Researchers have observed that on the island of Gotland in Sweden, honey bee colonies that were abandoned and left untreated for ten years led to the emergence and selection of spontaneously varroa-resistant strains. Subsequently, several other naturally resistant colonies were discovered in other parts of the world. This article presents the state of the literature on the three phenomena influencing resistance behaviours of the honey bee in response to the varroa mite.
Compliance with recommended Varroa treatments improves winter survival
This field study assesses how adherence to recommended Varroa destructor control regimens affects winter survival of honey bee colonies. The focus is on the Swiss strategy based on two formic acid treatments after honey harvest and an oxalic acid treatment during the broodless winter period.
Over two years, 300 colonies managed by 30 beekeepers in three Swiss cantons were monitored. Beekeepers were classified as compliant (correct number and timing), almost compliant (correct number but incorrect timing), or noncompliant (too few treatments). Varroa infestation, brood production, honey yield and winter mortality were recorded.
Results show a very strong link between compliance and survival. Fully compliant colonies had around 2 % winter losses. Even minor deviations from the recommended timing increased mortality risk about tenfold, while clear noncompliance increased risk up to 25-fold. Varroa infestation levels in October were critical: at roughly 10 mites per 100 adult bees, the probability of winter loss reached about 50 %.
Compliant colonies showed lower autumn Varroa infestation, larger amounts of capped brood and significantly higher honey yields. Reducing the number of treatments did not protect brood; high Varroa infestation had a much stronger negative effect than the treatments themselves.
After beekeepers were informed of the results following the first year, compliance improved and winter losses declined in the second year.
In conclusion, colony survival depends less on whether Varroa treatments are applied than on whether they are applied fully and at the correct time. This study provides strong field evidence that deviations from recommendations are a major driver of winter colony losses.
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