The Brown Marmorated Stink Bug (Halyomorpha halys) is an invasive species which in recent years has caused damages to South Tyrolean apple orchards. The usual control strategies – based on the use of synthetic chemical plant protection products as well as on physical methods (e.g., covering the orchards in netting) – have proven inadequate in preventing damages. Against this background, a search was launched in the places of origin of the Brown Marmorated Stink Bug for natural antagonists which could be used for the biological control of this pest. Among these natural antagonists, foremost the Samurai Wasp (Trissolcus japonicus) (Fig. 1 and 2) revealed itself to be an especially efficient egg parasite to suppress the population of H. halys. In 2020, the release of this wasp in certain regions and provinces of northern Italy – including South Tyrol – was approved by the Italian Ministry of the Environment.
Release of the Samurai Wasp (Trissolcus japonicus)
The release of the antagonist was carried out starting on June 22 2020, following the guidelines of the technical-scientific committee established by the Italian Ministry of Agriculture. A total of 42 locations in South Tyrol were selected for this task (Fig. 3); most of them were in areas with a high prevalence of the Stink Bug. These release locations included especially green areas, ecological corridors, but also hedges in the vicinity of fruit orchards. During the summer, two to three releases could be conducted at these locations; each time, 100 females and 10 males were released. To be able to determine the parasitization rate of the egg clutches of the Stink Bug, egg clutches were collected at the 42 sites both before and after the release dates. They were then incubated in incubation chambers and the hatching rates of the egg parasitoids determined.
Proof of the role of the Samurai Wasp as a natural antagonist
During the subsequent checks, more than 800 egg clutches were collected. The Samurai Wasp was found at 20 of the 42 release locations. The results show that the wasp apparently was able to successfully reproduce under the different local conditions of the release locations. Approx. 50% of the egg clutches displayed signs of parasitization by one or even several different species of the genus Trissolcus. Apart from the released Samurai Wasps, the two most frequent species were T. mitsukurii and A. bifasciatus.
Generally, the parasitization rate at sites with a low density of Stink Bugs was low. In the case of high Stink Bug densities, in contrast, the parasitization rate for the Samurai Wasp was comparatively higher. Parasitized egg clutches were found in particular in public green zones. Only in exceptional cases was the Samurai Wasp successful in parasitizing other Stink Bug species: This is an indication of a strong preference for the eggs of the Brown Marmorated Stink Bug.
