The virulence of 25 Metarhizium anisopliae isolates was tested under laboratory conditions and the two most effective isolates were evaluated in the field for control of the Ailanthus defoliator, Atteva fabriciella. A bioassay was carried out to determine the dose and time mortality responses. The LC50 of the isolates ranged from 3.16 to 647.81 × 105 conidia mL−1. Toxicity tests of the isolates MIS7 and MIS13 and 0.5% Pongamia pinnata seed oil, individually and in different combinations, indicated improved efficacy of the isolates when used in combination and also when combined with seed oil. Evaluation of these formulations in the field showed 66.36% reduction of infestation with MIS7 + MIS13 + 0.5% P. pinnata seed oil and 61.15% reduction with MIS7 + MIS13. The study indicated a possibility of employing combined formulations of M. anisopliae and also combination with P. pinnata seed oil for augmenting the effectiveness of the fungus.