New study confirms minimal impact of Samurai wasp on native stink bugs.

A new CABI-led study confirms that the samurai wasp (Trissolcus japonicus) – a natural enemy of the brown stink bug (Halyomorpha halys) – has little effect on the native stink bugs.

Dr Tim Hay, chief of biological control of arthropods based at the CABI Center in Switzerland, teamed up with colleagues to ask whether the primary host population of T. japonicus matches the host range investigated in Europe – including Switzerland, Italy and Germany where it has already been detected. .

Scientists, including those from the University of Turin, Agriculture and Agri-Food Canada and the Swiss Ministry of Finance, Economy and Agriculture, found that T. japonicus was most likely to attack species in the same ecological niche as H. halys.

Originally from Asia, the brown stink bug can cause significant damage to fruit and nut crops in Switzerland as well as elsewhere in the world.

The Guardian reported in 2020 how scientists fear the brown stink bug could spread to eight cities in Turkey, mainly across the Black Sea region, endangering around 70% of the world’s hazelnut supply.

T. japonicus, which was first released into a pear orchard in Zurich, Switzerland, in 2020, as part of a field trial conducted by Agroscope and supported by CABI, is seen as a potential enemy of the pest that also attacks fruits including cherries.

The most recent study published in Journal of Pest Science, over a period of three years, the investigated host range of T. japonicus was evaluated. She did this by exposing the egg masses of H. halys and 18 non-target species and collecting the masses of eggs laid naturally in Switzerland and Italy.

In all, 15 of the 18 non-target species were successfully parasitized by T. japonicus in the field, confirming its broad primary family range. However, most non-target species were less parasitized by T. japonicus than H. halys, taking advantage of either partial temporal or spatial refuges from parasitism.

Dr Hay said: “Species with an unusual life cycle and the same ecological niche as H. halys, such as Pentatoma rufipes, which was the most frequent non-target parasitoid in both countries, is likely to face an increased risk of parasitism.

In contrast, beneficial off-target effects may occur for the invasive pest, Nezara viridula, which has experienced a high rate of non-reproductive mortality caused by T. japonicus. In either case, life-table studies will be required to determine the effect of non-target parasitism and potential consequences at the population level. “.

Dr. Hay and his colleagues learned that the vast majority of European stink bug species lay eggs in the spring when T. japonicus numbers are low, and can thus gain a partial temporary refuge from parasitism.

In addition, species that live on herbaceous ground-cover hosts can escape intense parasitism due to T. japonicus’ preference for woody habitats, thus providing a spatial refuge for those species that occupy these habitats.

On the other hand, species that share the same ecological niche as H. halys (trees) are more likely to be attacked by T. japonicus, but the potential increased risk should only be considered for those species with an unusual life cycle, such as P rufipes or Picromerus bidens (L.) (Hemiptera: Pentatomidae), which lay their eggs in late summer when the majority of parasitism by T. japonicus occurs.

“Incorporating ecological factors such as habitat specificity, host and parasite phenotypes, host density and competition with parasites or native predators can help produce a more realistic scenario of potential risks to non-target species,” added Dr Hay.

“However, such field studies would only be possible if biological control agents are present—either through deliberate release or accidental establishments.”