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.