a Big Appetite for Waterlettuce
July 2, 2001
The state of Florida spends about
$650,000 annually to control waterlettuce, Pistia stratiotes, an aquatic
weed that has spread throughout waterways in the southeastern United States and
abroad. Now, three different species of the Argentinorhynchus weevil
have recently been found to feed on this mobile weed pest that is thought to
have originated in South America.
Agricultural Research Service
scientists at the South
American Biological Control Laboratory in Buenos Aires, Argentina, are
conducting tests to determine just how effective A. bruchi, A. breyeri
and A. squamosus might be as waterlettuce biocontrols.
According to Hugo A. Cordo, who leads the ARS effort in Argentina,
preliminary indications are that the weevils--especially A. breyeri--can
kill the weed if enough larvae develop on one plant. But this is a trickier
prospect than expected, because very specific environmental conditions are
necessary for the weevils to pupate.
Until now, details about pupal development--the most critical stage in the
weevils life cycle--have been a mystery. The researchers found that
full-grown larvae actually abandon the plant and bury themselves in the muddy
bottom of the water body. They also found that the drying out of the water body
seems to contribute positively to the underground development of the pupae.
This adaptation may increase the weevils effectiveness in
waterlettuce-infested areas that are exposed to alternating wet and dry
spells--conditions that also lead to weed seed germination.
Both larvae and adults inflict significant damage on waterlettuce, with each
species preferring a different part of the plant during their nocturnal
feedings. For example, A. breyeri larvae feed on tissues within the
central part of the plant rosette, while A. bruchi and A.
squamosus feed, respectively, on the upper and lower parts of the rhizome,
or root. So far, this three-tiered attack has been successful in controlling
the weeds spread in lab tests in Argentina. Further success could soon
bring this formidable biocontrol threesome to the United States.
ARS is the U.S. Department of
Agricultures chief scientific research agency.