Environmental Engineering Reference
In-Depth Information
10.4.3 Herbicidal control
Herbicidal control of aquatic plants is usually most successful against small infest-
ations accessible by land, air, or boat, but is relatively expensive, although it has
the advantage of being quick and temporarily effective. Like manual/mechanical
control, though, new infestations develop from untreated plants, and reproductive
structures that remain in the water column or hydrosoil, and so repeated applica-
tions are often necessary.
Hydrilla is a major economic and environmental weed in the USA, with control
measures amounting to millions of dollars annually (Milon et al. 1986; Center
et al . 1997). h e most widespread method of control is chemical, using the herbi-
cide fl uridone, which selectively and economically controls hydrilla, particularly
in large Florida lakes (Fox et al . 1996; Langeland 1996). At low concentrations,
fl uridone has off ered selective treatment for large areas of hydrilla at a relatively
low cost, but recent research has revealed that several populations of hydrilla,
particularly in large Central Florida lakes, have become resistant to these low
concentrations of fl uridone (Michel et al . 2004; Dayan and Netherland 2005).
Hydrilla can still be controlled at higher, sustained doses, but these high doses
impact non-target native aquatic macrophytes, and the cost of control becomes
much greater at higher concentrations. Despite its apparent success, herbicidal
control provides only short-term relief and, subsequently, must be regularly and
frequently reapplied. Moreover, the recent occurrence of herbicide resistance in the
fi eld of invasive aquatic plant management is concerning because management of
these species often relies heavily on a single chemical tool due to e cacy, cost, and
environmental considerations (Dayan and Netherland 2005).
Control of water hyacinth in many parts of the world still relies on herbicides.
Water hyacinth is very susceptible to herbicides such as 2,4-D, diquat and para-
quat, and glyphosate (Gopal 1987). Successful control can be obtained using
these chemicals in small, single-purpose water systems such as irrigation canals
and dams (Wright and Purcell 1995). In South Africa, the terbutryn herbicide
Clarosan 500FW was used to control a severe water hyacinth infestation on the
Hartebeespoort Dam in the late 1970s (Ashton et al . 1979). As with hydrilla,
herbicidal control against water hyacinth is usually temporary, requiring regular
reapplication. Furthermore, the use of chemical sprays contaminates sites used for
drinking water, for washing, and for fi shing, thereby threatening human health
(Julien et al . 1999).
10.4.4 Biological control
The aim of biological control is to reduce weed populations to manageable levels
through a balance between populations of the host plant and its natural enemies
which are host specifi c, depending entirely upon their host plant for survival (DeBach,
1974). Biocontrol is the preferred method of control for large infestations of aquatic
plants because it is both environmentally sustainable and economically effective. In
many instances, complete control of major aquatic weeds can be achieved through
 
 
 
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