I think you would be better off using a systemic insecticide like imidaclorpid. Should be available at any garden center -- look for a "rose and flower" type product, may be a combination fertilizer/insecticide compound.

Thanks for answering my question. I actually just heard about this insecticide that you mentioned...I'm a bit wary about using imidacloprid since recent findings have linked it to colony collapse disorder in bees.