Environmental Engineering Reference
In-Depth Information
Aquatic weeds have made good candidates for biological control in compari-
son to the success achieved against terrestrial weeds (McFadyen 1998). Hoff mann
(1995) divided biological control success into three broad categories: complete con-
trol where no other control measures are needed to reduce the weed population to
acceptable levels; substantial control where other methods such as herbicide appli-
cation or mechanical control are still required but less frequently; and negligible
control where other forms of intervention are required to reduce the weed popula-
tions to acceptable levels. In the biological control of aquatic weeds worldwide,
there are a number of species, including water lettuce (Fig. 10.3), salvinia, and red
water fern that have been brought under complete control through the introduc-
tion of host-specifi c natural enemies to a point where they need no longer pose
a threat to aquatic ecosystems (Hill 2003). Furthermore, these weeds have been
brought under control with the introduction of a single agent on each of them and
the time required to achieve control has been relatively short (less than 2 years).
Water hyacinth has been brought under complete control through the intro-
duction of a suite of agents in some areas of the world, most notably Lake Victoria
where the introduction of the two weevils, Neochetina eichhorniae and N . bruchi
reduced the weed infestation from 20,000ha to 2000ha in a period of 5 years
(Moorhouse et al . 2001). However, in other regions the biological control against
water hyacinth has been less successful and this has been ascribed to highly
eutrophic waters that allow luxuriant growth of the plant, cooler climates that slow
the build-up of the biological control agent populations, and inappropriate appli-
cation of other control methods such as herbicide application that may directly
aff ect the agents or cause a catastrophic reduction in the weed population thereby
decimating the agent population (Julien 2001). Under these conditions water hya-
cinth infestations regenerate from seedling recruitment or from unsprayed plants
which fl ourish in the absence of the agents.
Biological control of alligator weed is considered the fi rst aquatic weed biocon-
trol success story (Buckingham 1996). h e introduction of the fl ea beetle ( Agasicles
hygrophila ) has been highly successful in controlling this weed where it grows in
an aquatic environment, but less successful where it grows on the banks of rivers,
lakes, and impoundments (Buckingham 2002). Agents have also been released
against parrot's feather ( Myriophyllum spicatum ) in South Africa (Cilliers 1999)
and hydrilla in the USA (Balciunas et al . 2002) with varying levels of success.
Additional agents are being considered for the weeds where control is not consid-
ered complete.
Chinese grass carp have also been used in a number of countries around the
world for the control of submerged aquatic plants. In many cases these are indi-
genous plants that have become problematic due to eutrophication (Schoonbee
1991), but hydrilla has been controlled in some impoundments in the USA by
grass carp (Langeland 1996), and they were considered for control of lagarosi-
phon in New Zealand, but studies showed that their feeding would not be spe-
cifi c enough, threatening indigenous New Zealand fl ora (Howard-Williams and
Davies 1988).
 
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