Bacillus thuringiensis (Bt) occurs naturally in the soil and on plants. Different varieties of this bacterium produce a crystal protein that is toxic to specific groups of insects. Bt has been available in North America as a commercial microbial insecticide since the 1960s and is sold under various trade names. These products have an excellent safety record and can be used on crops until close to the day of harvest. Bt can be applied using conventional spray equipment but, because the bacteria must be eaten to be effective, good spray coverage is essential.

so you want the variety called "san diego" registered for use against larvae of Colorado potato beetle larvae. that's critical, buy the other type and it is not going to work! bt kills caterpillars so if butterflies are laying eggs nearby (like on milkweed) be careful.

i buy the liquid concentrate at blue seal or agway but any decent garden store should carry bt (mine is over 10 years old as i got a 16 oz bottle and it is still effective just store it in a cool dark place).... just be sure you get the bt FOR potatoes. bt kills larvae only it is totally useless against the adults. bt breaks down in the sun after a few days and will wash off after rain. apply when leaves are dry in the morning when no rain is predicted for a few days. i had pick the adults and larvae and crush the eggs. it is time consuming but i have 1 40' row so it is doable for me. i spray bt if there is a lot of leaf damage indicating a lot of larvae are out there.

google bt and you can read all about it.