The psyllid
The sap-sucking psyllid Aphalara itadori
On 9 March 2010 approval was granted by Defra to release the psyllid. The earliest time at which release could be realised is Spring/Summer 2010.
The psyllid Aphalara itadori is a true knotweed specialist that sucks the sap from the plant. It is about 2mm in length and capable of causing significant damage to the target weed.
It is the juvenile nymphs that cause the most damage to the plant and so where the adult psyllids choose to lay their eggs is highly important. In order to find out where the psyllid lays its eggs and whether any damage results, extensive tests were carried out on over 90 species of plants. In total, the location of more than 145,000 eggs were recorded. Only 0.6% of these were laid on non-target species or varieties and not one of those eggs was able to develop successfully to adulthood.
These findings were supported by further studies in which nymphs were physically transferred onto non-target plants and again, no adults developed. Research also shows that adult psyllids simply cannot survive even on the most closely related species to Japanese knotweed in Britain.
Based on this research, following an intentional release in Great Britain, the A. itadori psyllid should pose no threat to anything other than Japanese knotweed and the damaging hybrid variety – bohemian knotweed (Fallopia x bohemica). The rate and nature of its reproduction should mean establishment and multiplication would be successful. Furthermore, it should also be possible to integrate the psyllid control programme with current management regimes which would improve control efficacy at sites where traditional control remains necessary.
Other potential agent: the leaf-spot fungus Mycosphaerella polygoni-cuspidati
This leaf-spot fungus devastates Japanese knotweed in the field in the warmer months from June to October. It has never been recorded on any plant other than Japanese knotweed.
The fungus is much harder to work on than many of the potential insect agents since its lifecycle is complex and the infection process is intricately linked to environmental conditions. Nonetheless, host range testing has progressed well over the last couple of years and this fungus remains highly promising. A great deal of work has also been carried out by our Japanese collaborators, one of whom has completed an MSc and a PhD on the pathology of knotweed.
While research has shown the leaf-spot fungus could be a viable option, further studies are still underway.