Beetles Have a Big Appetite for Waterlettuce

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 weevil’s 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 weevil’s 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 weed’s 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 Agriculture’s chief scientific research agency.