Weevils Numbers Increasing on Caddo

As a part of the efforts to use biological control to suppress giant salvinia at Caddo Lake, monthly surveys are conducted to evaluate weevil populations at the salvinia weevil release site. During the last sampling event conducted on September 20, 2012 by Texas A&M AgriLife Extension Service scientists Dr. Allen Knutson, Dr. Abhishek Mukherjee and Mr. Lee Eisenberg discovered considerable giant salvinia browning. While this is not the drastic reduction in salvinia like what was seen at Lake Steinhagen, this is still proof that the weevils are working.

Lee Eisenberg collects salvinia samples on 9/20/2012 to evaluate salvinia weevil density. The browning of the salvinia here is caused by salvinia weevil damage.

Samples are collected by gathering salvinia in large Ziploc bags. These samples are returned to the lab at the Caddo Lake National Wildlife Refuge where they are weighed and subsequently dried in Berlese funnels. The salvinia is held in a metal funnel and a light/heat source is applied above the salvinia which dries the salvinia and causes the weevils to migrate away from the heat. In this case, the weevils move down and into a Mason jar where they are easily counted.

Berlese funnel used to count salvinia weevils. 

In this and all sampling events, 16 samples are processed each weighing in at 500 grams, or 0.5 kilograms. In total, 178 adult weevils were collected during this event yielding a density of 22 weevils per kilogram of salvinia. This is really good news and shows that weevil numbers are rapidly increasing on the lake. In fact, the weevil population has increased over 500% during the month since the August sampling when only 33 weevils were collected. Another positive finding is that almost 19% of the weevils collected were brown in color indicating that they are very young. These young weevils and other weevils hatched between now and the end of the growing season are those most likely to survive all the way through the winter due to their early stage in life.

So, we are hopeful that the winter will provide hospitable conditions to the weevils and that numbers on the lake will reach levels where effective control is seen. For now, we are pleased with the success that we are seeing and hope it continues.

Steinhagen Success

In a recent news release, the successful application of the salvinia weevil to control giant salvinia on Lake Steinhagen was showcased (http://today.agrilife.org/2012/09/20/weevils-successfully-destroy-acres-of-lake-invading-plants/). In a coordinated effort, the US Army Corps of Engineers (USACE) has been working with the Texas Parks and Wildlife Department (TPWD) Aquatic Habitat Enhancement Program staff from Jasper to use the salvinia weevil to combat giant salvinia on the lake. In late 2010, TPWD began releasing weevils on one area of the lake and through March of 2012 had released about 112,000 weevils. During this time, USACE did not apply chemicals to this area in an effort to see what the weevils were capable of. The pictures clearly describe their capabilities under good conditions.

Giant salvinia coverage on upper end of Lake Steinhagen, April 2012

In April 2012, giant salvinia was estimated to cover about 300 in this area of Lake Steinhagen. By August, the coverage area was conservatively estimated at only 150 acres and had led to open water in the middle of that area. What giant salvinia remains is isolated to the edge of the lake and mats of other vegetation. While the weevils haven't removed all the giant salvinia, they do show the ability to manage giant salvinia when it is present. 

Same area of Lake Steinhagen, August 2012

In an effort to get some more research bang for the buck, Dr. Abhishek Mukherjee of the Center for Invasive Species Eradication team worked with TPWD and USACE personnel to study the weevil population dynamics and document their impacts. Some of their findings describe why and how this successful reduction in giant salvinia came about this year.

In February and March 2012, documented weevil densities were in the 20 to 30 weevils per kilogram of giant salvinia range indicating that a sizable population of weevils had survived the winter on Lake Steinhagen. These relatively high numbers of weevils coming out of the winter enabled the population to expand rapidly. By late July, weevil numbers had more than doubled to just over 60 weevils per kilogram of giant salvinia. This number of weevils is widely considered the weevil density needed to suppress giant salvinia growth.

Ultimately, we hope to see similar success with biocontrol at Caddo Lake. The challenge is maintaining adequate weevil populations on the Lake through the winter. With Caddo Lake being more that 100 miles farther north than Lake Steinhagen, this may be difficult. Efforts at Caddo continue to explore ways to enhance the cold tolerance of the salvinia weevils and some live weevils were found late this past winter on Caddo, but their numbers were much lower than those on Lake Steinhagen.

So for now, we will continue releasing weevils on Caddo and hope for another mild winter that will allow their populations to persist and flourish early next spring.