The key to all of the critters is to remember they have eggs that continue
to hatch for many weeks after you spray. If you kill the adults this week
and don't follow up for the next couple of weeks then you have hatchlings
come back in two weeks and slowly your problem returns.

Systemics help that quite a bit because it makes the plant toxic to the
critter.

I believe some of these eggs can last all winter only to hatch in the spring
when the environment gets favorable for the hatchling. For years I got rid
of the scales and mealies for a month or two only to find them back a couple
of weeks later.

I know you shouldn't spray when you don't see anything but if you had them
in the past month then you should take into account that there are eggs
waiting to hatch. Mother nature designed them this way so they would
survive.

Good growing,
Gene