Invasives Blog

Himalayan balsam (Impatiens glandulifera), an invasive species native to the foothills of the Himalayas, is an extremely problematic weed in the British Isles, and one of the species CABI is working to help control in the natural environment.

Imported to the British Isles for its alluring pink flowers, over the past 50 years the plant has flourished, to the peril of native species, as the invasive plant battles for light, space, nutrients and water – taking it from those that originate in the area.

Since 2014, two strains of the rust fungus, Puccinia komarovii var. glanduliferae, from the plant’s native range in the Himalayas have been introduced into England and Wales to help tackle the growing population of Himalayan balsam. This technique, known as classical biological control does not involve chemicals, pesticides, or have a negative impact on the surrounding species and environment and has been implemented widely across the globe.

However, despite the relative success of the two rust strains initially introduced, it was discovered that not all populations of Himalayan balsam were infected by the rust, and additional strains are now needed to infect resistant populations and achieve successful control of the plant.

In a new paper written by CABI scientists, titled Chloroplast DNA analysis of the invasive weed, Himalayan balsam (Impatiens glandulifera), in the British Isles, the authors set out to investigate the alien species in more depth in the hope of building additional knowledge on introductions of the plant species into the British Isles. In turn, this can be used to help discover new rust strains that could have the potential to be effective on those resistant to current strains used.

In the study, scientists Daisuke Kurose, Kate Pollard, and Carol Ellison analysed samples of  Himalayan balsam that originated from both the native range and from across the British Isles. Those from the native Himalayan region, were sourced from locations such as India and Pakistan, and included samples which were collected as far back as 1881. Samples from the British Isles also include seven herbarium samples from the Natural History Museum, collected from across England more than 100 years ago. The molecular analysis compared six different chloroplast DNA sequences from leaf material from the native and introduced range to gain a better understanding of the plant.

The study found that Himalayan balsam in the British Isles and the native range form two distinct genetic groups, and this could help to understand why the two rust strains currently used do not infect all plants. The results provide an insight into the genetic diversity of Himalayan balsam in the British Isles and importantly present crucial data concerning where to focus future surveys for additional rust strains.

In a mountainous region such as the Himalayas, it is probable that the rust has evolved in isolation and as a result, distinct strains of the rust exist on individual populations of Himalayan balsam. The results of the releases of two strains of this rust in England and Wales have demonstrated the need to consider the introduction of more strains of the rust, since there are weed populations resistant to both strains.

There are numerous examples where matching biotypes of a target invasive alien weed with that of a fungal biological control agent has proven to be critical, and this may be the case for tackling the Himalayan balsam in the British Isles. However, there are other instances where this appears not to be a factor for successful control.

Taking into account the limited sample number used in this particular study and that the number of plants sampled at each site in the British Isles was low, caution should be taken when interpreting these results. However, they do provide evidence of where to find additional strains of rust that may be more suitable, infecting the dominant biotypes of Himalayan balsam in British Isles, and these should be targeted in future research endeavours.

Read the full paper in full with open access: Chloroplast DNA analysis of the invasive weed, Himalayan balsam (Impatiens glandulifera), in the British Isles

Read our project page on Himalayan balsam