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.