The potential of Beauveria bassiana to suppress Amblyomma americanum and Dermacentor variabilis populations was evaluated. In the laboratory the fungus was pathogenic to both species. Two (A. americanum) or five (D. variabilis) weeks was required for significant mortality. In the field there was significant mortality of A. americanum exposed to B. bassiana (F 124.45; p <0.001; df 1,6). This was not observed for D. variabilis (F 0.60; p 0.47; df 1,6).
The experimental design of the field trial is recommended for evaluations of fungus-arthropod interactions as the majority of the test organisms could be recovered. The enclosures did not have biologically significant effects on relative humidity and temperature conditions.
Suitability of the central Ohio mesoclimate for B. bassiana was determined. Optimal conditions for fungal infection (20-30°C; RH >90%) usually occurred concurrently after 18h00 from June-August. To avoid inactivation by ultraviolet radiation and maximize the chances of infection applications should occur after this hour during months when ticks are active.
Beauveria bassiana increased the respiration rate of A. americanum (F 7.76, df 5,36; R-square 0.52), although this was only significant two weeks post-exposure (p 0.02). The fungus negatively impacted the water balance of A. americanum as determined by weight (water) loss. When all fungus-exposed ticks had died, they had lost 25.2% ± 0.84 of their starting weight. Control ticks by comparison had lost 14.08% ± 0.85 of their starting weight. This difference was significant (p 0.01; df 96). The difference in percentage weight change at the time of death was significant (p 0.011; df 83). Exposed ticks lost less weight (µ 25.2% ± 0.84) compared to the controls (µ 27.8% ± 0.55). Most weight loss occurred in the two days following inoculation. The physical state of ticks was affected by exposure. Ticks became morbid quicker and died more rapidly than individuals in the control. These physiological costs may reduce survival and/or contribution to the next generation. This may make entomopathogenic fungi useful, even with low direct mortality, with population suppression over time following repeated applications. Beauveria bassiana is recommended for management of A. americanum, but not D. variabilis.