I actually buy 1.5k at a time , leaving diluted sugar water and somewhere for them to drink/hide. Once per week. I get them from a local nursery.
Why the 7 spots? Lucky number?
You might try getting a UV light for the back yard. There attracted to UV I believe.
Also, I'm thinking clipped wings might be better, as I've bought them but they just fly off. (I tried it myself, but there small little guys, and wont sit still)
Now a team of researchers at the Graduate School of Bioagricultural Sciences in Nagoya, Japan, has effectively clipped the beetles' wings without resorting to either genetic modification or, erm, actually clipping the cute li'l critters' wings.
Wingless ladybugs will control your pests naturally
www.clarkpest.com
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