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Preference of Varroa Jacobsoni Oudemans for Different Cell Types and Some Factors Affecting Reproduction

Apiacta, Feb. 1984 – pages 165-167

DE RUIJTER, A. (Netherlands)

Reproduction of Varroa only occurs inside capped worker and drone brood cells. Prior to reproduction, female mites leave the adult bees and enter brood cells just before capping. The mites enter the larval food and stay there until the bee larva frees them by eating the food. At this time, the cell has been capped. More mites are found in drone cells than in worker cells. There seems to be chemical attraction which is stronger in drone larvae than in worker larvae. In queenless colonies, more mites were found on drone pupae in drone cells than on drone pupae in worker cells. In Brazil, more mites were found in large worker cells than in smaller ones. These two observations indicate that features of the cell are important in respect of distribution of Varroa on the comb.

In our first experiment, we tried to establish this by offering different types of cells on one comb to Varroa-infected colonies.

Feeding on the larva inside the capped brood cell initiates oviposition. It has been demonstrated that application of juvenile hormone to honeybee larva, just after cell capping, stimulates the reproduction of Varroa. Oviposition starts 60 hours after cell capping. An egg is laid about every 30 hours. As a rule, the first egg is female, the second male, the third and following eggs are also female. Females develop from fertilized eggs and are diploid, males develop from unfertilized eggs and are haploid.

In our second experiment we studied factors influencing the pace of oviposition and the success of reproduction.

Experiment 1.

Material and methods

We put drops of melted beeswax in a worker comb in a regular pattern into certain cells: in every fourth horizontal row of cells, every fourth cell was made 3 to 4 mm less deep than neighbouring cells. This comb was placed inside the broodnest of a Varroa infected colony. When most of the brood was capped, we examined the contents of treated and untreated cells. We scored the number of adult Varroa females that had entered the different cells. This was done separately for five age classes of the bee brood, according to Ifantidis.

Results and discussion

The preference is always bigger than one (mean=6.97), except for one observation where the number of treated cells was very low. This means that adult female mites enter protruding cells more frequently than normal worker cells, though all cells contain worker brood.

Conclusion

Apparently chemical attraction is not the only factor that determines the distribution of mites in available cells; cell type is also important. The mechanism by which the mites discriminate between different cell types is not clear. Probably. the behaviour of the bees that carry the mites to the brood cells is different towards different cell types. The process of leaving the bee and entering the brood cell needs further study. Intervention in this process might lead to biological control.

Table 1.
Number of Cells Examined and Mean Numbers of Adult Female Mites per Cell for Each Age Class of Bee Brood
Age
class of the bee brood*
Untreated cells Treated cells Preference**
(B/A)
Number of cells Mean number of adult
female mites per cell (A)
Number of cells Mean number of adult
female mites per cell (B)
I 153 0.013 25 0.080 6.1
III 199 0.005 44 0.091 18.2
IV 133 0.038 10 0.200 5.3
II 52 0.019 9 0.222 11.6
III 49 0.041 11 0.564 8.9
III 106 0.283 18 0.556 2.0
IV 159 0.264 50 1.280 4.8
II 47 0.0532 6 2.667 5.0
III 26 0.731 3 0.667 0.9
* I=prepupa, II=pupa with white eyes, III=pupa with red eyes, IV=pupa early tanning, V=pupa with brown thorax.
** The mean number of mites in treated cells divided by mean number of mites in untreated cells is given as “preference”.
Table 2.
Mean Offspring of Females Confined in a Newly-capped Drone or Worker Cell and then Put in a Newly-capped Worker Cell
Period in brood cells: Number of cells Egg/

larva

Protonymph Deutonymph Adult
Mobile Immobile Mobile Immobile
24 hrs drone + 9 days worker n=19 0.0 0.5 0.6m 0.9m 1.8m 0.9f

0.8m

48 hrs drone + 9 days worker n=10 0.2 1.0 0.1m 1.3m 1.1m 1.7m
48 hrs drone + 7 days worker n=15 0.7 0.7 0.2m 1.7m 1.1m
m = male
f = female

Experiment 2.

Material and methods

Open worker brood cells were marked by a sheet of transparant plastic, attached to the frame by two drawing pins. The comb was then put back into the colony. After 4 hours, newly-capped cells were numbered and opened by using a razor blade. A female Varroa mite was introduced into each cell, the cells were carefully closed and the comb placed back in the colony again. After a desired period the cells were opened and the content studied.

We tried to increase the stimulation of oviposition by transferring mites 24 or 48 hours after cell capping into newly-capped cells. In this way, the mites were able to feed on round larvae and spinning larvae twice and would get a double amount of any factor in the haemolymph of the bee.

Results and Discussion

After 24 hours extra in a worker cell or drone cell, a male is produced in most cases. Apart from an adult female, in many cases there are 3 female deutonymphs. Compared to 10 days, the intervals between the successive eggs must have been shorter.

After 48 hours extra in a worker or drone cell, no males are produced at all. In many cases there are three deutonymphs (means are 2.4 and 3.0). This again suggests that the interval between successive eggs is shorter than in the normal situation.

Conclusion

Oviposition, as well as sex determination are affected by stimuli during the first two days inside the capped brood cell.