In vitro susceptibility of Varroa destructor and Apis mellifera to native strains of Bacillus thuringiensis

Fifty-four Bacillus-like strains were isolated from dead Varroa destructor collected in 24 colonies of bees from seven apiaries. The total proteins of 22 isolates were evaluated against adult female V. destructor (100 μg/mL), but only nine caused over 80 % mortality. The LC₅₀ of EA26.1, EA3 and EA11.3 isolates were calculated. The most virulent isolate was EA26.1, as 96.7 % of the mites were killed 36 h after treatment, and the LC₅₀ was 1.50 μg/mL. EA3 produced 93.3 % mite mortality after 48 h treatment, with an LC₅₀ of 7.1 μg/mL. Finally, EA11.3 produced 93.3 % mite mortality after 60 h treatment, with an LC₅₀ of 22.8 μg/mL. Additionally, the EA3 and EA26.1 isolates were used to determine mortality on adults and larvae of worker bees of A. mellifera. When the 16S rRNA gene was amplified and sequenced, the three strains were found to belong to the species B. thuringiensis. The results showed that honey bee adults and larvae are not negatively affected by identified B. thuringiensis isolates, indicating that they could be successfully used for the biological control of V. destructor without adverse effects on adults and larvae of A. mellifera, and reduce the negative impact of this mite on colonies.