There has been much talk about biological control on the Hartbeespoort Dam the past couple of years, but many residents are still unsure about what exactly this entails. The Centre for Biological Control Rhodes University answers frequently asked questions from the community.
What is biological control? When a plant is removed from its native range it is also removed from its natural enemies which help to keep it in check. Therefore, when the plant gets into a new habitat it can outcompete native species. The aim of biological control is to reunite the plant with its natural enemies from its native range so that they can reduce the plants vigour, and therefore its growth and spread.
Why should I use biological control? Biological control is cheaper, safer, and more effective in the long-term than either herbicide spraying or manual removal of invasive plants.
Is biological control safe? The biological control agents cannot complete their lifecycle on anything other than their host plants, so when the weed populations die back, the excess insects will either disperse to a new site or die.
How much do the insects cost? The Department of Forestry, Fisheries and the Environment have historically funded research into and implementation of biological control, allowing scientists and mass rearing stations to supply insects to invaded sites with no cost to the landowner.
Will biological control eradicate the plant? Biological control is unlikely to lead to eradication as the insects rely on the plants for survival. Instead, the aim of a biological control programme is to reduce the weed population to a level where it is no longer damaging to the environment and the local stakeholders. Some plants will remain, and these will ebb and flow seasonally with the natural cycle of biological control agent populations, just like a natural system in equilibrium.
How long will it take for biological control to work? This can be variable. It depends on how many insects are released, how often, and the size of the invasion. Local climate plays a role, the cooler the climate, the slower the control agent population growth. Effective biological control can often take multiple seasons before success.
Why do the plants come back in spring? Cold winters have severe impacts on aquatic weeds and their biological control agents. However, the plants recover quickly in spring, but the insect populations take longer to recover. Furthermore, plants such as water hyacinth have vast seed banks which germinate from the sediment in the spring time. This can lead to periodic reinfestation which requires further releases.
Can I use a biological control agent on several different weeds? Each biological control agent is host-specific which means it can only feed on one species of plant. The CBC has several different biological control agents available for a range of weeds, see the list here: https://www.ru.ac.za/centreforbiologicalcontrol/mass-rearing/waainekmassrearingfacility/
How do I release biological control agents? This is the easy part. Once you have received a consignment of insects they must be kept out of direct sun and released as soon as possible. To release, simply find a healthy patch of the target plant, preferably in an undisturbed site where they won’t be removed, sprayed with herbicides/insecticides, or otherwise disturbed. Once a site is found, carefully tip the insects out of the containers and send a GPS coordinate and other release information to the person that sent the insects to you.
Do I need permission to release biological control agents? You are allowed to release biological control agents on any invasive species as regulated by NEM:BA https://www.ru.ac.za/centreforbiologicalcontrol/resources/frequentlyaskedquestions/