Leaf-roll gall formation in Fallopia bohemica and its implications for biological control

The psyllid, Aphalara itadori Shinji (Hemiptera: Psyllidae), is a biological control agent against invasive knotweed species in Europe and North America. Despite many releases, successful establishment of the agent has never been confirmed. The slow establishment of A. itadori 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 the psyllids from the Murakami prefecture in Japan, where it was observed causing severe leaf-roll galling on Fallopia bohemica Chrtek & Chrtková (Polygonaceae). The induction of leaf-roll galls by Murakami psyllids on F. 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. Artificial leaf-roll galls were consistently utilized by developing A. itadori nymphs, and contributed to their survival. Leaf-roll galling in F. 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 suggest that releases of A. itadori should be conducted early in the growth season, or in conjunction with cutting treatments, so that developing nymphs have access to newly emerging leaves. Additionally, our results provide motivation to seek additional lines of A. itadori that may impact other invasive knotweed species in the similar ways.