Population dynamics of Cyrtobagous salviniae on common salvinia in South Florida

Authors: Tipping, Philip; Center, Ted

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2005
Publication Date: 6/1/2005
Citation: Tipping, P.W., Center, T.D. 2005. Population dynamics of cyrtobagous salviniae on common salvinia in south florida. Journal of Aquatic Plant Management. 43:47-50

Interpretive Summary: Common salvinia is a growing weed problem in fresh water ecosystems in the southeastern United States. However, this species does not achieve the same pest status in Florida, perhaps because of the presence of a small weevil, Cyrtobagous salviniae. This weevil has not been detected in common salvinia in other states. A related weed, giant salvinia, has been controlled around the world by this same weevil. We surveyed six sites in south Florida from 1999 through 2001 for the presence of this weevil on common salvinia growing in ditches, rivers, canals, and sloughs in order to determine the density and abundance of this insect throughout the year. Since common salvinia is a floating fern, it is completely dependent on the water conditions at each site, especially water depth. During the course of this study, every site experienced at least one period when the water dried up. Despite these dynamic conditions, the weevil was able to colonize common salvinia whenever it appeared, even after salvinia populations were temporarily eliminated by drought conditions. The weevil populations built up to more than 100 adults per square meter of common salvinia in some sites, levels that are probably acting to regulate the salvinia. We submit that the presence of the weevil in Florida is responsible for the reduced aggressiveness of this plant compared to other areas of the U.S.

Technical Abstract: Adult populations of the biological control agent Cyrtobagous salviniae Calder and Sands (Coleoptera: Curculionidae), were surveyed every 2 to 3 weeks over a three year period from common salvinia, Salvinia minima Baker, in Florida. This weevil was found at every site and date when S. minima was present. Sites that were less likely to dry up, flush out, or experience wider fluctuations in water levels, generally supported higher densities of C. salviniae. The mean number of adults per square meter of common salvinia ranged from 40.6 to 11.0, depending on the site. Peak densities of adults were found during the fall and winter for the first two years of the survey, with a summer peak found during the third year. Local extinctions of the plant occurred regularly when water levels dropped to zero. However, these sites were quickly recolonized by both the weed and the weevil when water levels rose. Two other herbivores were commonly collected during the survey: Samea multiplicalis (Guenee) (Lepidoptera: Pyralidae) and Synclita obliteralis (Walker) (Lepidoptera: Pyralidae). Both are generalist species that appear to have a negligible impact on common salvinia populations. Weevil densities exceeding 100 adults per square meter were not uncommon and we submit that these higher densities effectively regulate populations of common salvinia in south Florida.