Goyer and Stark (1984) reported that beetle densities as low as one individual
per plant had an adverse effect on plant height and number of
Plant harvesting or herbicide application can sharply reduce the effec-
tiveness of an established population of
on waterhyacinth.
After
such treatments, insects disperse and the plants regrow in log phase without
control.
It may then take several years for insect densities to reach
tive levels (Center and Durden 1984, 1986).
Haag (1985) has found that
diquat, 2,4-D, and glyphosate were nontoxic to the weevils at low doses and
that adult insects will migrate to nearby untreated plants.
Integrated
methods of aquatic plant management may ultimately prove to be among our most
successful
approaches.
Haag (1986) describes an experiment in which insects
and a herbicide were used together to control a waterhyacinth infestation in a
About 75 percent of the weed mat was sprayed with 2,4-D, in increments
pond.
of 20 percent, and both species of waterhyacinth weevils were "herded" into an
unsprayed area where they overwintered, increased sharply in density, and
exerted complete control of the weeds in the next spring.
Additional experi-
ments with this approach are in progress in larger lakes and reservoirs.
Some native insect species are known to damage aquatic
Work is
now in progress to identify these species and to determine their usefulness in
plant control (Buckingham, Haag, and
1986; Haag,
and Buckingham
1986;
Haag, and Buckingham 1986).
Plant pathogens should be ideal for control of rooted aquatic plants
because they are numerous and diverse, usually are host specific, easily
disseminated and self-maintaining, exert a limiting influence on target plants
without eradication, and normally are not dangerous to man and domestic ani-
mals (Zetter and Freeman 1972; Freeman, Charudattan, and Conway 1975; Freeman
Only one plant pathogen has been significant in plant control, the
1977).
fungus Cercospora
which was isolated from waterhyacinth in
Reservoir, Florida.
Pilot field tests were held in Louisiana in 1977-80, and
a commercial formulation has been developed by Abbott Laboratories (Freeman
et al. 1981; Perfetti 1983).
The effectiveness of the fungus in Louisiana
cannot be separated from the impacts of the beetles, but it is believed that
its effect on waterhyacinth has been less than expected (Sanders 1984).
Another species, Cercospora piaropi, was generally believed to produce only
moderate plant damage.
however, has documented a situation at
Lake
Texas, where this fungus has apparently caused widespread damage
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