Gene-knockdown in the honey bee mite Varroa destructor by a non-invasive approach:
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Gene-knockdown in the honey bee mite Varroa destructor by a non-invasive approach: studies on a glutathione S-transferase
Ewan M Campbell,1 Giles E Budge,2 and Alan S Bowman 1
At the time of the oxalic acid treatments the bee colonies' strength and brood amount were average for Danish conditions. Neither eight days after the March treatment, nor at the first honey harvest in June a significant difference could be detected in oxalic acid concentration in food or honey between the treated groups and the control group. Eight days after the treatment the maximum level of oxalic acid was found in the sprayed group with a mean concentration of 0.0062%. For comparison, the natural concentration of oxalic acid (oxalates) based on fresh weight in spinach is 0.3-1.2%, in rhubarb 0.2-1.3%, in tea 0.3-2.0% and in cocoa 0.5-0.9% (Fassett 1973). Since oxalic acid is not fat-soluble no residues will build up in the wax in the treated colonies (Imdorf et al. 1998). Thus, residues in honey and wax after spring treatment with oxalic acid seems not to be problematic.
If a difference in GST activity from treated vs. non-treated colonies was found it could be an indication of a physiological effect on individuals in the treated colonies. In this study trickling treatment of colonies with oxalic acid does not seem to have a prolonged negative effect on the level of GST activity in pupae or newly emerged adults as no difference in GST activity in treated vs. non-treated colonies could be demonstrated. However, a lack of effect on the level of GST activity does not rule out that individuals in oxalic acid treated colonies were physiologically affected by the treatment.
During the season 1998, the treated colonies seemed to develop normally compared to the control colonies. Other Danish trials using the same methods in springtime confirm this finding (Hansen 1999). Only one of the treated colonies in the present study did not survive the winter 98/99. In spring 1999, the colonies in the present study had strength ranging from 4.93 to 6.25 comb gates and a brood amount of 0.89 to 1.53 dm2 which corresponds with the average in bee colonies at that time the country.
The results of this study do not give a direct measure of the efficacy of the two treatments. Oxalic acid spraying does not have an effect on varroa mites in the sealed brood (Radetzki 1994). Also trickling is only recommended in broodless colonies as experiments have suggested that the efficacy of one treatment in colonies with brood was too poor and several treatments weakened the colonies (Imdorf et al. 1998). Because of the brood present in our colonies, we assume that the efficacy of these spring treatment is lower than the approximately 98% found by Radezki (1994) and Imdorf et al. (1998) for spraying and trickling, respectively. In spring in Denmark, it is not possible to cut out the brood to increase the efficacy as it is done in the autumn by lactic acid treatment (Brødsgaard et al. 1997). Removing the brood in spring would most likely weaken the colonies. A proper efficacy test should of course be followed by a total count of mites in the colonies or a treatment with a pesticide with a well-documented effect. But since there are no pesticides registered for use in Denmark the honey, wax and equipment from the treated colonies would have to be destroyed and this was not possible in this preliminary study. Nevertheless, assuming that the varroa infestation was evenly distributed between the colonies there seems to be a tendency that the spraying treatment was more efficient than the trickling based on the observed varroa drop-down although the difference was not significant (FIGURE 1). The lack of difference in efficacy between trickling and spraying treatment corresponds with the findings of Imdorf et al. (1998) who found no differences when treating broodless colonies in autumn with the two methods. The varroa drop-down after the trickling treatment in the autumn showed that the varroa population in the colonies treated in spring seemed to have developed as well as in the control. That result could be explained by a poor efficacy of the spring treatment, reinvasion from the control colonies to the treated colonies combined with a very short persistence of oxalic acid or the very few control colonies.
Neither the residues of oxalic acid in honey, the GST activity, nor the colony development after spring treatment with either trickling or spraying with oxalic acid seem to indicate any problems. However, before the use of oxalic acid as spring treatments is recommended in Denmark it is necessary to put more effort into efficacy tests with a large number of control colonies in field trials.
In another apiary, the glutathione S-transferase (GST) activity was measured in individual pupae and adult bees from trickled and control colonies. The result showed that 15 days after treatment the GST activity in pupae and adult bees from the trickled colonies was not different from the GST activity found in non-treated colonies indicating that trickling treatment of colonies with oxalic acid does not seem to have an effect on the level of GST activity in pupae or newly emerged adult bees.
The varroa mortality was recorded after the spring treatments with oxalic acid trickling and spraying and again in the autumn after an oxalic acid trickling treatments. Furthermore, the bee colony strength and brood amount were recorded prior to the spring treatment and again a year after the treatments. A significant difference in varroa mortality was seen after the spring treatment between the treated colonies and the controls. In the trickling group the total mite drop-down per colony was in average 61.53, in the sprayed group it was 145.47 and in the control group 1.50. After the autumn treatment, no significant difference was found between the three groups and the mite drop-down ranged between 936 and 1,400 mites. In 1998, the mean bee colony strength was approximately 5.5 comb gates before the treatment. At the same time the mean brood amount ranged from 1.77 to 3.25 dm2. During the 1998 season, no difference in colony development was observed among the three trial groups. One year after the treatments the mean colony strength ranged from 4.93 to 6.25 comb gates. The brood amount ranged from 0.89 to 1.53 dm2. There was no significant difference between the treated groups and the control group at any time.