The red spider mite, Tetranychus merganser Boudreaux (Acari: Tetranychidae), is an important agricultural pest in Australia, China, Thailand, the United States, and Mexico because feeding causes severe damage and defoliation of plants. The aim of the study was to evaluate effects of Equisetum arvense L. ethanolic extract on biological parameters of T. merganser in a laboratory. Different concentrations (10, 50, 100, 250, 500, and 1,000 µg/ml) of ethanolic extract were applied to T. merganser females. E. arvense extract did not kill T. merganser, but increased the number of eggs by 19.9, 22.5, and 21.2% compared with a check at 24, 48, and 72 hours, respectively. It also augmented the growth rate and finite rate of increase, and decreased doubling time of the red spider mite. The effects require further study because the plant extracts could have considerable implications for natural enemies and also might be suitable for mass production of red spider mites for other experiments in a laboratory.