INTRAGUILD INTERACTIONS BETWEEN EGG PARASITOIDS: FROM LABORATORY TO FIELD INVESTIGATIONS

Field studies on egg parasitoid guilds of herbivore stink bugs, serious pests for a wide number of crops, have shown that Trissolcus spp. and Ooencyrtus spp. can naturally co-occur in the same host, and generally the former parasitize more eggs than the latter. In many countries, to control such pests, biological control programs based on egg parasitoids have been used obtaining inconstantly success. Thereby, understanding the competitive interactions among Trissolcus and Ooencyrtus species may be useful in order to improve biological control of such pests but only few researchers have investigated the effects of competitive interactions among these egg parasitoids. The aim of this thesis was to address on extrinsic and intrinsic competitive interaction that occur between Trissolcus basalis (Hymenoptera: Platygastridae) and Ooencyrtus telenomicida (Hymenoptera: Encyrtidae), two egg parasitoids of the southern green stink bug (SGSB), Nezara viridula (Heteroptera: Pentatomidae) in laboratory conditions and field and semifield conditions. In laboratory conditions intraguild interactions between O. telenomicida and T. basalis, as the former has the potential to be a facultative hyperparasitoid of the latter has been investigated. In 2nd chapter Has been assessed the suitability of N. viridula eggs for the development of O. telenomicida as a function of egg age when they were unparasitized, or had been attacked by T. basalis at different times prior to exposure to O. telenomicida females. Ooencyrtus telenomicida can exploit healthy N. viridula host eggs up to 5 days of age, just prior to the emergence of N. viridula. This window of opportunity can be extended for an additional 6–7 days through interspecific competition or facultative hyperparasitism. While there are minor fitness costs for O. telenomicida as the result of interspecific larval competition, those costs are greater with facultative hyperparasitism. In choice assays O. telenomicida females discriminated between different quality N. viridula eggs, avoiding those where their progeny would have to develop as facultative hyperparasitoids of T. basalis. Results are discussed with respect to the possible effects that the costs of intraguild parasitism might have on biological control programmes. In 3rd chapter the interactions between egg parasiotids in natural conditions has been analyzed. Infact research on interspecific competitive interactions among insect parasitoids has often been characterized by laboratory studies in which host insects are exposed to female parasitoids of different species in various sequences and combinations. Therefore, the lack of bioassays carried out under natural conditions is a key limiting aspect in understanding how interspecific interactions could affect pest suppression. In this scenario, for two-year has been investigated interspecific interactions between Trissolcus basalis (Wollaston) (Hymenoptera: Platygastridae) and Ooencyrtus telenomicida (Vassiliev) (Hymenoptera: Encyrtidae), two egg parasitoids of the pest Nezara viridula (L.) (Heteroptera: Pentatomidae) that co-occur in cultivated crops. Under semi-field (in out-door mesh cages) and field conditions, the follows parameters have been investigated: 1) the seasonal occurrence of competing parasitoid species on sentinel egg masses under natural conditions; 2) the parasitism impact achieved by competing species on the shared host on naturally laid egg masses under natural conditions; 3) the outcome of manipulating intraguild interactions. Results from sentinel egg masses showed that T. basalis occurs in May and successfully parasitizes hosts until the end of September/beginning of October, whereas O. telenomicida is mainly occurring in July-August. In both years, it was found that T. basalis is predominant. From naturally laid egg masses, results indicated that T. basalis achieves the highest parasitism impact on the hosts, even in those egg masses which are parasitized by more than one female of different species (= multiparasitism). Results from manipulating intraguild interactions showed that T. basalis achieves the highest parasitism impact on N. viridula when released alone, but it suffers from competition with O. telenomicida. The ecological factors that play a role in intraguild interactions in the context of biological control perspective are discussed. In 4th chapter it has been studied how the presence of synthetic Methyl salicilate can influence the intraguild competitive interaction between Trissolcus basalis (Wollaston) (Hymenoptera: Platygastridae) and Ooencyrtus telenomicida (Vassiliev) (Hymenoptera: Encyrtidae), two egg parasitoids of the southern green stink bug, Nezara viridula (L.) (Heteroptera: Pentatomidae). Field experiments has been conducted, using sentinel egg masses placed randomly on the pepper plants. The following parameters have been evaluated: 1) the proportion of parasitized egg masses out of the total to assess the ability of the parasitoid to locate the host; 2) the proportion of individual eggs exploited out of the total egg number of the located egg masses to evaluate the ability of the parasitoid to exploit the host. The results demonstrated that T. basalis has a high ability to find host compared to O. telenomicida and that this ability is not affected by the presence of Methyl salicylate. Furthermore, it was shown that T. basalis is able to parasitize a higher number of eggs than O. telonimicida.