Year: 1997

COOPERATORS: Mark Dumesnil, Coastal Region Preserves Manager

Dr. Jim Bergan, Coastal Texas Land Stewart

The Nature Conservancy of Texas, Clive Runnells Family Mad Island Marsh Preserve

COORDINATING AGENTS: John A. Jackman, Professor and Extension Entomology

Specialist, College Station

James Engbrock, County Extension Agent, Matagorda County 

SUMMARY: Alligator weed flea beetles were collected and released at Clive Runnells Family Mad Island Marsh Preserve to control alligator weed in November 1996 and May 1997. Released beetles became established at the preserve and completely defoliated the aquatic form of this weed in June 1997. The beetles continued to be well established in November 1997 and are contributing to the control of alligator weed on the preserve and surrounding areas. As they continue to migrate into new areas, biocontrol of this weed will provide no cost control to adjacent landowners.

PROBLEM: Alligator weed inhibits navigation and water flow in the canals at the Nature Conservancy of Texas’ Clive Runnells Family Mad Island Marsh Preserve. Chemical control of alligator weed is expensive therefore alternative control programs should be considered. Alligator weed is controlled in many areas by the alligator weed flea beetle, Agascicles hygrophila. However, this beetle has died out at Mad Island in part due to cold winter temperatures of the recent past. Re-establishment of the beetle would contribute to low-cost non-chemical control of this pest weed.

OBJECTIVE: Re-establish a viable long-term population of alligator weed flea beetle at Mad Island Marsh Preserve resulting in low-cost control of this weed.

MATERIALS AND METHODS: Contacts were made with personnel at Texas Parks and Wildlife Department (TPWD), TAMU - Extension Service, and Waterways Experiment Station, Vicksburg, Mississippi, to locate a source of alligator weed flea beetles.

On November 19, 1996, a collection trip was conducted at the Wallisville Reservoir near I-10 and the Trinity River in Texas. Approximately 150 adult alligator weed flea beetles were collected from the plants using an aerial net. Navigation to the collecting sites was facilitated by the use of an air boat provided by TPWD. Collected adult beetles were held in cardboard cartons in slightly cooled conditions and returned to Mad Island Preserve the following day. Beetles were then released from the containers onto alligator weed within the main freshwater marsh system. On May 12, 1997, an additional collection and release of 130 beetles was made using the same protocol and release site.

Site evaluations of weed infestations were periodically made using photographs, visual estimates of defoliation and measurements of stem length above the water line. Estimation of defoliation may have been confounded somewhat by damage from a caterpillar which also feeds on alligator weed. Adult beetle counts may have been underestimated because the adult beetles move off of handled stems. Defoliation estimates indicate the current damage but do not show the impact of stems that have been completely removed or damage that allowed competition from advancing native plants. 

RESULTS: While completing the release on May 13, 1997, adult beetles were found to be already present at Mad Island Preserve and probably already established from the previous release in November.

On June 27, alligator weed flea beetles had defoliated at least 95% of the aquatic population of alligator weed. Evidence of this defoliation was apparent at least one mile in each direction from the release site encompassing the whole waterways on the northern portion of the preserve. Although defoliation was evident on the terrestrial form of the weed, only about 10% or less of this form appeared to be impacted. At the same time, only adult beetles were evident. Apparently most of the recently emerged adults had migrated to new areas.

In August, the alligator weed had re-invaded previously impacted areas and covered most of the fresh water marsh waterway system. There was little evidence that alligator weed flea beetles were still present at the Preserve.

On November 7, 1997, the waterways were once again open and areas previously impeded by alligator weed were free-flowing. The openness of the waterways was due in part to previous flooding caused by heavy precipitation during the preceding weeks. The weed was commonly found in it’s terrestrial form along the banks and into the water’s edge. Many of the aquatic plants present were native and are preferred in this habitat. Adult beetles, egg masses, larvae, and damage were present on the alligator weed in the refuge and especially common in sheltered areas near the original release site.

Table 1. Alligator weed and alligator weed flea beetle observations at four sites at Mad Island, on November 7, 1997. (All samples had 10 stems measured).

ECONOMIC ANALYSIS: It was estimated that the area impacted is about 3.5 miles of waterways averaging about 30 feet wide or a total area involved of about 12.7 acres. Estimates of control costs are $ 130 per acre with a typical application cost of $1000 per day. Thus, the control cost would be about $2651. The actual impact of this effort will be much greater because the biological control program will assist with avoiding treatment in future years and in areas surrounding the preserve. The biological control approach in this habitat is particularly significant because the wildlife area is an ecologically sensitive area. The sensitive nature of the habitat is noteworthy because the refuge was the site of one of the most diverse Audobon Christmas Bird Counts in the nation in 1997.

CONCLUSION: The reduction of alligator weed has already allowed the re-establishment of native plants within the existing waterways at the preserve. Continuous long-term suppression of alligator weed is expected by the flea beetles provided they continue to survive despite meteorological perturbations. Periodic fluctuations of the weed and insect can be expected. It is commonly observed that alligator weed flea beetles slow down their reproduction during the hot summer months and may potentially be eliminated from an area due to cold temperatures.

ACKNOWLEDGMENTS: Texas Parks and Wildlife Department provided an air-boat for the collecting trips and Bill Johnson piloted the boat. We thank Ed Riley and Allen Knutson for assistance in collecting flea beetles and advice during this project.

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