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Biological control of Himalayan balsam

Himalayan balsam has rapidly become one of the UK’s most invasive weed species. A lack of natural enemies allows it to successfully compete with native plants for space, light, nutrients and pollinators, reducing biodiversity and contributing to erosion. Traditional control methods are inadequate. This project involves identifying an insect or plant pathogen that exclusively attacks Himalayan balsam, which can be released into the UK to control the plant while leaving indigenous species intact.

Project Overview

So, what’s the problem

Himalayan balsam (Impatiens glandulifera) has rapidly become one of the UK’s most invasive weed species, colonising river banks, waste ground and damp woodlands. It successfully competes with native plant species for space, light, nutrients and pollinators, and excludes other plant growth, thereby reducing native biodiversity. As an annual, Himalayan balsam dies back in the winter, and where the plant grows in riparian systems this can leave river banks bare of vegetation and liable to erosion. Dead plant material can also enter the river, increasing the risk of flooding.

Like most introduced plant species Himalayan balsam arrived in the UK without any of the natural enemies that help keep the plant in check in its native range (the foothills of the Himalayas, India and Pakistan). Without these natural enemies, Himalayan balsam is able to grow more aggressively and has a greater ability to reproduce, giving it an advantage over native species. Traditional control methods are currently inadequate in controlling Himalayan balsam in the UK. This is often because the plant grows in inaccessible areas or sites of high conservation status where chemical and/or manual control is not an option.

What is this project doing?

The ultimate aim of the project is to find a co-evolved insect or plant pathogen that exclusively attacks Himalayan Balsam, which can be released into the UK to control the plant whilst leaving indigenous species intact, so that the ecosystems can be restored.

Since 2006, surveys have been conducted throughout the plant’s native range to identify natural enemies that could be considered as biocontrol agents in the introduced range. Many of the natural enemies, both fungal and arthropod species, collected and identified during the survey have been rejected as suitable control agents. We undertook safety testing procedures in our UK quarantine facility, and found they were able to attack other closely related to Himalayan balsam.

One natural enemy, a rust fungus, which was observed causing significant impacts on Himalayan balsam in the Indian Himalayas, was exported to our quarantine facility in the UK in 2010 to undergo extensive safety testing. The rust, a Puccinia species, is an autoecious, macrocyclic (completing its entire life cycle on a single species), five spore staged rust fungus which infects the stem and leaves of Himalayan balsam throughout the growing season.

Watch a photodiary of the 2008 survey.


We tested the safety of this Himalayan balsam rust following strict internationally recognized testing procedures. We compiled a test plant list comprising 84 plants; this consisted of 74 species and an additional 10 varieties of three widely grown ornamental species in the UK. On the list were 26 native plant species to the UK, 52 ornamental plants, three economically important crop/ fruit species and three introduced/ invasive species. This safety testing showed that the rust is highly specific to Himalayan balsam.

Our research was all undertaken in quarantine conditions and through this we clarified the lifecycle of the rust. We were able to prove that all spore stages observed on Himalayan balsam in the native range belong to the same species. We backed up this research with molecular evidence.

Interestingly, we found that the rust species is new to science. We therefore renamed it following the International Code of Nomenclature (see: Puccinia komarovii var. glanduliferae var. nov.: a fungal agent for the biological control of Himalayan balsam (Impatiens glandulifera)).

We compiled all of the scientific research into a dossier (a Pest Risk Assessment (PRA)), which we submitted to UK regulators for consideration, and the European Commission’s Standing Committee on Plant Health in Brussels (see: First release of a fungal classical biocontrol agent against an invasive alien weed in Europe: biology of the rust, Puccinia komarovii var. glanduliferae). Both regulators supported the release of the Himalayan balsam rust in the UK. On the 23 July 2014, the Himalayan balsam rust was approved for release by Defra Ministers making this the first fungal biological control agent to be released against a weed in the European Union. It was released at three authorized locations in Berkshire and Cornwall in August 2014, and the rust was found to spread onto field plants and complete its life cycle under UK climatic conditions.

During 2015-2018, the rust was released at 36 sites, in 17 counties in England and Wales. In 2015, field infection was lower than anticipated at a number of sites, and investigations in 2016 found that the poor results could, in part, be explained by unfavourable environmental conditions. However, the genetic resistance of some weed populations to the introduced strain of the rust from India appeared to be the fundamental issue. Inoculation experiments conducted under controlled conditions, revealed significant variation in the susceptibility of plant populations to the rust, with some showing immunity (no infection overserved).

Subsequently, a second strain of the rust from Pakistan was tested, and was found to infect a different cohort of Himalayan balsam populations. However, the presence of weed genotypes in the British Isles, not susceptible to either rust strain, requires additional strains to be identified in the native Himalayan range. The results of the field releases show that the rust is able to overwinter and establish populations in stands of Himalayan balsam in England. Future work will look at the impact of the biological control agent.

A summary of the results can be found at our Himalayan balsam website.

For an update of our current work, please see the Water Framework Directive document below.


Project Manager

Carol Ellison

Principal Scientist, Invasive Species Management CABI, Bakeham Ln, Englefield Green, Egham TW20 9TY, UK