How can two competing egg parasitoid species coexist in a host patch?

Interspecific competitions among parasitoids can affect community structures, and, as a consequence at applicative level, biological control programs. For example, competitions can cause local displacement of inferior species or niche separation. However, the coexistence of species attacking the same host is possible when they adopt different strategies to exploit the resource. In this work we evaluated in field and semi-field conditions intraguild interactions between two egg parasitoids, Trissolcus basalis and Ooencyrtus telenomicida exploring egg masses of Nezara viridula. In semi-field trials, pepper plants were covered with mesh net creating small cages with five plants inside. In each cage, egg masses were placed on plants and then T. basalis and O. telenomicida were released individually or simultaneously. After one week egg masses were recollected. The results showed that T. basalis achieves a high parasitism level when released alone but suffers competition by O. telenomicida in case of simultaneous releasing. Field trials were conducted in tomato fields from May to October in two consecutive years, by monitoring 8 sentinel egg masses placed weekly on plants and recollected one week later, and by collecting naturally laid egg masses that were parasitized only by T. basalis, only by O. telenomicida or by both species. The outcomes showed that T. basalis occurs from June to October, whereas O. telenomicida mainly in July and August, and the former achieves a higher impact on the natural host population than the latter, even when multiparasitism occurs. The ecological role of these results is discussed.