Bioassays were designed to evaluate Beauveria bassiana (Balsamo) Vuillemin strain GHA against Listronotus maculicollis (Kirby) adults. B. bassiana and its “inert” carrier oil in the product BotaniGard and the inert carrier oil alone provided 99 and 96% mortality, respectively, in petri dish assays 1 d after treatment when applied in 1 ml water. When the same treatments were applied in 0.5 ml of carrier water, mortality was only 1.4 and 0.7%, respectively, 1 d after treatment. After 10 d in petri dishes, B. bassiana and its inert carrier oil and the inert carrier oil alone applied in 0.5 ml water showed 77 and 9% mortality, respectively. When one-tenth the label dosage of B. bassiana and inert carrier oil was combined with neonicotinoids applied in 1 ml water, there were significant increases in weevil mortality over the neonicotinoids alone 1 d after treatment. When 88.7% of one-tenth the label dosage of inert carrier oil alone was combined with neonicotinoids clothianidin, imidacloprid, and dinotefuran applied in 1 ml water, there was also a significant increases (38%) with clothianidin in weevil mortality over clothianidin alone 1 d after treatment. B. bassiana and its inert carrier oil provided 28, 50, and 78% mortality at the highest label dosage and 47, 76, and 89% mortality at 4× the highest label dosage in turf plug assays at 7, 10, and 14 d after treatment. Addition of 5 or 20% MycoMax (a nutrient source for B. bassiana) did not significantly increase mortality.