On Mosquito dunks: though they are Bti, mosquito dunks are only about 10% Bti toxins and are pretty low-mass. As far as I can tell, if root aphids are susceptible to Bti, it is only VERY slightly (compared with gnats) and require HIGH doses of Bti to achieve control. Even powdered Gnatrol (which I believe to be far more potent and cost effective than mosquito dunks) is not very efficient for this purpose: berger who suggested it as an eradication method said he used 4g/gal... this makes using powdered gnatrol about $1/gal of mix (buying 224g on Ebay for $50 w/ shipping)... if you have a big system and you plan on using it at the regular consistency berger suggested for eradication, this simply is not cost-effective.
Vectobac (liquid) for $150/2.5 gal, however, used at 20 mL/gal (equivalent to berger's suggested 4g/gal of powdered Gnatrol) is only $.31/gal - less than 1/3 the price if you throw down the $150 - unless you only use about 5 gal a week and only plan to use Bti for one round, liquid Vectobac or liquid Gnatrol are the ONLY cost effective way to throw Bti at this problem. (Search first post for link to purchase Vectobac)
Vectobac (liquid) for $150/2.5 gal, however, used at 20 mL/gal (equivalent to berger's suggested 4g/gal of powdered Gnatrol) is only $.31/gal - less than 1/3 the price if you throw down the $150 - unless you only use about 5 gal a week and only plan to use Bti for one round, liquid Vectobac or liquid Gnatrol are the ONLY cost effective way to throw Bti at this problem. (Search first post for link to purchase Vectobac)