Environmental Engineering Reference
In-Depth Information
which prompted the start of an awareness campaign highlighting the danger of
the weed, in an attempt to curb the spread (Gunasekera and Bonila 2001). An
alternative to alligator weed had to be found to ensure public participation in the
eradication programme, and the native lesser joy weed (
A
.
denticulata
) was consid-
ered suitable and distributed to Sri Lankan families (Gunasekera 1999).
There are a number of options available for the control of aquatic invasives, offering
varied success, and in most cases, an integrated management approach, combining
more than one method, is essential for acceptable control. However, deciding on
the acceptable level of control must be the fi rst step in an integrated management
approach and will vary greatly between the species of plant being controlled and
the function of the water body.
Because of the sheer biomass of aquatic plant infestations, utilization of these
infestations is often encouraged, particularly in poorer rural areas where local
communities are perceived to benefi t from the use of such infestations. In most
instances, however, utilization is not suffi cient to control invasive aquatic plants,
and may even promote their spread, as was the case with alligator weed in Australia.
In addition, most aquatic plants have a very high water content; for example, water
hyacinth is nearly 95% water (Harley 1990), and to gain 1t of dry material, 9t of
fresh material have to be collected (Julien
et al
.
1996), making the cost of process-
ing water hyacinth commercially unviable (Julien
et al
.
1999).
Manual and mechanical control methods, involving removal by hand or special-
ized machines, are generally effective only for small infestations, as they are often
labour intensive, and ineffectual against large, dense infestations (Fig. 10.1). These
methods are only temporarily effective and require repeated follow-up treatments
because often removal of the infestation is not complete, so any plant fragments or
buds that remain are capable of regenerating into new infestations. Furthermore,
seeds remaining in the hydrosoil germinate as a result of light penetration follow-
ing removal.
A plant similar in structure to hydrilla, and similar in the eff ects that it has
on freshwater systems in the USA, is spiked water milfoil, a submerged aquatic,
native to Eurasia. Control of this plant is extremely di cult once it has estab-
lished, and although herbicidal control is the preferred method of control using
2,4-dichlorophenoxyacetic acid (2,4-D) or fl uridone, infestations in oligotrophic
small systems have been eff ectively controlled physically by means of hand harvest-
ing, suction harvesting, and benthic barriers. However, typical of these methods,
