Answer: I think you're describing an oyster shell scale. These pests sttach themselves to plant stems and feed by sucking the fluids from the tissues. There are three generations per year and the little ones move only a small distance away from the parent before they attach themselves to a stem and begin feeding. Because of the hardened shell most pesticides won't penetrate, so picking them off, or smothering them with dormant oil are about the only things you can do to control them. There is a parasitic wasp that can control the scale population but they generally only show up if there are many, many plants affected. You did the right thing in destroying the affected plants. This will keep the pests from migrating to other plants in your garden. Those that fell into the planting bed will probably die - they don't crawl too well on soil and there's nothing for them to eat. They need to attach themselves to green stems in order to suck the juices out of the tissues.
I suspect you imported these pests on the plant you brought home.
Hope that's the last scale insect you see!
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