Responses to Varroa destructor and Nosema ceranae by several commercial strains of Australian and North American honeybees (Hymenoptera: Apidae)

The potential impact of varroa (Varroa destructor, Anderson & Trueman) on Australian beekeeping and agriculture depends in part on the levels of resistance to this parasite expressed by Australian commercial honeybees (Apis mellifera). The responses of seven lines of Australian honeybees to V.destructor were compared with the responses of a stock of Italian honeybees from the United States known for its susceptibility to V.destructor and two stocks known for their resistance to V.destructor, Russian honeybees (RHB) and a stock expressing the varroa sensitive hygiene trait (VSH). The experiment began in May with uniform colonies having uniform infestation of V.destructor. V.destructor infestations measured as the percentage of adult bees infested in the Australian lines and the Italian stock rose from less than 10% in August to over 25% in October. From August to November, 44% of both the Australian and Italian 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 August to 10% (RHB) and 14% (VSH) in October. Likely, some of this increase resulted from invasion pressure by mites from the dying Australian and Italian colonies. During the August to November period, 4.4% of the RHB and 14.3% of the VSH colonies died. In comparisons of the seven Australian lines, only nonsignificant and trivial differences were found for infestation and mortality rates. All Australian lines were highly susceptible to V.destructor. Additionally, evaluations of rates of Nosema 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.