Tritrophic relations within the parasitoid guild of Halyomorpha halys (Hemiptera: Pentatomidae) and implications for biological control

Abstract

Each year, invasive species cause significant ecological and economic damage globally, and their numbers are expected to increase due to the effects of climate changes and globalization. The brown marmorated stink bug, Halyomorpha halys (Stål) is a recent example of a serious invasive pest of worldwide importance. In the invaded area H. halys has caused severe damage to agricultural crops leading to an increase in the use of chemical insecticides and therefore a negative impact on the environment and human health. The low efficacy of insecticide treatments, combined with the lack of effective indigenous natural enemies, led to considering classical biological control a promising method for the long-term management of this pest in the invaded areas. Among the natural enemies of H. halys, the Hymenoptera egg parasitoids play a significant role. This Ph.D. thesis focused on the assessing distribution of the egg parasitoid species in those invaded areas of Europe where H. halys is abundant, providing at a simple morphological diagnostic tool that could support the identification of species during field surveys (Chapter 1). The results of monitoring evidenced the wide distribution of two exotic egg parasitoids Trissolcus japonicus, the main natural enemy of H.halys in the area of origin, and Trissolcus mitsukurii as well the presence of the native Anastatus bifasciatus and the exotic hyperparasitoid Acroclisoides sinicus. The presence and the high impact of both Trissolcus species on H. halys egg masses led to consider these exotic species as valid candidates for the biological control of H. halys in the invaded area. For this reason analysis of risk assessment on non-target hosts (in Chapter 2) was conducted on both Trissolcus species through different multidisciplinary approaches. Although no choice tests have shown the polyphagous habitus of both species under laboratory conditions, tests that include the effects of the interactions between the pest and host plant, such as in the olfactometer bioassays, demonstrate the preference for the co-evolved host. The co-presence of the two Trissolcus species in the invaded area also provides the need to investigate ( in Chapter 3) how these species interact with each other and the way these species exploit the common resources under different competition scenarios. The behavioral responses and the different reproductive success of each species within the different cases provided information that could be useful to understand how the dynamic populations will evolve and how to improve the strategies in the biological control program against H. halys. In conclusion, following the discovery of the exotic parasitoid A. sinicus, frequently found associated with H. halys and its parasitoid guild, detailed studies were conducted on the reproductive and developmental biology of this species, until now completely unknown (in Chapter 4). The study of its biology confirmed a previous hypothesis of an obligate hyperparasitoid, in particular of Scelionidae species. Since obligate hyperparasitoids occupy the highest level in multitrophic systems it may interfere with the populations densities of the primary parasitoid, including Trissolcus species, and, therefore, with the biological control program.