The coronavirus Covid-19 is causing a massive impact at the moment. Our thoughts are with anyone and their families who may have been affected by this virulent disease; not just the fatalities but the many who have recovered from infections with impaired health too. Plus, of course, all the many homes/households and businesses that have had to cope with the interruptions to their lives caused by the global ‘lockdown’. Inevitably, questions get asked about whether we should have been better prepared. Could we have been ‘wiser before the event’? Well, perhaps…… Here’s a view from the invasive weed perspective!

The link between COVID-19 and Invasive Species

It might seem, at first, to be a little far-fetched to make a link between Covid-19 and Japanese knotweed or Himalayan balsam but bear with me because they are not really so very different! Covid-19 and Japanese knotweed etc. are both alien species taking advantage of the opportunity to infect vulnerable targets which have little or no natural immunity. They are novel organisms taking advantage of ‘fresh fields’ in which to grow. In many respects we are seeing evolutionary processes taking place on a very short time scale. Covid-19 itself is the product of a genetic mutation which has enabled an organism to switch host; genetics also plays a part in the way that human populations respond to Covid-19.

Variability in the population ensures some immunity but most of us are susceptible albeit with huge differences in symptoms and outcomes. As a result of all this, if the virus doesn’t die out completely, eventually we will develop the much sought after ‘herd immunity’ (measles is making a comeback because we lost ‘herd immunity’ due to a decline in immunisation rates – viruses don’t disappear, they just bide their time!).

Japanese knotweed spreads in a remarkably similar way

Not so different with Japanese knotweed! It is able to exploit most temperate ecosystems that haven’t been exposed to it before (so no ‘immunity’) but it tends to be more successful in some climates than others. The way that Japanese knotweed spreads can be described using ecological models which are remarkably similar to those used by epidemiologists to describe disease transmission.

This last point reminded me, in a roundabout way, that the recent Environmental Commons Select Committee report on Invasive Species included in their enquiry the diseases that are causing existential challenges for many of our native plants (Ash die-back etc.). They concluded that there is a need to tighten-up import controls to screen for insect vectors and fungal pathogens to which our plants/animals have no natural defence (the Government’s forest creation ambitions ought to be fulfilled entirely from UK-sourced seeds/saplings where at all possible).

But there is one distinct and interesting divergence from the ‘norm’ with Japanese knotweed and that is its almost complete lack of genetic diversity. It is widely accepted that virtually every single plant we see, from John O’Groats to Lands End is a clonal ‘sister’ of the original plant specimen brought to Kew Gardens in the mid-19th century. There are some other problematic relatives like Giant Knotweed and Compact knotweed that can fertilise Japanese knotweed’s female flowers and produce hybrids (the Giant/Japanese hybrid is called ‘Bohemian’ knotweed) but they represent only a small fraction of the total knotweed biomass in the UK.

There is much to learn from the study of invasive plants

So whilst there is much we can learn about viral disease transmission from the study of invasive plants (and vice versa) actually Japanese knotweed has a massive achilles heel which we may one day be able to exploit to achieve effective biological control. Its lack of genetic diversity may be its ultimate undoing. CABI – the international research centre for the control of invasive species and disease in agriculture – are involved in the development of a fungal pathogen as a biocontrol agent, a ‘mycoherbicide’ (knotweed-specific non-chemical spray), already patented in the EU and undergoing field trials at locations unknown!! If it works it could be the proverbial silver-bullet although we should remember that any biological agent applied in the field is bound to produce variable results simply due to site conditions/climate factors etc.

Final point. Has the invasive weed sector gained something from the international crisis caused by Covid-19? We think the answer is tentatively ‘yes’. It’s a wake-up call to legislators; even in a global economy we mustn’t play fast and loose with our ecosystems which are vulnerable to disruption by outside factors.

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