Preventing Varroa Reinfestation From Drifting and Robbing
Varroa does not stay where you put it. Mites spread between colonies when bees drift to neighboring hives or engage in robbing behavior. A colony that you treated to a clean 0.5% mite count in September can be back above 2% by November if it is receiving drifting bees from an untreated neighbor's collapsing hive. Understanding how reinfestation works is essential for maintaining the gains from your treatment program.
How Bees Move Mites Between Colonies
Drifting is the normal navigation error that worker bees make when returning from foraging flights. Bees orient to their own hive using sun position, landmarks, and hive odor. In apiaries with multiple hives in close proximity, particularly when hives are arranged in straight rows and look similar, worker bees routinely enter the wrong hive. These drifting bees carry their phoretic mite load with them into the recipient colony.
Drifting happens in every apiary with multiple colonies. It is a normal behavior, not a pathology. The problem arises when colonies with very different mite loads are in close proximity. A low-mite colony that receives drifting bees from a high-mite colony is continuously receiving mite imports.
Robbing is more aggressive and more effective at moving mites. When a colony is weak, dying, or recently collapsed, neighboring bees raid it for its honey stores. Robber bees return to their home colony carrying honey and varroa mites. A collapsing high-mite colony that gets robbed out by its neighbors is essentially a mite bomb that distributes its mite population across the local area.
Scale of Reinfestation
Research has shown that reinfestation can account for a significant portion of mite population growth in managed apiaries, particularly in areas with high apiary density. In some studies, 30 to 50% of mite increase in treated colonies during late summer was attributable to reinfestation from neighboring colonies rather than in-colony reproduction.
This means that treating your own colonies correctly is a necessary but not always sufficient condition for maintaining low mite levels. Your neighbors' management practices affect your mite counts.
Identifying Reinfestation in Your Records
Reinfestation produces a recognizable pattern in mite count data: mite counts that rise faster than expected after a successful treatment, particularly in colonies near the downwind or downdrift side of an apiary, or in apiaries located near other managed or feral colonies.
If your post-treatment count shows 0.4% efficacy of 90%, and six weeks later the same colony is back at 2%, that rate of increase is faster than typical in-colony reproduction alone would produce. Reinfestation is the likely explanation.
Look for patterns in your VarroaVault data: which colonies in a yard consistently rebound fastest? Are they the ones at the end of the row where drifting is more common? Are they the colonies closest to a neighboring apiary? Location patterns in high-reinfesting colonies suggest drifting or proximity to an outside source.
Reducing Drifting
Reducing drifting within your own apiary reduces internal mite exchange between your colonies. Practical steps:
Visual differentiation. Paint hive boxes different colors or mark them with different entrance patterns. Bees use visual cues to orient to their home hive. More distinctive hives see less drifting.
Spatial arrangement. Avoid straight row arrangements where all hives face the same direction and look identical. L-shaped, curved, or scattered arrangements reduce drifting. Hive entrances facing slightly different directions also help.
Entrance size standardization. If some hives have large entrances and others have reduced entrances, drifting bees preferentially enter the larger openings. Consistent entrance sizing reduces this asymmetry.
Landmark placement. Plant distinctive vegetation near individual hives. A beekeeper who places a rosemary bush next to one hive and a stone marker next to another gives bees better orientation cues.
Managing Late-Season Drifting and Robbing
Late summer and fall are the highest-risk periods for robbing and mite-carrying drifting. The nectar flow has ended, foragers are not finding much, and colonies are in a heightened robbing state. Collapsing colonies are most common in this period as the summer mite buildup peaks.
Reduce entrances in late summer and fall to limit robbing exposure. A full-width entrance on a weak colony is an invitation to robbers. Narrow the entrance to one bee width for any colony that is not at full strength.
If a neighboring colony collapses, close it immediately. Do not leave a dead-out open for robbing. Either wrap the equipment until you can deal with it or seal it completely. A slowly dying hive open for robbing is the most efficient mite distributor in your apiary.
Community-Level Reinfestation
The most significant reinfestation source for some beekeepers is feral bee populations or neighboring managed apiaries. In areas with high bee density, such as urban settings or areas with many hobby beekeepers, reinfestation pressure can undermine even excellent individual management programs.
The most effective response is engaging your local beekeeping community. Beekeepers who share information about management practices, who conduct coordinated treatment programs within a region, and who support responsible swarm removal of feral colonies reduce the regional mite burden for everyone. Individual excellence in varroa management is more effective in a community context of general competence.