Date of Award

12-2001

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

Advisor

Eleanor Groden

Second Committee Member

Francis Drummond

Third Committee Member

Seanna Annis

Abstract

This work is an investigation of:( 1) mortality of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), under different modes of exposure to the pathogen Beauveria bassiana (Bals.) Vuill., (2) host and environmental factors affecting sporulation, production, and viability of B. bassiana conidia from infected cadavers of CPB larvae, and (3) transmission of the disease in CPB populations. The research leads to an improved understanding of the process of transmission of B. bassiana in CPB populations. This may enhance its use as a biological control agent for this pest insect. The highest rate of mortality of CPB larvae (77 percent) was achieved through direct spray of the insects with conidial suspensions of B. bassiana together with exposure to inoculated foliage. Direct spraying alone resulted in 76 percent mortality and exposure to inoculated foliage resulted in 34 percent. B. bassiana killed larvae sporulated within 2 days after death at 100 percent relative humidity and 25°C, and produced 1.75 x l04 to 8.7 x 108 conidid/insect. Cadaver size had the largest impact on the amount of conidia produced. Larvae treated as third and fourth instars produce significantly lower densities of conidia than larvae treated as first and seconds instars. Lower dosages of B. bassiana increased conidia production. Cadavers did not produce conidia at humidity levels lower than 95 percent. Exposure to intermediate levels of humidity resulted in a longer time to sporulation when at a later date the cadaver were transferred to optimal conditions for sporulation. Thus lower relative humidities are detrimental to the fungus. An optimum range of temperature for maximum production of conidia was observed to be between 15 and 30°C. Cadavers exposed to soil produced on average more conidia than cadavers incubated without soil for the same period of time. In the field, the most favorable environmental conditions for sporulation, production and viability of B. bassiana conidia occurred under a potato canopy. High relative humidity (>95%) and soil moisture benefit sporulation and conidia production. Cadavers can persist in the field for more than 30 days and viability of conidia decreased over time but recovered after a rain event. Transmission of the disease in the potato canopy was too low to detect. Transmission was more likely to occur at the soil level where pre-pupae may encounter fallen sporulated cadavers while searching for a site to burrow for pupation.

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