An ethrel treatment of olive trees in two sites differing in altitude caused a decline in the infestation of Prays oleae, an important pest in this crop. Since the ethylene treatment did not affect natural enemies of P. oleae, we conclude that this natural compound can be used as an alternative to the use of pesticides in olive orchards. The results of this work indicate that ethrel affects P. oleae attack not by increasing ethylene synthesis, but that other more complex defence mechanisms are involved in such a process.