Initial screening of 12 Beauveria bassiana (Balsamo) Vuillemin isolates against larvae of the lesser mealworm (Alphitobius diaperinus [Panzer]) resulted in the selection of two isolates, GHA and 707, for further testing under field conditions. Three formulations of each strain were prepared: an EC, a ground corn granular formulation, and waste product of fungal propagation containing spent media, mycelia, and unharvested conidia (“residue” formulation). Two field trials were conducted in commercial caged-layer houses in Georgia with 5–6 mo of manure accumulation and established populations of A. diaperinus and hide beetles (Dermestes maculatus DeGeer). In the first trial field, B. bassiana was applied a single time to the manure surface at either 10⁹ (EC and granular cornmeal bait formulations) or 10⁸ (residue formulation) fungal spores per square meter. In the second trial, two successive weekly treatments were applied, using a total of 6X the rate of application used in the first trial, Significant treatment effects were short-lived and only detected 2 wk after treatment in both trials. The granular formulations of both strains and the residue formulation of the GHA strain provided the greatest degree of suppression (60–90%) of beetle larvae. A laboratory bioassay confirmed that the granular bait was the most effective formulation. More frequent applications made earlier in the manure accumulation cycle may be necessary to achieve satisfactory control of these beetles.