BIOLOGICAL CONTROL

Biological control is an attempt to introduce the plant’s natural enemies to its new habitat, with the assumption that these natural enemies will remove the plant’s competitive advantage until its vigour is reduced to a level comparable to that of the natural vegetation.

Natural enemies that are used for biological control are called biocontrol agents.

In the control of invasive plants, the biocontrol agents used most frequently are insects, mites and pathogens (disease-causing organisms such as fungi). Biocontrol agents target specific plant organs, such as the vegetative parts of the plant (its leaves, stems or roots) or the reproductive parts (flowers, fruits or seeds).

The choice of biocontrol agents depends on the aim of the control project. If the aim is to get rid of the invasive plant species, scientists select the types of biocontrol agents causing the most damage that are available. In such projects, scientists may use agents that affect the vegetative parts of the plant as well as agents that reduce seed production. However, if the target plant is useful in certain situations but becomes a pest when uncontrolled, conflict of interests arises regarding biological control.

This conflict is usually resolved by avoiding biocontrol agents that have the ability of causing damage to the useful part of the plant, and instead using only seed-reducing agents. These reduce the reproductive potential of the plants, curb their dispersal and reduce the follow-up work needed after clearing, while still allowing for the continued utilisation of the plant. For instance, trees are normally grown for their wood, but the seeds are seldom utilised. If seeds are needed to replant a plantation, a seed orchard can be specially protected against the biocontrol agents in the same way as other crops are protected against insect pests. If, on the other hand, the pods are the most valuable part of the tree, as in the case of mesquite (Prosopis spp.), no biocontrol agents can be selected that will prevent pod production. The seed-feeding beetles that were introduced against mesquite prevent only the germination of seeds from the animal droppings, without significantly reducing the nutritional value of the pods. They do not prevent pod or seed production.

Biocontrol agents are mostly introduced from the country of origin of the plant.

Before the official release of a biocontrol agent in South Africa, extensive studies are carried out in a quarantine facility to ensure that the agent will not damage other, nontarget plants. A biocontrol agent is only released once it has been proved as sufficiently host-specific for release in this country. Tested and approved biocontrol agents therefore do not pose a threat to our own crops or indigenous vegetation, or to those of neighbouring countries. No cases occurred of weed biocontrol agents changing their host plant affinities after their release in a new country to include plants other than those known to be acceptable hosts.

Probably without exception, biocontrol agents do not completely exterminate populations of their host plants. At best, they can be expected to reduce the weed density to an acceptable level or to reduce the vigour and/or reproductive potential of individual plants. The fact that a few host plants always survive, in spite of the attack by a biocontrol agent, actually ensures that the agent does not die out as a result of a lack of food. The small population of biocontrol agents that persists will disperse onto any regrowth or newly-emerged seedlings of the weed. For this reason, biocontrol can be regarded as a sustainable control method.

Biological control works relatively slowly. On average, at least five years should be allowed for a biocontrol agent to establish itself successfully before causing significant damage to its host plant.

Unfortunately, not all growth of invasive plant species can be curbed purely by biological control. It could happen that effective biocontrol agents do exist, but cannot be released in South Africa because they are not sufficiently host-specific. Alternatively, the invasive plant might be a man-made hybrid between two or more species, and is no longer an acceptable host to the natural enemies of either of the parent plants. It could also happen that the natural enemies of some plants are not adapted to all the climatic regions in which the plant is a problem in South Africa, or that the habitat already contains predators or parasitoids that attack the biocontrol agents. In such cases, biological control will have to be replaced or supplemented by chemical or other control measures.

Biocontrol is:

• environmentally friendly because it causes no pollution and affects only the target (invasive) plant
• self-perpetuating or self-sustaining and therefore permanent
• cost-effective
• does not disturb the soil or create large empty areas where other invaders could establish, because it does not kill all the target plants at once. Instead, it allows the natural vegetation of the area to recover gradually in the shelter of the dying weeds.

In some instances, biocontrol agents may effectively control a weed on their own. In other cases, the biocontrol agents should be incorporated into a more comprehensive weed control programme that might include other methods of control such as chemical and mechanical control as well as utilisation of products of the weed. To make optimal use of the available biocontrol agents, the following points should be considered:

The possible use of biocontrol agents should already be kept in mind during the planning phase of any weed control programme. The person in charge of planning must find out which agents are available, what they do and how to use them. One then has to consider how best to integrate the use of the biocontrol agents with the other control methods.

The mechanical or chemical clearing of large weed infestations may eliminate any biocontrol agents present on the weed in that area. It is therefore essential to establish small reserves of healthy, mature plants on which the agents can survive and reproduce and from which they can spread onto plants that may have escaped the clearing process. Some agents disperse rapidly on their own and can readily colonise extensive areas, while others—such as cochineal insects and mealybugs—have to be collected manually from the reserves and released in the target areas. Persons involved in cactus biocontrol should always remove some insect-infested cactus plant material and distribute it to healthy cactus before the cochineal or mealybugs have destroyed their host plants in a specific area. This will ensure that the biocontrol agents do not become extinct locally, but maintain their presence in the area to colonise regrowth.

Some weed species are at present under effective biological control. Further time and money should not be wasted on other clearing methods. Examples are:

• Silky hakea (Hakea sericea) in areas where gummosis disease and the other agents are very active
• Sesbania (Sesbania punicea) after the introduction of all three insect agents
• Port Jackson acacia (Acacia saligna) when the gall rust fungus is present
• Harrisia cactus (Harrisia martinii) after the establishment of the mealybug
• Australian pest pear (Opuntia stricta) after the establishment of cochineal.

The chemicals and labour costs saved in this way can rather be used for the control of invasive plants where there are no effective biocontrol agents, or in areas where biological control is less effective.