Mite bombs or robber lures? The roles of drifting and robbing in Varroa destructor transmission from collapsing honey bee colonies to their neighbors

Abstract

When honey bee colonies collapse from high infestations of Varroa mites, neighboring colonies often experience surges in their mite populations. Collapsing colonies, often called "mite bombs", seem to pass their mites to neighboring colonies. This can happen by mite-infested workers from the collapsing colonies drifting into the neighboring colonies, or by mite-free workers from the neighboring colonies robbing out the collapsing colonies, or both. To study inter-colony mite transmission, we positioned six nearly mite-free colonies of black-colored bees around a cluster of three mite-laden colonies of yellow-colored bees. We then monitored the movement of bees between the black-bee and yellow-bee colonies before, during, and after mite-induced collapse of the yellow-bee colonies. Throughout the experiment, we monitored each colony's mite level. We found that large numbers of mites spread to the black-bee colonies (in both nearby and distant hives) when the yellow-bee colonies collapsed from high mite infestations and became targets of robbing by the black-bee colonies. We conclude that "robber lures" is a better term than "mite bombs" for describing colonies that are succumbing to high mite loads and are exuding mites to neighboring colonies.

Fig 1. Birds-eye view of study site.

It shows the array of three mite donor colonies (MDCs) and six mite receiver colonies (MRCs). All hive entrances faced south.

Fig 2. Measurements of mite levels in study colonies.

Phoretic mite infestation levels for MDCs (A) and MRCs (B), and rates of adult (dark colored) mites dropping onto oiled boards underneath MRCs (C). The vertical black lines at 21-Sept and 26-Sept indicate the first day we observed fighting between workers of different color morphs at the entrance to any colony (dotted line) and the first day we observed intense robbing at any colony (dashed line). Phoretic mite measurements of each MDC ended when it died.

Fig 3. Records of fighting and robbing at MRCs and MDCs from 1 Sept to 20 Oct.

An empty circle indicates when any case of worker-worker fighting at the hive entrance was seen. A filled square indicates when intense robbing of a colony was seen.

Fig 4. Foreign workers at MRCs.

Percent of workers at the hive entrance of each MRC that were yellow (from the MDCs). Flat colored rectangles denote instances when no yellow bees were seen.

Fig 5. Drone drift into MDCs and MRCs over course of experiment.

Data collection ended when all drones were evicted from the study colonies in late September. No yellow drones (from the MDCs) were recorded entering MRCs 300E or 50E.