The development of ‘Varroa destructor’ in native ‘Apis mellifera ruttneri’ and in introduced ‘Apis mellifera ligustica’ colonies on the island of Malta

Abstract

Very little information is available in the scientific literature on Apis mellifera ruttneri, the endemic honeybee of the Maltese Islands and information about its behaviour and development is lacking. In June 2017, a standardized study was set up for the evaluation of colony survival, population dynamics of Varroa destructor and hygienic behaviour in A. m. ruttneri and A. m. ligustica, and it continued until March 2020. A total of 33 colonies (A. m. ruttneri n = 15, and A. m. ligustica n = 18, sister queens) of roughly the same size, were distributed across two apiaries, one in Siġġiewi and one at the University of Malta, followed shortly after by an initial treatment against V. destructor. A. m. ruttneri colonies expressed a consistent and significantly (Mann-Whitney: P = 0.003 < 0.05) lower infestation level of Varroa during the entire duration of the experiment. The mean infestation level in A. m. ruttneri did not surpass 10%, while the highest values of A. m. ligustica increased to 40%. Hygienic behaviour towards pin-pierced brood was higher in A. m. ruttneri (Mean = 60%) than in A. m. ligustica (Mean = 44%), but there were no significant differences at 16 hours and 15 hours interval; however, there was a significant difference at 14 hours interval (ANOVA: P = 0.000 < 0.05). Honey production was also significantly higher (ANOVA: P = 0.032 < 0.05) in A. m. ruttneri (Mean = 6.2 kg) when compared to A. m. ligustica (Mean = 2 kg), even though this was evaluated once. Within a year from the study, all A. m. ligustica were lost, while (n = 12) ruttneri colonies still survived. After 23 months from the beginning of the study. The remaining survivor colonies (n = 10) were evaluated for the traits of uncapping/recapping behaviour and SMR. High SMR values were found (Mean = 57%), while Varroa-targeted recapping was expressed at even higher frequencies (Mean = 90%). All this implies that A. m. ruttneri shows a certain degree of tolerance and resistance to V. destructor and, above all, is better suited for the beekeeping conditions of the Maltese islands.