This work describes the insecticidal activity of two halogenated monoterpenes, (mertensene and violacene) isolated from the red alga Plocamium cartilagineum, and two derivatives (dibromomertensene and dihydromertensene), towards the tomato moth Tuta absolute (Povolny) and the greenbug, Schizaphis graminum, a cereal aphid. The toxicity of violacene to aphids was higher than other compounds causing 92 % aphid mortality after 48h and was similar to insecticides used as aphicides. Violacene, dibromomertensene and dihydromertensene reduced the reproduction index of aphids. Dibromomertensene and violacene protected tomato plants against the tomato moth.