Ellen Klinger

Date of Award

8-2003

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Eleanor Groden

Second Committee Member

Francis A. Drummond

Third Committee Member

Seanna Annis

Abstract

Factors influencing the susceptibility of adult Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), to the fungal entomopathogen, Beauveria bassiana (Bals.), were studied. In an assay comparing trials between a): laboratory reared, non-diapausing beetles and b): field collected, post-diapausing beetles, survival of B. bassiana treated beetles was higher for non-diapausing adults, however, control mortality was higher for post-diapausing adults. In a similar assay with field collected pre- and post-diapausing beetles, survival of B. bassiana treated individuals was higher for post-diapause adults and control mortality was higher for post-diapausing adults than pre-diapausing beetles. In a third experiment, the effect of time from eclosion from the pupal case on susceptibility of laboratory-reared, non-diapausing adults was evaluated. A trend of increasing susceptibility up to 60-degree days (base 10° C) was be seen, and a significant decline of mortality occurred after 125-degree days. The effect of sublethal and lethal doses of B. bassiana were evaluated on the survival of overwintering beetles. B. bassiana dose had no effect on the proportions of beetles sporulating or dylng. Behavior and infection of newly emerged adult CPB in the presence of B. bassiana infected cadavers was studied to determine the likelihood of transmission of disease as beetles emerge from the soil and colonize host plants. In 2001, arenas were constructed to accommodate potted greenhouse grown potato plants surrounded with soil to simulate the field environment. B. bassiana-killed, sporulating adult beetles were placed in varying patterns surrounding a release point for healthy beetles in the center of the arena. Laboratory reared, newly eclosed beetles were buried just below the soil surface at the release point and were observed for 30 minutes as they emerged and colonized one of four plants. The study was replicated in 2002 using a similar grid in a potato field. In both the arena and field, emerging beetles showed no preference for movement in any cardinal direction, and direction was not affected by the presence or absence of B. bassiana sporulating cadavers, nor did the presence of cadavers affect the time taken to colonize a plant or the distance traveled by a beetle. Relative humidity (RH) was a significant factor for distance traveled to the plant, with longer travel distances as the RH declined. The plant colonization behavior of newly emerged Colorado potato beetles does not appear to be altered by the presence of B. bassiana cadavers in the immediate environment. The likelihood of emerging adults contacting sporulating cadavers on the soil surface was quantified at different cadaver densities. A curvilinear relationship exists between density of cadavers on the soil surface and the square root number of encounters, with encounters increasing with increasing density. Proportional mortality and sporulation also have a curvilinear relationship with density, both values increasing with cadaver density. Thus, beetles show no avoidance behavior to sporulating cadavers. Density of sporulating cadavers on the soil surface is an important factor in the horizontal transmission of B. bassiana from cadavers to healthy, post-pupation adults.

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