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By Otto Boecking, Institut für Landwirtschaftliche, Germany
Varroa jacobsoni is a predatory mite that lives on honeybees. In recent years it has been spread throughout the world by beekeepers. It is now the worst bee disease of Apis mellifera. However, Varroa jacobsoni does not show notable disturbance of the Asiatic hive bee Apis cerana. Therefore in Apis cerana beekeeping, treatments against Varroa are not necessary. Varroa jacobsoni regularly infest Apis cerana colonies, yet many Apis cerana beekeepers have never seen a Varroa mite in their colonies! Apis cerana and Varroa have built up a balanced host-parasite interrelation in which neither the bees nor the mites completely kill the other species. This is because during their long, evolutionary process in Apis cerana colonies the growth of Varroa populations is restricted to the time when drone brood is present. Varroa jacobsoni cannot reproduce in worker brood. In contrast to this, Varroa does reproduce successfully in both drone and worker brood of Apis mellifera bees and can build large mite populations during the whole brood rearing season. For this reason chemical treatments against Varroa are used regularly in Apis mellifera beekeeping world-wide.
How do Apis cerana keep Varroa in check?
Non-reproduction of Varroa in worker brood
In colonies of Apis cerana the main reason for the restricted population growth of the mite is partially explained by physiological barriers in the worker brood and/or in the mite itself.
Apis cerana actively defend themselves by removing mite-infested brood. This may also lead to the lack of Varroa jacobsoni in worker brood.
If Varroa mites enter Apis cerana worker brood cells, many of these infested brood cells are detected by adult worker bees running over the comb. To hinder the mites from reproducing, the bees uncap these cells and remove the bee larvae or pupae. Consequently the mites fail to reproduce in these cells.
This removal behaviour is shown much less in Apis cerana’s Varroa-infested drone brood cells. Perhaps worker bees are hindered from uncapping mite-infested drone cells because of the unique, thick cocoon capping of Apis cerana drone cells. Maybe this allows Varroa to reproduce in only drone brood. But in the case of multiple-infested drone brood cells (more than two invaded mother mites per cell) the parasitised drones are weakened and they are not able to open the cell capping from the inside. Since adult worker bees also do not open these highly-infested drone cells from the outside, the parasitised drones have to die together with the whole number of infesting mites in the cell. We can describe this as a natural “Varroa-trap”. This non-removal behaviour of Apis cerana towards multiple-infested drone cells is sometimes also supplemented by the worker bees plugging the central pore with wax material from outside. This plugging of the central pore in Apis cerana drone cells by the worker bees can also be seen during infestation with European foulbrood, isolating this pathogen within the brood nest. If the colony now absconds, many pathogens are left behind in the old nest, isolated in the non-removed and plugged cells.
Another known defence behaviour of Apis cerana that keeps Varroa in check is the bees’ grooming behaviour towards mites. Sometimes worker bees realise that mites are hiding on them. They try to get rid of them by self-grooming. If they fail to remove the mites they ask for help from nestmates using a specific shaking dance. Self- and nestmate-grooming leads to a disturbance and sometimes killing of the mites by the bees. The distinct phoretic positions preferred by the Varroa mites on the bodies of the bees show some adaptation towards escape from the grooming behaviour of the bees.
During evolution Apis cerana bees have developed a complex of these several defence behaviours to keep Varroa in check. “How clever they are!”
Varroa jacobsoni has only recently been spread into populations of Apis mellifera bees that fail to keep Varroa in check. This is because the defence behaviours present in Apis cerana exist to only a small extent in Apis mellifera, and its reproduction system is highly favourable for the Varroa mite to build large populations.
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[Bees for Development Journal #48]