Environmental Engineering Reference
In-Depth Information
long-term monitoring as well as management of disturbances that might reintro-
duce invasive species or shift competitive balance away from natives.
Biological weed control is the use of living organisms to lower the population
level or competitive ability of a plant species so it is no longer an economic prob-
lem (DeLoach 1997; Cruttwell McFadyen 2000; Coombs
et al
. 2004). Biological
control can have longer lasting effects on weed populations than other forms of
control; however, a longer time frame, often years, is required for biological control
agents to become established and exert their effect. The object of biological control
is not to eradicate the weed; ideally, some of the weed population should always be
present to maintain a population of the natural enemy. However, such control can
be permanent once the weed and natural enemy populations are in equilibrium
(Julien and Griffi ths 1998).
Biological control does not necessarily kill plants directly; rather their competi-
tive ability and fecundity are typically reduced. h us, biological control should not
be attempted when the eradication or rapid removal of a weed is the goal. Control
agents must be host specifi c for the weed that they aff ect to prevent damage to
crops or native plants. h is specifi city is an advantage when control is directed at
only one weed species but a disadvantage when several plant species must be sup-
pressed in the same area. Closely related native species in the vicinity of the target
weed may also be sensitive to the control agent and should be tested for host spe-
cifi city (Wittenberg and Cock 2005). Because of the potential for problems when
organisms, even presumed benefi cial ones, are deliberately introduced into new
areas, biological control is tightly regulated in some countries. Where no protocols
are in place, the International Plant Protection Convention's Code of Conduct
for the Introduction of Exotic Biological Control Agents provides such guidance
(IPPC 1996; Wittenberg and Cock 2005).
h ere are several approaches to biological control, including the introduction
of natural enemies from the original range of the target species to the new invaded
range; augmentation of natural enemies during pest outbreaks; or habitat manage-
ment to enhance native populations of predators and parasitoids (Wittenberg and
Cock 2005). Grazing is perhaps the oldest and most common form of biological
weed control. It can be accomplished using a wide array of animals that eat vege-
tation, including large ruminants and ungulates, birds, insects, and fi sh. However,
grazing can be an agent of weed propagule dissemination and can suppress native
plant populations as well as the weed. h e use, timing, and rotation of grazing
animals for invasive plant suppression should, therefore, be done with care to min-
imize their negative impacts on native vegetation (Radosevich
et al
. 2007).
Chemicals, like other methods of weed control, have been used for centuries to
suppress or remove weeds in agroecosystems; their use for control of invasive plants
