Queen survival and oxalic acid residues in sugar stores after summer application against Varroa destructor in honey bees (Apis mellifera)
Bram Cornelissen, Jeroen Donders, Pam van Stratum, Tjeerd Blacquière, and Coby van Dooremalen
Methods using oxalic acid (OA) to control Varroa destructor in honey bee (Apis mellifera) colonies are widely applied. In this study, the effects of an OA spray application in early summer on the survival of young and old queens, and on OA residues in sugar stores were investigated. A questionnaire among beekeepers was used to determine the ‘normal’ and ‘acceptable’ queen mortality as a result of beekeeper activities. ‘Acceptable’ queen mortality (4.1 ± 0.1% (n = 11)) did not differ from queen mortality after OA spray application (2.7% for old and 3.8% for young queens). ‘Normal’ queen mortality (1.1 ± 0.4% (n = 11) and 4.2 ± 0.1% (n = 11) for old and young queens, respectively) also did not differ from queen mortality after spraying OA. OA found in sugar stores of colonies sprayed with OA (94 ± 7 mg/kg (n = 8)) did not differ from control colonies (80 ± 4 mg/kg (n = 9)). Finding OA residues in both groups was probably due to bees foraging on chestnut (Castanea sativa) trees. We conclude that OA spray application in periods without brood during spring and summer poses little danger to honey bee queens and that in sugar stores harvested in summer, OA residues are within the limits of natural variability.