You could try a systemic insecticide.

Bayer makes a fert. with insecticide in it [granule form]. This works really well for me. The plant soak up the insecticide just as they do the fert. Dahlias love the stuff -- as do all the plants and shrubs I've used it on. Really does cut down on pests without the worries of spraying. Bought it from HD. Rose & flowering plant specific but have used it on other plants with the same great results.