OPTIMIZING THE BIOLOGICAL CONTROL OF INVASIVE INSECTS: EVALUATING NATURAL ENEMY HOST SPECIFICITY, PREY BEHAVIOR, AND VIBRATIONAL COMMUNICATION

The effects of globalization have led to the spread of invasive alien insect species, whose dispersion is continuously increasing and are usually characterized by a negative impact on the new environment and biodiversity. Moreover, such species can also have disruptive consequences on the economy of the new habitat. The absence of biotic factors, such as natural enemies, is one of the key aspects of their success. Therefore, introducing their natural enemies to control the phytophagous populations is an effective and sustainable management strategy for invasive insects. However, biological control can be complex and requires detailed studies on insect ecology. To assess the efficacy, safety, and environmental impact of introducing a new parasitoid for a long-term strategy, a hierarchical risk assessment procedure must be followed. The risk assessment of a natural enemy involves a series of targeted analyses to evaluate the effects of its introduction in order to minimize potential risks and maximize the benefits of biological control. The tests on parasitoid host specificity include studies on chemical ecology. In this regard, we investigated and compared the ability of two egg parasitoids, Trissolcus japonicus and Trissolcus mitsukurii, associated with the phytophagous invasive species Halyomorpha halys. In order to evaluate their potential impact on non-target species, the behaviors of these exotic parasitoids were also tested on the traces of the cosmopolitan stink bug Nezara viridula. The exposure of singular female parasitoids to contact kairomones of both pests elicited modifications in orthokinetic and locomotory behavior, highlighting a preference of T. japonicus for H. halys chemical cues. In contrast, a higher attraction of T. mitsukurii for the traces released by N. viridula was found. Such preference of T. mitsukurii was also confirmed by the bioassay involving the synthetic chemical cues of the aforementioned host. The investigation of the interaction between the prey and its natural enemy is also an important milestone that should be evaluated in assessing the efficacy of the parasitoids. Therefore, a choice test to determine host-finding capacity was performed on Ganaspis brasiliensis, which is a larval parasitoid of another important invasive species, Drosophila suzukii. The experiment was conducted in semi-field conditions, exposing the parasitoid to blueberry branches infested with three different SWD density rates (i.e., 10 females, 20 females, 30 females). The data obtained from this experiment showed a significant preference of G. brasiliensis for a medium infestation rate (i.e., 20 SWD females), suggesting that such infestation rate is optimal for progeny success. Another important factor to be evaluated in biological control is the impact that the use of natural enemies can have on prey behavior. Behavioral plasticity is one of the key aspects that was found to have a great impact on the success of pest stability in a new environment. Indeed, the presence of the parasitoid induced a modification in the oviposition behavior in two different conditions: semi-field and laboratory. While egg laying was significantly higher in the presence of the parasitoid under laboratory conditions, the position in which the eggs were laid within the used substrate was also indicative of the induced impact in the case of contaminated substrates by the parasitoids only. The biological control of an invasive species could involve strategies other than the use of natural enemies. Among these, the use of vibrational communication between conspecifics could be employed to develop tactics for mass trapping or mating disruption. However, developing such strategies requires studying the substrate-borne vibration of the species in consideration. With the aim of characterizing the substrate-borne vibrational communication that mediates courtship and leads to mating, we recorded the vibrational signals of adult males and females of another invasive species, Bagrada hilaris. We identified three types of signals: a male and female signals emitted when alone or before contact with the opposite sex, and a mating signal produced by the male after contact during courtship. In conclusion, in order to prevent and limit the negative impact of the introduction of an invasive phytophagous, different strategies could be implemented. Introducing natural enemies as a biological control strategy can be effective and sustainable, but requires detailed studies to minimize potential risks and maximize benefits. In particular, host specificity, host-finding capacity, and prey behavior are important factors that should be evaluated. Furthermore, alternative strategies such as vibrational communication between conspecifics can be explored but require thorough understanding of the target species. Ultimately, careful consideration and evaluation of all the abovementioned factors are necessary to develop novel and effective sustainable management strategies for invasive insect species.