Do Trichoderma treatments affect the Asian chestnut gall wasp (Dryocosmus kuriphilus) and its natural enemy?

Fungi of the genus Trichoderma are beneficial microorganisms with multiple positive effects on plants. Many studies have demonstrated these microbes are able to promote plant growth and improve plant health. In fact, they can stimulate plant defense responses against various plant parasites, such as fungi, nematodes, and insects. Although these beneficial fungi have been mainly investigated as biocontrol agents against fungal diseases, an increase of studies focusing on their efficacy toward insect pests has been recently recorded. These studies showed that some Trichoderma species (particularly T. harzianum, T. viride, and T. atroviride) may affect insects both directly and indirectly. They act directly, through the parasitization of the insect, which is exploited as food source, or through the production of bioactive compounds with insecticidal, repellent, or antifeedant effects. On the other hand, Trichoderma may act on the insect indirectly, for example as mycoparasites of insect-symbiotic fungi. In addition, being endowed with an endophytic lifestyle, Trichoderma fungi can also stimulate plant defenses or promote the production of volatile organic compounds attractive to the natural enemies of phytophagous insects. Considering the growing demand for eco-friendly tools to control plant parasites, an increase in the use of Trichoderma-based products as biocontrol agents is expected. Therefore, it is critical to understand the multiple yet unexplored interactions between Trichoderma and plant-parasites. In this regard, a novel case study is represented by chestnut groves, where Trichoderma treatments may contribute to control the numerous fungal diseases affecting this tree species, but at the same time may affect also the main chestnut insect pest, Dryocosmus kuriphilus, the Asian chestnut gall wasp (ACGW), and its natural enemy, the biocontrol agent Torymus sinensis, largely employed in Italy and other European countries in ACGW biological control. In this study, two chestnut groves were treated in June with a conidial suspension (1 x 10 7 conidia/mL) of Trichoderma spp. (T. viride, T. atroviride e T. harzianum) by endotherapic applications with the BITE® tool. In each chestnut grove, a given number of chestnut trees were selected for the treatment, and an equal number were left as controls. To verify the effect of the treatment, chestnut branches were collected from treated and untreated trees, before and after the treatment. Three types of samples were collected: buds and ACGW withered galls in winter; ACGW fresh galls in June. In the laboratory, buds and galls were counted and dissected to verify the presence and health status of the ACGW and T. sinensis. Results indicate that the treatment did not act directly against T. sinensis as no difference was found between its mortality percentages on treated and untreated chestnuts. The hypothesis that Trichoderma, by its endophytic properties, can affect ACGW and T. sinensis interactions is discussed.