You might try soapy water. It destroys the surface tension on the water which the wrigglers hang on to to breathe. Could be a cheap answer to the problem. If you want to see what I mean, put a drop of petrol or something similar in a glass of water so that it covers the top. Then get a bit of soap and just touch it to the water and watch what happens. It should drive all the oil to the edge of the glass. It's got to be worth a try.
Bigal