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A temporal survey of relative abundance of two parasitic wasps Leptopilina boulardi and L. heterotoma was conducted in an oasis in Tunisia. These two species infest the larvae of Drosophila simulans which live on the prickle pear of Opuntia. We observed an alternate occurrence of the two parasitoids during the year. Experiments on competitive ability of the two species reveal that L. boulardi is the better competitor. This is confirmed by continuous observation of the two parasitic species in population cages, in which L. boulardi appears always the winner. The disappearance of L. boulardi during the autumn is due to temperature induced diapause which does not exist for L. heterotoma. L. heterotoma is maintained only by the disappearance of its competitor. During the early spring, L. boulardi emerges and competes successfully against L. heterotoma. We demonstrate in this work that a non-competitive parasitic species can survive in a community due to environmental conditions (i.e. temperature).