This is the thread where people ask Beth Parker

Sam, I don't know anything about outdoor growing in your zone. Are they house plants? I had terrible trouble with scale on my indoor grapefruit, despite all my soapy-water ministrations—hand-sponging each affected leaf. When I put it outside in the spring the problem went away, natural predators, I guess. But then the scale would come back when the plant was moved back inside, and by the end of winter things were dire, with sticky scale honeydew dripping all over. I finally euthanized the plant by putting it on the compost heap mid-winter, which was hard—I had grown it from a seed! But it was infecting other plants. The scale spread to my bay laurel and ficus, but not to such an insane degree.
As to agave—the only problem I can think of would be excess moisture?