Biological control is an attempt to
introduce the plants natural enemies to its new habitat, with the assumption that
these natural enemies will remove the plants 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.
BIOLOGICAL CONTROL SAFE?
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
IS BIOLOGICAL CONTROL?
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.
OF BIOLOGICAL CONTROL
- environmentally friendly because it causes no pollution
and affects only the target (invasive) plant
- self-perpetuating or self-sustaining and therefore
- 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.
BIOLOGICAL CONTROL INTO WEED MANAGEMENT
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.
RESERVES OR REFUGIA
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 otherssuch as cochineal insects and
mealybugshave 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.