Eichhornia crassipes (water hyacinth) possesses several biological properties. Periplaneta americana (cockroach) is a household insect pest that can disseminate pathogens. The present study analyses the chemical constituents of water hyacinth leaf methanol extract and its in vitro effects on antioxidant and esterase enzymes in cockroaches. Water hyacinth methanol extract (WHME) was subjected to gas chromatography-flame ionization detection (GC-FID). Crude enzyme preparations were obtained from nymph and adult cockroaches. In vitro effects of different concentrations (10–80 μg/ml) of WHME on superoxide dismutase (SOD), catalase (CAT), glutathione- S-transferase (GST), acetylcholinesterase (AChE), and carboxylesterase (CE) in crude enzyme preparations were spectrophotometrically measured using cypermethrin (CYP) and chlorpyrifos (CPF) as reference insecticides. The GC-FID chromatogram of WHME showed the presence of gallic acid, para-hydroxybenzoic acid, eritosin, gentisic acid, catechin, protocatechuric acid, quercetin, and eugenol. The WHME and CPF reduced catalase in adults and SOD in nymphs, while CYP and CPF reduced SOD activity in the adults of P. americana. GST was significantly increased by WHME in both nymph and adult cockroaches. AChE was reduced by WHME, CYP, and CPF in nymphs, while CE was increased by WHME in nymphs and adults as compared to the control. In conclusion, our study indicates a high phenolic content in the water hyacinth leaf methanol extract, which increased the activities of the antioxidant enzymes AChE and CE in P. americana. The results also show that the efficacy and the mode by which the extracts could induce toxicity depend on the growth stage of the cockroach.