Defoliator pests often cause severe damage to commercially valuable timber trees, such as teak, grown in plantations, affecting the quality and quantity of the wood produced. Twenty-five isolates of an entomopathogenic fungus, Metarhizium anisopliae (Metsch.), derived from different forest habitats were tested for their efficacy against the Teak Defoliator, Hyblaea puera Cram. The median lethal concentration (LC50) for the isolates ranged from 0.65 to 947.41 × 105 conidia mL−1. Four isolates, viz. MIS2, MIS19, MIS7 and MIS1, were promising, having low LC50 values. A median lethal time (LT50) of 3.8 d was recorded for the isolates MIS2 and MIS7 when the moth larvae were exposed to a spore load of 107 conidia mL−1. Formulations composed of “MIS2 + MIS7 + 0.5% Pongamia pinnata seed oil” and “MIS2 + MIS7” proved to be superior against the Teak Defoliator, causing 94.73% and 93.93% mortality, respectively. Application of the formulation “MIS2 + MIS7 + 0.5% P. pinnata oil” resulted in 63.6% and 56.2% reduction in infestation in nurseries and plantations, respectively, while the formulation “MIS2 + MIS7” recorded 56.18% and 51.60% reduction. Our results indicate that M. anisopliae formulations could be an effective means of managing Teak Defoliator in plantations, particularly when combined with P. pinnata oil.