That’s a great question! The best thing to do is to include lots of plants that provide ladybugs with nectar and pollen in your garden. This will support your indigenous ladybugs and encourage them to stick around. Here’s a list of great plants to start with: http://savvygardening.com/plants-beneficial-insects/
Also, if you still want to release a beneficial insect if an aphid, mite, or mealybug outbreak occurs, you’re much better off releasing lacewing larvae that were reared in an insectary. The larvae don’t have wings and are very voracious predators (they don’t call them aphid lions for nothing). Insectary reared ladybugs are hard to find, but insectary reared lacewing larvae are easy to get. My favorite mail order insectary is Roncon-Vitova. But there are many others.

Patrick, Chilliwack, BC