Field surveys indicate that declining colonies of honey bees, Apis mellifera L. (Hymenoptera: Apidae), suffer simultaneously from multiple stress factors, raising concern that multiple stressors could be interacting to compound bee stress in an additive or synergistic fashion. We tested two null hypotheses: 1) Varroa destructor Anderson & Trueman (Acari: Varroidae) (=varroa) and Aethina tumida Murray (Coleoptera: Nitidulidae) do not interact such that the number of one affects the number or density of the other and 2) bee damage from one does not change in response to changing levels of the other. In a split-split plot design replicated in 2 yr and two states, experimental apiaries were established and each manipulated to achieve one of five average ± SE colony adult A. tumida populations: 0; 285 ± 6; 721 ± 5; 1,544 ± 14; or 3,175 ± 90. Within each apiary, the population of varroa mites in each colony was manipulated to achieve one of three average ± SE colony mite populations: 763 ± 121; 1,111 ± 155; or 1,856 ± 300. On a one-way basis, there was a predictable increase in measures of bee morbidity with increasing densities of each pest. Colony varroa mite levels decreased as apiary-wide A. tumida levels increased. In contrast, colony levels of the honey bee mite, Acarapis woodi (Rennie) (Acari: Tarsonemidae), increased as colony varroa levels increased. Concerning measures of bee morbidity, varroa and A. tumida did not interact such that damage by one was affected by changing levels of the other. A treatment threshold established for varroa before the arrival of A. tumida has not changed during the years since A. tumida has become established in the region.
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Vol. 103 • No. 5