No, Ladybugs won't eat your MJ plants.
Here's some pics and a little information about them:
Lady Bugs (Hippodainia convergens) The Lady Bug is the most widely recognized of all Beneficial Insects. The Lady Bug, actually a beetle, is known mostly for aphid control but also feeds on chinch bugs, asparagus beetle larvae, thrips, Colorado potato beetle, whitefly & mites. The Lady Bug is effective in gardens and greenhouses and will attack pests in both it's larvae and adult stages. Ladybugs will consume over 1,000 aphids in it's lifetime. Available year round Lady Bugs are shipped as adults and can be stored for several weeks in the refrigerator. 1500 is enough for an average garden, 1 gallon will cover an acre depending on the level of infestation.
These rotund little beetles are harmless to plants but voracious eaters of soft-bodied insects such as aphids, scale insects and mealybugs. Often gardeners release them as a natural biological control for insect pests. Wing covers of most ladybug species are covered with characteristic color patterns. Ladybugs live longer than one year and in the fall they gather in protected places to hibernate. Sometimes they make their way into homes, annoying homeowners, but unlike cockroaches that eat and make messes, ladybugs sleep and live off their own stored body fat. Almost 500 species live in a variety of habitats in North America.
Check out the picture of some aphids munching on some Dortmund climbers. (Look at those two little greenish critters on the red leaf on the left side of the picture.) But look at that ladybug working her way towards them. Watching her, I see she moves very quickly in apparent random directions. But when she finds an aphid, it stops her in her tracks as she enjoys the feast. They can eat 50 to 60 aphids per day.
