Seeking better versions of the same agent Aphalara itadori – the knotweed story continues

Japanese knotweed, Reynoutria japonica, Giant knotweed, Reynoutria sachalinensis and the hybrid, Reynoutria x bohemica have been problematic invasive plant species in Europe and North America since their introduction from Japan in the 19th Century. Aphalara itadori Shinji (Hemiptera: Psyllidae), a sap-sucking psyllid specific to these knotweed species, collected from Mt. Aso, Kyushu Island, Japan (Kyushu line), has been released into the United Kingdom (UK) and subsequently into North America, as a classical biological control agent after approval by Department for Environment Food and Rural Affairs in 2010. Reproduction of the Kyushu line has been observed on R. japonica at several release sites, with some overwintering; however, long-term establishment has proven challenging. The application of CLIMEX modelling led to a further survey in 2019 which focused on collecting psyllids from further north on Honshu Island, Japan, with higher Composite Match Index values to the UK. Psyllids were collected from Murakami, Niigata Prefecture, northern region of Japan (Murakami line) where extensive and severe leaf curling damage attributable to psyllid nymphs was observed, quite unlike anything seen with the original Kyushu psyllids. The Murakami psyllid was confirmed as A. itadori based on both molecular analysis and morphological characteristics. A molecular screen for disease contamination in the Murakami line found that it was not infected with any of Candidatus Liberibacter spp. nor Candidatus Phytoplasma spp. which other psyllid species have been known to vector, indicating that the curling damage was likely to be caused by the physical interaction of the psyllids. Host-specificity testing of the psyllid using the most important nine selected non-target plant species showed the host-specificity of both the Murakami and Kyushu lines are very similar. The results also suggest that the Murakami line performs better on R. x bohemica than on R. japonica. Based on those results, releases of the Murakami line were granted in the Netherlands in 2020, and in the UK and Canada in 2021. The Murakami line has been released onto R. japonica and R. x bohemica populations in the UK and showed more curling damage on R. x bohemica than on R. japonica in the field. In this presentation, our research with the new Murakami psyllids on knotweed species is reported