Samurai wasp has minimal impact on native stink bugs, new CABI-led study confirms

Newswise — A recent CABI-led study has verified that the Samurai wasp (Trissolcus japonicus) - a predator of the brown marmorated stink bug (Halyomorpha halys) pest - exerts minimal influence on indigenous stink bugs.

Dr. Tim Haye, the Arthropod Biological Control Department Head at CABI's Swiss center, collaborated with peers to investigate if the inherent host range of T. japonicus aligns with its observed host range in European countries such as Switzerland, Italy, and Germany, where its presence has already been confirmed.

The researchers, comprising experts from the University of Turin, Agriculture and Agri-Food Canada, and the Swiss Department of Finance, Economy, and Agriculture, discovered that T. japonicus demonstrates a higher propensity to target species occupying a similar ecological niche to H. halys.

Originating from Asia, the brown marmorated stink bug poses a significant threat to fruit and nut crops not only in Switzerland but also in various other regions worldwide, leading to substantial damage.

In 2020, The Guardian newspaper highlighted concerns raised by scientists regarding the spread of the brown marmorated stink bug across eight cities in Turkey, predominantly in the Black Sea region. This alarming expansion puts approximately 70% of the global hazelnut supply in jeopardy.

T.japonicus made its debut in a pear orchard in Zurich, Switzerland, in 2020, marking its first release. This event was part of a field trial led by Agroscope and supported by CABI. The introduction of T. japonicus is considered a promising endeavor as it has the potential to combat the pest that poses a threat to various fruits, including cherries.

The recently published study, featured in the Journal of Pest Science, investigated the realized host range of T. japonicus over a span of three years. This research employed a methodology that involved exposing sentinel egg masses of H. halys, as well as 18 non-target species, and collecting naturally laid egg masses in Switzerland and Italy.

Out of the 18 non-target species examined, a total of 15 species were found to be successfully parasitized by T. japonicus in the field. This finding confirms the wide fundamental host range of T. japonicus. However, it was observed that most non-target species experienced lower levels of parasitism by T. japonicus compared to H. halys. These species seemed to benefit from either partial temporal or spatial refuges, which provided some degree of protection against parasitism.

According to Dr. Haye, "Species with unique life cycles and occupying the same ecological niche as H. halys, such as Pentatoma rufipes, which happened to be the non-target species with the highest parasitism rate in both countries, may face an elevated risk of being parasitized by T. japonicus."

"On the other hand, there could be beneficial non-target effects observed in the case of the invasive pest, Nezara viridula, as it experienced significant non-reproductive mortality caused by T. japonicus," explained Dr. Haye. "In both situations, conducting life table studies will be necessary to assess the impact of non-target parasitism and understand the potential consequences at the population level."

Dr. Haye and his colleagues discovered that the majority of European stink bug species tend to deposit their eggs during the spring season when T. japonicus populations are at a lower level. As a result, these species can benefit from partial temporal refuge, reducing their vulnerability to parasitism by T. japonicus.

Furthermore, species residing on herbaceous hosts within the ground cover have the ability to evade intense parasitism, as T. japonicus exhibits a preference for woody habitats. This creates a spatial refuge for those species that inhabit such habitats, offering them protection from high levels of parasitism by T. japonicus.

Conversely, species that share the same ecological niche as H. halys, particularly tree-dwelling species, have a higher likelihood of being targeted by T. japonicus. However, it is important to note that an increased risk should be primarily attributed to those species with atypical life cycles, such as P. rufipes or Picromerus bidens (L.) (Hemiptera: Pentatomidae), which deposit their eggs in late summer when the majority of T. japonicus parasitism occurs.

Dr. Haye emphasized the importance of considering ecological factors such as habitat specificity, the timing of host and parasitoid activities, host population density, and competition with native parasitoids or predators. Incorporating these factors into the assessment can provide a more comprehensive and realistic understanding of the potential risks posed to non-target species. By taking into account these ecological dynamics, a more accurate picture of the potential impacts on non-target species can be obtained.