What is natural control?

Natural (or biological) control, is the use of living organisms to control pest populations.

Increasing trade and travel between countries has increased the number of plants and other non-native species arriving in the UK. The majority arrive without their natural enemies that keep them in check in their native range. In the case of plants almost all the insects and fungal pathogens that would normally inflict damage on the plant are lost, giving the plant an unfair advantage over its new neighbours.

Natural control is a means of redressing the imbalance by re-introducing one or some of the specialist natural enemies that exert control on the species in its native range.

Surveying Himalayan balsam in the native range

Surveying Himalayan balsam in the native range

A sustainable alternative
The use of self-replicating and co-evolved natural enemies for the long-term management of invasive alien species is a sustainable solution. Once established an effective agent provides control indefinitely without further cost or intervention.

Biological control has been used effectively against invasive species for over one hundred years.

What is known as the ‘classical approach’ is being proposed for Himalayan balsam. This method involves going back to where the species originated (in this case the foothills of the Himalayas, both India and Pakistan) and finding the specialized natural enemies that keep the species in check.

Only after intensive research and vigorous safety testing to ensure a natural enemy is specific to Himalayan balsam, will it be considered for release. In effect, this method utilizes nature’s own in-built mechanisms to ensure equilibrium.

It is obviously important that potential control agents such as insects or pathogens don’t attack other plants, especially crops or endangered species. Scientists therefore spend considerable time (at least three years) studying the host range of any potential agent within a secure quarantine facility adhering to the International Code of Conduct.

This testing enables scientists to predict which, if any, other plant species might be at risk. Only once these stringent tests have been carried out to prove the control agent is safe, is an application for release made.

Natural control – the advantages
Environmental– Natural control is exactly that – natural – and does not rely on the use of man-made chemicals that can impact adversely on the ecosystem in which they are used. Furthermore, the amount of herbicides required for weed control can be reduced.

Cost – After the initial research costs there is virtually no need for further expenditure once the agents are established and having an impact on the weed, bar monitoring activities.

Sustainability – It is permanent and therefore completely sustainable. The weed is continually attacked by the biocontrol agent.

Spread – The control agents, be they insects or pathogens spread through the weed population until stopped by physical, environmental or chemical barriers, just as they have done in their native range.

Safety – Natural control agents should pose no threat to human health, crop production or beneficial organisms.

Landscape – Whilst the agents are doing their job on the Himalayan balsam, the native flora, which was previously out-competed, should be able to gradually recover and re-colonise areas without the need for extensive replanting.

Disadvantages
Control not eradication – A successful agent should not eradicate the weed on which it depends, but should reduce it to acceptable levels. There may be costs associated with alternative control methods.

Timescale – It takes time. It can take five to ten years from release to achieve successful control.

Impacts – The complete impact on the target weed is not always predictable.