After a long hot summer, many custodians of the countryside will be breathing a sigh of relief as the winter months will provide a rest bite from battling with infestations of non-native plant species. Unfortunately, the battle is too often one sided and the weeds are winning! Himalayan balsam (Impatiens glandulifera) is one of the UK’s most invasive non-native plant species and an incredibly difficult plant to control, due mainly to the inaccessible and sensitive habitats where it grows. As a weed of riparian habitats chemical control is often not a viable option, manual control is labour intensive, and for any control effort to be successful control must take place on a catchment scale else seeds from populations upstream will colonise cleared areas during the late summer months.
Himalayan balsam invading pasture-land in the Camel Catchment, Cornwall, UK (CABI)
So what other options are there to combat this alien invader? Well in the UK, since 2006, CABI has been researching the potential of controlling Himalayan balsam using biological control. Our aim is to identify co-evolved, host specific, natural enemies from the plants native range which could be introduced into the UK to control the plant in both a sustainable and ecological manner. To-date our research has shown that in the foothills of the Himalayas, Himalayan balsam is attacked by a multitude of natural enemies including both insects and plant pathogens and some of these may have the potential to be used as biological control agents in the plants introduced/invasive range.
Over the course of the project we have surveyed Himalayan balsam throughout the Himalayas and in 2008 we identified a number of sites in the Kullu Valley, Himachal Pradesh, India, where the plant was common and the damage inflicted by natural enemies was high. You can view a photo diary of our 2008 survey via the CABI website. As well as surveying in India we also surveyed the plant in Pakistan, and although populations of the plant were abundant, the diversity of natural enemies was lower than that of the Indian Himalayas. Based on these findings, CABI has been working in collaboration with scientists from the National Bureau of Plant Genetic Resources, New Delhi, where we have been researching the life cycle and host range of some of the plant pathogens and insect species identified during previous surveys.
In 2010, CABI secured the export of potential biocontrol agents from India and over the summer months our entomologists and pathologists have been testing the suitability of the potential agents in a series of tests under quarantine conditions. At present, research is currently focusing on testing a rust pathogen (a Puccinia species) which was observed as being highly damaging in the plant’s native range. The rust initially infects the stem of young plants early in the season resulting in abnormal and stunted growth. Often infected seedlings do not survive to reach maturity. Field observations in the native range suggest the rust pathogen has a high level of host specificity. Himalayan balsam is often found growing in close proximately with other closely related species (other Impatiens species), and where Himalayan balsam is infected with the rust other species are uninfected.
Throughout the winter months our research will concentrate on evaluating the infection parameters and host range of the rust by testing it against a selection of plant species which include UK natives, closely related ornamental species and economically important species. Although a time consuming process the research is progressing well and hopefully in future years we will have a biocontrol agent which will halt the spread of Himalayan balsam along our rivers.
By Rob Tanner, Principal Investigator of the Himalayan balsam biocontrol research
Tanner, RA., Ellison, C., Shaw, RH., Evans, HC. & Gange AC. (2008). Losing patience with Impatiens: are natural enemies the solution? Outlooks on Pest Management. 19: 86-91.
Tanner, RA. (2008). A review on the potential for the biological control of the invasive weed Impatiens glandulifera Royle in Europe. In Plant Invasions: Human Perception, Ecological Impacts and Management (B Tokarska-Guzik, JH Brock, G Brundu, LE Child, C Daehler & P Pysek (eds)) Backhuys Publishers, Leiden, The Netherlands. 343-354.
Shaw, RH. & Tanner, RA. (2008). Weed like to see less of them. Biologist. 55 (4): 208-214.