Chemical control of the diamondback moth, Plutella xylostella (L.), is constrained by resistance and residue concerns, whereas botanical products often show limited persistence on foliage. This study evaluated whether nanochitosan (NC) can support the delivery of Millettia pachyloba Drake leaf ethanolic extract (MPEE) against P. xylostella and whether laboratory responses translate to Brassica juncea (L.) Czern. & Coss. under greenhouse conditions. Laboratory leaf-dip/spray bioassays compared MPEE, NC, and a fixed-ratio MPEE+NC co-formulation. Greenhouse performance was assessed in a randomized complete block design using plant survival rate (SR), stem durability (SD), and harvest efficiency (HE). Potency and efficacy were determined with four-parameter log-logistic models, and combination effects were evaluated a priori using Bliss independence, Loewe additivity, and highest-single-agent criteria. MPEE produced clear dose-dependent deterrent responses, while NC alone showed negligible intrinsic toxicity within the tested range. Greenhouse effective-dose estimates for MPEE and MPEE+NC were similar across observation times, and mixed/generalized models showed a positive but borderline treatment effect for SR after false-discovery-rate adjustment, with no significant Treatment × Time interaction. These results support NC as a delivery-compatible carrier that may improve the practical use of MPEE, but stronger claims of greenhouse synergy require a complete co-formulation dose-response and factorial interaction dataset.