The psyllid, a natural enemy of knotweed in Japan, is one of the best hopes to control the invasive in the UK. It sucks the plant's sap and stunts the knotweed's growth, and is known to thrive in a wide range of climates. Dr Dick Shaw of the Centre for Agriculture & Biosciences International said extensive testing has shown the psyllid poses no risk to plants other than Japanese knotweed.
Research has instead focused on whether the psyllid could disrupt the equilibrium of other predator-prey relationships. For example, if the natural enemy of another pest such as aphids developed a preference for A. itadori, its original prey might run wild or the predator population could boom, an effect the scientists are keen to avoid.
However, the real risk is the struggle to get it established. The programme has just finished a five-year test. It found that the psyllid could overwinter successfully in the UK. So far psyllid populations have failed to build to sufficient numbers to demonstrate their effect. Last year researchers resorted to augmenting the population of psyllids in field cage trials, adding 5,000 per cage to prove the insect is safe.
The most likely negative outcome will be that the psyllid fails to establish, said Shaw. So far results have been lacklustre, although the signs of psyllid damage are clear on the target plants. "It will be years before it is effective. Once we've cracked what it is that's halting it - there are 100 possible reasons. For example, we've been releasing them in batches and the psyllids then lay tens of thousands of eggs in sync. If it then rains it could knock all of them off in one go, so we're now releasing them sequentially."
The current population has probably suffered from being bred in a lab for 120 generations, so they are looking at bringing in some "new blood" from Japan, said Shaw. "We're confident there will be no bad effects but it is a long-term game, not a quick fix. Plenty of programmes have done nothing for 10 years then they explode and do the job."
In the long term, a leaf spot fungus may also hold promise. A Mycosphaerella species that only attacks knotweed has been "effectively dismissed" for classical biocontrol - releasing the fungus into the wild. However, Defra has applied for a patent and could eventually create a mycoherbicide that could be sprayed directly onto the plants.