FEATURE
Great care , however , must be taken to ensure that the selected biocontrol agent is completely host-specific and that it will not attack closely related indigenous plants of the host region and , thereby , itself becoming a new invasive problem organism . Potential biocontrol agents must thus be carefully screened in quarantine while conducting exhaustive trials that will ultimately ensure that the agent is completely host-specific and therefore “ safe ” to use . The Plant Protection Research Institute of South Africa is the agency tasked with this long-term and extremely important research .
When a suitable biocontrol agent is declared “ safe ” it can be released onto the invasive plants to multiply and prey on their host plant . The procedure of screening a potential biocontrol agent is , unfortunately , a time-consuming and lengthy process and therefore also rather expensive .
Once cleared and declared safe for introduction , successful biocontrol agents can become the most cost-effective method of alien vegetation control and , in some cases , may result in the complete eradication of the problem plant . This ideal situation , however , takes considerable time to develop and biocontrol should not completely replace mechanical and chemical control methods , but should rather be introduced simultaneously to assist with the overall control strategy . Biocontrol is thus not a miracle “ quick-fix ” option .
The advantages of biocontrol are numerous :
• There is no pollution , as no chemicals are used .
• The biocontrol agents are only released if completely host-specific – there is thus no danger of introducing a new alien problem organism .
• Biocontrol agents are self-dispersing .
• Once successfully introduced , biocontrol is permanent – there is thus no need to regularly reinfest , as is the case with a chemical application which has to be continuously followed up .
It is important to appreciate that the aim of biocontrol is not to eradicate the invasive alien plant , but to reduce its competitiveness with the indigenous plant species , this reduces the density of the infestation and thus its overall impact on the environment .
Figure 4 . Prosopis glandulosa seed pods .
which are specially collected from the area of origin of the invasive plant . By introducing these natural plant enemies , which are also without their own natural controls , it is sometimes possible to decrease and even completely eradicate invasive alien plants . A good example of the successful use of biocontrol in South Africa is the Cochineal insect ( Dactylopius austrinus ) which was introduced from Central America to help control the spread of prickly pear ( Opuntia ficus indica ). The introduction has been so successful that the cactus has been completely eradicated in some areas .
The main disadvantage of biocontrol is that the level of control that the biocontrol agent will achieve is not known beforehand and there is a delay before biocontrol agents achieve their full impact . There is , however , a very positive benefit to cost ratio - the benefits of biocontrol normally outweigh the drawbacks and it still represents a comparatively cheap and safe option for the control of alien plant invasions .
Biocontrol agents for alien trees come in a variety of forms and operate in a variety of ways . It is thus often the best
Grassroots Vol 22 No 2 July 2022 24