The beet armyworm (Spodoptera exigua (Hübner)) is one of the most important vegetable pests in Thailand. After 24 hr of both dipping and sprayer bioassay, estimation of LC₅₀ of the ethyl acetate extract of Jatropha gossypifolia senescent leaves demonstrated toxicity to secondary instar S. exigua larvae because in this stage most armyworm start to move to other plants and it is also the first susceptible stage for toxicity tests. Ricinine, the main alkaloid separated from ethyl acetate crude extract, showed toxicity on secondary instar larvae by the sprayer method with an LC₅₀ of 3,215 ppm whereas the LC₅₀ value for ethyl acetate crude extract is 8,644 ppm. Thus, the ethyl acetate crude extract of Jatropha gossypifolia senescent leaves may have ricinine as the active ingredient and may be used as an alternative choice for the minimal application of chemical insecticides for Spodoptera exigua.