4.6. Breeding mites in colonies
A common problem for varroa research is obtaining mites in sufficient quantities for experiments. It is desirable to obtain mites already early in the season when their numbers in the colonies is still low and in large quantities for as long as possible thereafter. The method described here allows, within a short time, the regular harvesting of a high number of phoretic mites early in and throughout the season. The method is based on the trapping comb originally designed to control the mite (Fries and Hansen, 1993; Maul et al., 1998). It consists in caging the queen from an infested colony, and letting all the brood emerge. Once the colony is broodless and all mites are in the phoretic stage, a comb of open brood is introduced. Just before capping, most of the phoretic mites looking for reproduction opportunities will enter the cells provided. Once the brood is capped, the comb is removed and placed in an incubator until the bees emerge. The newly emerged non-flying and non-stinging bees will be highly infested with varroa, making mite harvesting easy and fast. The infested comb can also be retrieved at any time to obtain mites at a particular developmental stage. This method is further developed to optimise logistical aspects according to the following protocol:
1. Prepare several hives as breeder colonies during the season preceding the experiments.
2. Adjust varroa treatment during the season preceding the experiments to ensure the survival of the colonies, but to also allow the survival of a relatively high number of mites over the winter.
This makes it possible to keep a starter mite population for a fast growth in parasite numbers following the winter.
3. In the next year, when the colonies are well developed, the weekly natural mite fall is counted over a brood cycle (3 weeks).
4. Rank the colonies according to their mite load and strength.
The most infested hives should be used first since they are susceptible to collapse before less infested colonies. The parasite population can still be left to grow in the less infested colonies until they are used for mite collection. Among several hives with the same range of infestation, those closer to swarming stage can be used first. This makes it possible to prevent swarming and the loss of mites.
The breeding cycle can start:
5. Day 1: cage the queen from the colonies selected for mite rearing.
At day 22, all the brood present at day 1 will have emerged.
6. Day 12: prepare the trapping comb:
6.1. Select a strong colony (brood provider) with an actively laying queen.
6.2. Cage the queen on an empty dark comb (that queens prefer for egg-laying) and placed in the brood nest of her colony.
7. Day 13: after 24 h, remove the queen from the cage, but leave the comb in the cage to prevent further egg-laying by the queen.
This comb contains brood of similar age in which varroa mites will later be trapped. To increase chances of obtaining enough brood for trapping, queens of several colonies can be caged and the comb with the most brood is used.
8. Day 19:
8.1. Transfer the trapping comb that now contains 7 days old brood to the varroa rearing colony, of which the brood has emerged.
8.2. Release the queen of the rearing colony so that she can resume her egg-laying activity.
9. Day 22: the brood cells of the trapping comb have been capped.
9.1. Remove the comb from the rearing colony.
9.2. Transport to the laboratory.
9.3. Place in a well-ventilated bee tight box.
9.4. Keep in an incubator at 34.5 °C with 60-70 % relative humidity until adult worker emergence.
The comb should contain sufficient pollen and honey supplies so that the emerging bees can feed. Is it not the case, food should be supplied.
Work on the rearing apiary should end with the collection of the trapping comb so that it does not remain for too long outside the colony before being placed in the incubator. To avoid damage to the brood transport should be done in a thermoregulated and moist container.
10. Day 33: Start collecting mites from the infested workers emerging this and the following day.
For mite collection, bees can be held with forceps and the mites caught with a size 00 paintbrush or a mouth aspirator.
During the rearing cycle, the colony experiences 2 to 3 weeks without brood. After two subsequent brood cycles, the colony has usually regained strength and the varroa population will have increased again. Given that the varroa natural fall indicates a sufficiently large varroa population, the same colony can be used again to harvest mites. Furthermore, depending on the amount of mites needed, several colonies can be used at a time to increase the harvest. Breeding cycles on new colonies can be started every week. Thus, after 5 weeks, batches of mites can be harvested weekly. The additional time axis shown at the bottom of Fig. 21 illustrates how most of the working days can be combined for colonies or groups of colonies at different stages in the cycle. By starting on a Thursday for example, no work on a Wednesday, Friday or weekend day is necessary and mite collection always occurs on a Tuesday.
Pros: the method allows the collection of mites indoors rather than on the apiary, prevents the danger of robbing by neighbouring colonies since the colonies do not remain open for mite sampling, necessitates few visits to the breeding apiary, allows the collection of mites on a particular day, facilitates sampling as the density of mite per emerging bee is high.
Cons: logistic intensive; if mites are
collected from emerged workers older mother and young daughter mites are not
Fig. 21. Timeline of the rearing cycle. An additional cycle is depicted below the main timeline to illustrate how the various tasks (symbolized by arrows) can be combined between different rearing cycles to optimize the process.