Friday, February 18, 2011
Weevil Counting Using Berlese Funnels
These strange looking contraptions with lights on top are known as "Berlese Funnels". For our purposes, these funnels are used to estimate the density of weevils (numbers present per amount of giant salvina) located either on the lake or in our weevil rearing tanks.
To use these funnels, a pre-weighed amount of giant salvinia (fresh weight) is placed into the funnel(s) and the 40watt light bulb is turned on for 24 to 48 hours. The weevils, trying to get away from the drying light, fall through a screen and into a mason jar at the bottom of the funnel. By counting these weevils in the jar, we are able to extrapolate how many salvinia weevils are present in an area
of giant salvinia. Although a trip through the Berlese funnel is fatal for the weevils (there is ethyl alcohol at the bottom of the jars for preservation), their sacrifice is not in vain. The weevil numbers obtained from the funnels can tell us when we have sufficient weevil density to put out on the lake.
The funnels were run during the week of February 14, 2011 using samples from the tanks. Despite the very cold weather we've experienced in January and early February of 2011, we still have lots of weevils alive in three out of the four tanks. The one tank that is showing low weevil numbers never got a good population established. We should be able to restock it with weevils from another tank and get a good crop by the time we're ready to release them on the lake. For control of giant salvinia in a system, it's believe that 60 weevils per kilogram of giant salvinia is needed. However, it's believed that weevils should be released for control once they reach 30 per kilogram. One of our tanks is showing over 50 weevils per kilogram of giant salvinia, thus, the sooner we can get these bugs out on the water, the better. The important thing now is to keep these weevils alive and reproducing until it's time to release them on the lake. All in all, the fact that living weevils are present in the tanks shows that the greenhouses have done their job so far.