Responses to V arroa destructor and N osema ceranae by several commercial strains of A ustralian and N orth A merican honeybees ( H ymenoptera: A pidae)

The potential impact of varroa ( V arroa destructor , Anderson & Trueman) on A ustralian beekeeping and agriculture depends in part on the levels of resistance to this parasite expressed by A ustralian commercial honeybees ( A pis mellifera ). The responses of seven lines of A ustralian honeybees to V . destructor were compared with the responses of a stock of I talian honeybees from the U nited S tates known for its susceptibility to V . destructor and two stocks known for their resistance to V . destructor , R ussian honeybees ( RHB ) and a stock expressing the varroa sensitive hygiene trait ( VSH ). The experiment began in M ay with uniform colonies having uniform infestation of V . destructor . V . destructor infestations measured as the percentage of adult bees infested in the A ustralian lines and the I talian stock rose from less than 10% in A ugust to over 25% in O ctober. From A ugust to N ovember, 44% of both the A ustralian and I talian colonies died while strongly exhibiting symptoms of parasitic mite syndrome. In contrast, RHB and VSH colonies displayed comparative resistance to V . destructor . Their infestation rates rose from about 5% in A ugust to 10% ( RHB ) and 14% ( VSH ) in O ctober. Likely, some of this increase resulted from invasion pressure by mites from the dying A ustralian and I talian colonies. During the A ugust to N ovember period, 4.4% of the RHB and 14.3% of the VSH colonies died. In comparisons of the seven A ustralian lines, only nonsignificant and trivial differences were found for infestation and mortality rates. All A ustralian lines were highly susceptible to V . destructor . Additionally, evaluations of rates of N osema ceranae infections were made throughout the course of the experiment. Although high levels of infection were found across all stocks and lines, no stock or line exhibited an adverse effect from N . ceranae infection.