The systemic Insecticides I have looked at are silent about the frequency of
application.
Any suggestions? BRBR

As a rule, do not use insecticides unless you actually see a problem, or know
from experience that one is likely to occur at a specific time. With most
insects, like aphids, spray once and then not again until they recur. With
mites, scale, or mealy bugs, spray at weekly intervals three times. The
specific insecticide you have probably has an 800 number to call. You can ask
them for more information. A great deal depends on your climate. Here in the
North, where few pests winter over outdoors, I don't have to do a lot of
spraying.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)