Reduced density and visually complex apiaries reduce parasite load and promote honey production and overwintering survival in honey bees. PLoS ONE, 14, e0216286. https://doi.org/10.1371/journal.pone.0216286

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