How to combat invasive balsam
Since its arrival in the 1800s, Himalayan balsam has taken over many a riverbank in the UK. Could it finally have met its match?
Scientists are using fungi to help keep Himalayan balsam at bay in the UK, but how does it work?
There is something unsettling about the term ‘invasive non-native species’, as though there is something sinister going on, and these plants and animals have somehow sneaked into the country while we were looking the other way. But often, nothing could be further from the truth. Not only have we held the door open for them, we’ve thrown them a lavish welcome party.
Plants in particular were often brought in as exotic curiosities by those lovers of the strange – the Victorians. Many still brighten our gardens quite happily, but some have taken to their new home rather too well.
Don’t be fooled by these pretty flowers. In their own land, where they evolved, natural enemies keep their numbers in check. But in the UK, they have no such adversaries.
Inside those delicate blooms thus lies the blueprint for a ruthless competitor.
Some of these species are familiar to us all. Japanese knotweed and giant hogweed are common sights on recently disturbed ground, taking over railway embankments and riverbanks, particularly in urban settings. They are often found jockeying for position with a third invader – Himalayan balsam. Between them, they are smothering our native plant life.
A long way from home
In its home range in the foothills of the Himalayas, Himalayan balsam grows in small clusters. With its delicate, pink, orchid-like blooms, it is easy to see why it appealed to those intrepid collectors. But in dense stands that can engulf acres of riverbank, it more resembles an explosion in a confetti factory.
Himalayan balsam has been released many times across the UK since its arrival in the late 1800s, not least by beekeepers as a source of nectar. It produces more nectar per flower than any of our native species, leading some ecologists to believe that its presence is preventing bees from pollinating the native flora.
Plants often evolve a strategy of ‘dispersal by animal’. The Himalayan balsam’s strategy could easily be ‘dispersal by children’, as few can resist squeezing the seed capsule on autumn woodland walks, as it temptingly bulges like a coiled spring before firing its cargo with a dramatic ‘pop.’ It’s an impressive mechanism, shooting the seeds up to 7m. Should they land in a river, the seeds are carried downstream and can soon populate the whole catchment.
What seemed like a good idea when a local community group asked me last summer if I fancied a spot of ‘balsambashing’ soon felt like an impossible task once I was standing amidst acres of the stuff, seriously contemplating uprooting it all by hand. The closer to water it is, the bigger balsam seems to grow. Take hold of its thick, fleshy stem, give it a sharp pull and it comes up easily enough – those shallow roots don’t put up much of a fight. But it is those very shallow roots that can cause such a problem in riverine habitats. They fail to bind the riverbanks, which makes the bare soil prone to erosion in winter spates, when the plant dies back.
The options for controlling this invader aren’t promising, particularly at the riverside. Physically shifting balsam on any scale requires vast numbers of people, and it often grows in such difficult to access places that removal is simply not feasible. Chemical ‘enemies’ we might want to use are severely restricted throughout Europe. There is only one herbicide, glyphosate, that can be used near water, and the fate of that is under review.
There is, however, a third method – biocontrol. This means introducing the natural enemies of an invasive species, which co-evolved with it in its home range, into the new environment already colonised. The perfect agent will have evolved so closely with the invader that it poses minimal threat to native species. The aim is not eradication, but control.
The approach has its critics. Science journalist Fred Pearce argues in his book The New Wild that we should learn to accept invasive species as a consequence of the modern world we live in. A new plant and animal community will be created in time – we should stop trying to wage a war against the aliens.
According to Richard Shaw, country director at the Centre of Agriculture and Bioscience
International (CABI), something may indeed evolve to keep the numbers of these invaders in check, but it may take thousands of years – we just don’t know. What about the damage they are doing in the meantime, not least to our native biodiversity?
Focussing on fungi
Since 2006, CABI scientists have been conducting surveys throughout the balsam’s native range to identify potential biocontrol agents. Many have been discounted, but one, a rust fungus in the Indian Himalayas, was observed to have significant impacts on its balsam host. In 2010, it was brought to CABI’s UK lab for quarantined tests. Field trials began in 2014, and in spring 2015, the fungus was released at 25 sites in England and Wales.
Initially, the fungus was spread by introducing infected plants among existing stands of balsam, but the spraying of fungal spores, suspended in liquid, directly onto the growing leaves proved far more efficient. A patch of about 10m² would be zoned off, and plants in about half of the area treated three times during the summer, optimising the rust’s chance to establish itself.
Rust fungi co-evolved so closely with balsam that they preferentially attack plants from their region.
Rust fungus has a complex life-cycle and launches a two-pronged attack on its host. It targets the leaves throughout the growing season, weakening the plant and reducing its ability to set seed. Then, after overwintering in the soil, it awakens in the spring to attack the stems of any seedlings, rendering them weak and twisted, before hitting the leaves once again to continue the cycle. That overwintering stage is therefore a key part of the process.
“Little pustules on the underside of leaves are the easiest way to tell if a plant is infected,” explained the late Carol Ellison, who was a rust fungus specialist at CABI. “Wipe your finger across them, and you’ll see a spread of spores.”
Despite the early optimism from the lab, results in the field following the 2015 release were puzzlingly slow, with infection rates much lower than expected at a number of sites.
The scientists delved deeper, carrying out molecular analysis on 100-year-old samples of Himalayan balsam at Kew. They now think that the plant has made
three separate arrivals to the UK, from three different regions of the Himalayas – Pakistan, India and possibly western Nepal. There are now, therefore, at least three different forms, known as biotypes, established in Britain.
Special relationship
They also discovered that rust fungi have co-evolved so closely with balsam that they will preferentially attack plants from their own region. Since the strain of fungus introduced hailed from the Indian Himalayas, only those plants originating from that same home range were affected, which explains those early, disappointing results. The theory was proved when a second strain of rust fungus, this time from Pakistan, was released, and found to infect a different cohort of balsam in a different region of the UK.
Effectively controlling balsam therefore depends on matching the rust fungus to the strain of balsam with which it coevolved, from the geographical region it is found. The CABI team now wants to trial a further rust fungus from the Kashmir region, but the political situation there, not to mention the COVID-19 pandemic, is making collection nigh on impossible.
For now, both the Indian and Pakistan rust fungi are out at a total of 36 sites – with scientists trying to match each strain with its corresponding biotype – and the results have been more encouraging. In spring 2019, signs of infection were found on 30–40 per cent of plants at one site along the River Tees, while at another site – Harmondsworth Moor in West London – the rust has overwintered twice (and as such no longer requires physical release) and has spread 50m. At eight further sites, the life-cycle has been completed.
Despite these moderate successes, it is fair to say that CABI’s experiments with biocontrol in the UK have, so far, been unspectacular. The scientists suspect that the problem may be ‘climatic matching’ – the agents are not coping with the British weather. It is still a long way from Richard Shaw’s description of classic biocontrol: “Release once; walk away.”
We live in a world that expects instant results, but science is rarely like that. “A biocontrol agent of this type will spread kilometres in a year,” Ellison said, “but you need a decent density for it to really spread – to become ‘epiphytotic’ [destroying large numbers via disease]. There is a lag phase, which can be five to ten years, while it adapts to life in the UK.”
Given that the invasive plants CABI is working with in the UK grow in or beside water, which restricts chemical treatment, successful biocontrol methods might well be worth the wait.
In the meantime, there are always those balsam-bashing parties, I suppose. But if my river catchment is anything to go by, we will need one heck of a party.
is a writer and podcaster who focuses on the environment and conservation.
FIND OUT MORE
Our guide to invasive plants and their biocontrol agents: discoverwildlife.com/invasive-plants