Leaf‐roll gall formation in Reynoutria × bohemica and its implications for biological control with Aphalara itadori

Abstract

The psyllid Aphalara itadori Shinji (Hemiptera: Psyllidae) is a biocontrol agent against invasive knotweed species (Polygonaceae) in Europe and North America. Despite an extensive release program, successful establishment of the agent has never been confirmed. The slow establishment of A. itadori in its introduced range appears to be predominantly the result of nymphal mortality, either through desiccation, predation, or the adaptation of captive populations to low‐stress environments. In 2016, CABI UK collected a new line of psyllids from Murakami, Niigata Prefecture, in Japan, where it was observed causing severe leaf‐roll galling on one of the many varieties of native knotweeds. The induction of leaf‐roll galls by Murakami psyllids on invasive Bohemian knotweed, Reynoutria × bohemica, has the potential to alleviate some or all of the observed nymphal mortality factors. We conducted a series of growth chamber experiments to explore the psyllid behaviors that initiate leaf‐roll galls, and the ways in which the galls might benefit the psyllids and inhibit invasive knotweed. Reynoutria × bohemica exposed to A. itadori for 4 weeks suffered reduced stem elongation and leaf area compared to control plants. Artificial leaf‐roll galls were consistently utilized by developing A. itadori nymphs, and contributed to their survival. Leaf‐roll galling in R. × bohemica was predominantly initiated by early instar nymphs of A. itadori, and only occurred when leaves were attacked at a very early stage in their development. Our results can inform the knotweed biocontrol program in several key ways. Releases of A. itadori should be timed so that developing nymphs have access to newly emerging leaves. Monitoring efforts at release sites should focus on the growing tips of R. × bohemica, where the highly mobile nymphs appear to migrate. Finally, our results provide motivation to seek additional lines of A. itadori that may affect other invasive knotweed species in similar ways.

Biological control of weeds – Context dependency matters

“…Rahman et al (2023) show in a glasshouse experiment that impact of a jewel beetle differs between two life forms of a climbing vine, with one life form experiencing larger short-term per capita impact but the other one prone to higher impact at larger time scales. Jones et al (2023a) found in a laboratory study that all life stages of a psyllid, F I G U R E 1 Context dependency matters in insect biocontrol of weeds: interactions and contexts addressed in this journal issue dedicated to the biological control of weeds.…”

Exploiting exotic pathogens as mycoherbicides against invasive alien weeds: Japanese knotweed as a case study

“…Current management of the weed relies on chemical herbicides; however, these are only effective in the short term. They are also costly and unsustainable due to the extensive, deep‐rooted rhizome system of Japanese knotweed, and their application is often restricted, particularly in riparian environments 19 . In the UK, the annual cost for its management was estimated at > £165 million in 2010, 20 but this figure is likely to have increased significantly since this assessment was undertaken.…”