I am sure that a lot of you already know of knotweed – it is the big baddie of the invasive species, as its ability to dominate environments, by killing off and inhibiting the growth and reproduction of other plants (Clements et al., 2016), and by changing the soil chemistry to the benefit of invaders (Lavoie, 2017), make it a huge problem for native species communities. The IUCN picked Japanese knotweed as one of its top 100 worst plant and animal invaders (IUCN, 2000) – and like many other more familiar villains (take your pick: Voldermort, the Joker or Pokémon’s Team Rocket??), knotweed wreaks havoc wherever it appears.
The knotweed group which I am referring to includes Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinensis) and their ugly lovechild Bohemian knotweed (Fallopia bohemica). Though giant knotweed is less widespread, the other two species can be found all around the US and UK (Lavoie, 2017). Knotweeds reduce native plant biomass and species richness (Lavoie, 2017), colonise disturbed habitats and spread rapidly through floods, human activity and dumping of topsoil contaminated with fragments of its rhizome (a type of plant stem) (Shaw et al., 2009). It can also grow more in height by 1 metre every month (CABI, 2020), and has been shown to negatively affect mites, beetles, snails and springtails (Lavoie, 2017).
But some insects are not badly affected by knotweeds. In fact, one, Aphalara itadori, is suggested to be highly effective at controlling the spread of the invasive group of plants. At present, herbicides are the main way by which knotweeds are controlled (Clements et al., 2016), with mechanical control used on small-scale invasions. But chemical control is very expensive: a UK-wide control program has been estimated to cost up to £1.5 billion (Shaw et al., 2009). So research has instead been looking towards biological control.
Biological control is the use of an organism to control a target organism, in this case the knotweeds – this is where A. itadori comes back in. Through their 2003 trial biocontrol program, Shaw et al. (2009) found that when the insect was exposed to Japanese knotweed it inflicted significant damage to it, even in low numbers. A. itadori also targeted the knotweed almost exclusively, with only 1.52% of insect eggs laid outside of the knotweed species. This is really encouraging as it shows how biological control could be a new weapon against knotweeds. The UK must introduce a new biocontrol organism in this way as when Japanese knotweed first came to the UK from Japan, it escaped the natural enemies which prevented it from being such an issue in Japan. At the moment, neither A. itadori nor the fungus Mycosphaerella polygoni-cuspidati, which was also recommended as a potential biocontrol agent, are used in the UK, however research on their effectiveness continues (Environment Agency, 2009).
PS. Did you know you can eat knotweed? It’s actually pretty good! Here’s some sweet recipes if you ever get the chance to grab some.
CABI (n.d.) What is Japanese Knotweed? [online] Available at: https://www.cabi.org/japaneseknotweedalliance/what-is-japanese-knotweed/ (Accessed: 24/07/2020)
Clements, D.R., Larsen, T. & Grenz, J. (2016) Knotweed management strategies in North America with the advent of widespread hybrid Bohemian knotweed, regional differences, and the potential for biocontrol via the psyllid Aphalara itadori Shinji. Invasive Plant Science and Management, 9(1), pp.60-70.
Environment Agency (2009) The Biological Control of Japanese Knotweed. [online] Available at: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/290480/scho0209bphy-e-e.pdf (Accessed: 24/07/2020)
Lavoie, C. (2017) The impact of invasive knotweed species (Reynoutria spp.) on the environment: review and research perspectives. Biological Invasions, 19(8), pp.2319-2337
Shaw, R.H., Bryner, S. & Tanner, R. (2009) The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: potentially the first classical biological weed control agent for the European Union. Biological control, 49(2), pp.105-113