The parasite Aspergillus flavus Link and other fungi of the biosphere of the alkali bee Nomia melanderi Ckll., in eastern Oregon and Washington Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/g445ch99h

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  • The pathogenicity of A. flavus on alkali bee larvae was studied using sample materials from three nesting sites of these soil inhabiting bees along the Oregon-Washington state line. Soil saprophytes of the biosphere of the alkali bee were also studied. Field observations and direction of soil cores from bee beds provided a basis for estimates of A. flavus disease at the three sites. Fungi, including A. flavus and soil saprophytes were isolated from: soil samples, the surface of adult alkali bees, pollen balls, and larval feces. In vitro disease induction experiments were carried out comparing uninoculated larvae and larvae surface inoculated with conidia of A. flavus. The tests were conducted at five levels of relative humidity within each of three temperature treatments. Field observations and soil cores revealed A. flavus as the pre-dominante fungus pathogen in two of the three sites studied. One site was free of the pathogen. Studies of soil dilution plates and fungal isolations from adult bees, pollen balls, and larval feces disclosed propagules of A. flavus in the biosphere of the bees in two of the three beds. Saprophytic fungi of the biosphere did not vary significantly over a three year period. These fungi provided a useful index of soil contamination of adult bees, pollen balls, and larval feces. Studies of the effect of relative humidity and temperature on disease induction in larvae surface inoculated with A. flavus conidia revealed that temperatures of 25° to 30°C and relative-humidities from 90 to 100 percent favored disease development while lower temperatures and/or lower humidities inhibited disease induction. The increased incidence of A. flavus disease of larvae during the spring and summer is considered to result from increasing soil temperatures and relative humidities within the brood cells, Conversely, a reduced incidence of disease during the fall and winter results from lower temperatures. Higher incidence of A. flavus disease in one of the beds (the Garbe bed) was attributed to its age and higher bee population. Soil cores taken from the Garbe bed to establish the Harris bed account for the prevalence of disease in the latter. The lack of A. flavus disease in the Wallace-Key bed was not clear, but its younger age and lower population density offer a partial explanation. Control of A. flavus within the nesting site might logically aim at elimination of the fungus from the soil. Sub-irrigation of the beds with fungicide is one possible approach.
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