Linda Benedict, Sanders, Dearl E., Johnson, Seth J.
Seth Johnson, Dearl Sanders, Lee Eisenberg and Keith Whitehead
The 1958 science fiction cult classic movie “The Blob” featured an alien amorphous creature that consumed whatever it touched and was nearly impossible to stop. In 1998, science fiction became science fact when the invasive plant giant salvinia was first identified in Toledo Bend Reservoir in Louisiana. An initial infestation estimated to be less than 400 acres in 1999 has expanded to more than 70,000 acres in 20 Louisiana lakes, seven bayous or rivers, the Atchafalaya Basin, the Red River and the coastal fresh water marsh from Lafitte to Morgan City.
Giant salvinia is a floating fern native to southern Brazil. The plant consists of a series of horizontal stems with a pair of floating leaves at each node. The plant can be distinguished from other float- ing ferns by the presence of leaf hairs on the upper leaf surface that first divide and then rejoin at the tip in an eggbeater shape. As the plant matures, the older leaves begin to curl at the edges and can form dense mats several layers thick. With wind and water movement, these mats often become several feet deep – deep enough for people to walk on.
The plant has an extremely rapid growth and reproductive rate. Although it reproduces by fragmentation or budding, it is incapable of producing viable spores. An observer has reported, “A single plant has been described to cover 40 square miles in three months.”
In a controlled situation of optimum growth, a population covering 70 square yards was able to cover about 1,390 square yards in 14 days and 26,670 square yards in 42 days. In this trial the plant was able to achieve exponential growth with an estimated daily growth rate of 80 percent near day 35 (Figure 1). At this stage, the plant was able to double its coverage area every 36 hours. This underscores the ability of giant salvinia to rank in the top tier of invasive plants – aquatic or terrestrial.
Giant salvinia does its environmental damage in three ways. First, it displaces all other floating vegetation, native and nonnative. Second, once dense mats are formed, virtually all sunlight is blocked, killing all the submerged vegetation. Finally, as the submerged plants and older salvinia die and decay, dissolved oxygen levels in the water are depleted, forcing fish and other aquatic animals to flee or die. Observers have reported that entire fishing villages in New Guinea were abandoned when lakes were overwhelmed with the weed in the late 1960s.
The LSU AgCenter began herbicide-screening trials in 1999 and continued them through 2009, evaluating 20 potential herbicides for their effect on giant salvinia. Included in the various trials were all herbicides with existing aquatic labels, herbicides with potential aquatic labels and herbicides with active or pending labels for use in rice. (Herbicides labeled for use in rice can more easily be registered for use in aquatic situations.)
The common names of the herbicides tested that provided at least 90 percent control in small plots include diquat, glyphosate, glufosinate ammonium, fluridone and penoxsulam.
The herbicides fluridone and penoxulam are total-water-volume treatments in which the plants absorb the herbicide from the water. Control is a function of herbicide concentration and contact time. Loss of concentration or contact time results in reduced control. The remaining herbicides are foliar treatments that require direct contact with plant leaves. Because of the tremendous regrowth potential of the weed, any that escape must be treated to maintain control. Multiple applications are essential, but they increase the overall cost of control. So far, 10 years of herbicide trials indicate that chemical control of giant salvinia will be difficult and costly.
Introducing an aquatic weevil
Cyrtobagous salviniae – an aquatic weevil native to Brazil, Bolivia and Paraguay – has been used successfully for the biological control of giant salvinia in a number of countries, including Australia, Fiji, Ghana, India, Kenya, Malaysia, Namibia, Papua New Guinea, Republic of South Africa, Sri Lanka, Zambia, Zimbabwe and recently the United States. The adults, which average 2.5 millimeters in length, are sub-aquatic in nature and can be spotted on or under leaves, within the leaf buds or among the roots of giant salvinia plants.
The weevil is a specialist whose larvae burrow in the rhizomes of the plant. Adults consume leaves and buds. Feeding on buds inhibits growth of the plant. Feeding by the larvae causes the leaves to first darken to brown and then drop off. The combined feeding of larvae and adults kills the plant. Populations may reach a density of 250 adults and 750 larvae in one square yard, which Australian researchers consider necessary to achieve control. In 2001, researchers from the USDA Agricultural Research Service released weevils from the Brazilian strain in a major salvinia infestation at Toledo Bend Reservoir in Sabine Parish, La., as well as on the Texas side. The weevils established at a number of locations, and salvinia biomass was dramatically reduced at several of the infestation sites.
The Brazilian salvinia weevils have been described as light ivory-colored when they emerge, turning brown within 1-2 days and then darkening to black after 5-6 days. The adults deposit eggs 6-14 days after emergence, but not when temperatures are below a constant 70 degrees. After mating, females typically deposit one egg every 2-5 days in shallow holes excavated in plant tissue. Females can continue to lay eggs for 60 days. In the laboratory, eggs hatched in 10 days at a constant 78 degrees, but did not hatch below a constant temperature of 68 degrees or above a constant temperature of 97 degrees. The larvae rapidly begin to burrow within the rhizomes, where they undergo three developmental stages over 23 days. Larvae cannot survive at constant temperatures below 61 degrees. When they pupate, their cocoons can be found attached to roots or rhizomes under water.
The AgCenter established a salvinia weevil nursery on a 6-acre pond on the privately held Golden Ranch Plantation at Gheens in Lafourche Parish in May 2007. The pond was seeded with 1,000 pounds of giant salvinia collected from infested waters on the property in May 2007. The surface of the pond was cov ered with salvinia by July 15. The pond was fertilized with ammonium sulfate at regular intervals to maintain optimum nutrient levels for salvinia and weevil growth. In August 2007, the pond was seeded with 10,000-12,000 weevils collected from a heavily infested pond on the Texas side of Toledo Bend near Center, Texas. The population, however, was predominately male (about 9 to 1), and after the overwintering population died in spring 2008, the population was too low to be detected.
In May 2008, the pond was reseeded in several locations with a total of 880 pounds of giant salvinia with 11 weevils per pound collected from Toledo Bend near Egret, La. The weevil population increased over the summer, and the Louisiana Department of Wildlife & Fisheries began releasing them in other areas in October 2008, when the weevil population was two to three per pound of salvinia. The weevil population was monitored once or twice a week from November 2008 to August 2009 by collecting and processing six random samples and processing them in Berlese Funnels (Figure 3). A subsample of the weevils was dissected, and their reproductive status was determined.
The weevil population was stable at 1-1.3 per pound from November 2008 to February 2009. In February and March the population increased as weevils started to emerge after passing the winter as immatures. Weevils became reproductively active and started laying eggs in March and in April, but the population dipped to less than one per pound because of a die-off of weevils that passed the winter as adults.
The second-generation weevil population from eggs laid in March by overwintering and emergent weevils began an exponential increase in early May, continuing to a peak of 24 per pound of salvinia on June 16, 2009. In mid-May, when the weevil population reached about three per pound, approximately 20 percent of the salvinia had died and sunk.
By early July, approximately 67 percent of the pond was clear, and the weevil density was approximately 18 per pound. By early August more than 90 percent of the pond was clear, and the only salvinia was located around the edges, where the weevils were concentrated at more than 27 per pound (Figure 4).
The Department of Wildlife & Fisheries began to collect and transport the weevil-infested salvinia on June 15 when the weevil density peaked. More than 30 tons of salvinia with approximately 1.8 million weevils were transported to Lake Bistineau near Bossier City, where some weevil effects on the giant salvinia problem should be apparent sometime in 2010. An additional half million weevils were harvested and transported to four other north Louisiana lakes. Additional weevil nurseries started in September 2009 and planned to start in spring 2010 will produce weevils available to attack the giant salvinia problem in Louisiana in 2011.
Sterling Blanche from the LSU AgCenter Rice Research Station for statistical analysis and theLouisiana Department of Wildlife & Fisheries for funding.
Seth Johnson, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.; Dearl Sanders, Professor and Coordinator, Bob R. Jones-Idlewild Research Station; Lee Eisenberg, Research Associate, Department of Entomology; and Keith Whitehead, Research Associate.
(This article was published in the winter 2010 issue of Louisiana Agriculture.)