Insecticides and Aging

[Ashby Chadburn]

I grow orchids in a greenhouse in Northern California. I have had some pretty bad infestations of mealy bugs and scale. I have treated the plants with several products, but I have found Orthene and Merit to be especially effective. I mix it in a five gallon container and use it for weeks at a time. My question is whether the insecticide mix remains potent and effective or is it getting weaker and weaker and ineffective? ( I use one product at a time, not mixed together). Your thoughts?