Last Year's Weevil Making Their Presence Known

In the summer of 2012, the primary weevil release site was in the Back Lake area of Caddo Lake. This site was chosen as it is a relatively isolated area of the lake with little boat traffic, it maintained water throughout the 2011 drought and has a dense Cypress break that provided a relatively stable habitat for the weevils. Flow in this area is also relatively calm and typically results in floating vegetation staying in place. One downfall of this release site is the difficulty in getting to this area when lake level drop. Toward the end of 2012 when the water level dropped, it became impossible to enter this site by boat and the team had to wade in to sample weevil populations.

Fast forward one year to 2013. The Back Lake area was abandoned as a release site due to it inaccessibility during low lake levels. That said, this area has been checked out this year to see if the weevils released last year remained in the area and if they were having any impact on giant salvinia.

The short answers to those questions are Yes and Yes! Photos taken at this site show considerable damage to the giant salvinia at this site and a decent amount of open water present as well. We will be going out to this site in the next few weeks to collect samples so the weevil density can be counted and will also survey the surrounding area to see if any other weevil damage can be found.

Weevil damage at the 2012 weevil release site in Back Lake. No weevils were released at this location in 2013.

This find is really encouraging as it shows that the weevils released in earlier years survived the winter in large enough numbers to have a noticeable impact on giant salvinia. While this is only a small success, it is still a success!

Weevils Making an Impact on Caddo

Now that summer is officially over and we move toward the winter months salvinia weevil numbers in the release site have increased to the point where obvious plant damage from the weevils can be seen. Over the course of the spring and summer months, more than 50,000 weevils have been released into an isolated area of Caddo Lake. This may seem like a lot, but when you have about 7,000 or 8,000 acres of salvinia, 50,000 weevils don't cover much ground.

Controlling, or impacting salvinia with weevils is essentially a numbers game. You must have a high enough weevil density, a a sufficient number of weevils per kilogram of salvinia before visible damage is obvious. A weevil density of about 40 adult weevils per kilogram of salvinia is typically that magic level. And the good news is that the weevils we released this year have finally gotten to that point.

According to Lee Eisenberg, the weevil density in July was about 6 weevils per kilogram. By August the density was up to 15 per kilogram and increased to 34 in September. The October sampling hasn't been completed yet, but photo evidence suggests that weevil numbers have now surpassed 40 per kilogram.

The photo below was taken the first week of October and clearly shows a "sick looking" area of salvinia in the release site. Additionally, weevil damage is obvious moving away from the release site as well.

Weevil damage to giant salvinia at the release site in early October 2013

While this is only a small success, it is still a success that shows the utility of salvinia weevils. It also reiterates the need to have more weevils on Caddo Lake. Additionally, the facts that weevil reproduction is driven by temperature and that the point in the year that population numbers reach the 40 per kilogram threshold is driven by when reproduction begins highlight the need to find weevils that are adapted to colder climates and reproduce at colder temperatures.