First determine if there is a valid reason to do either of those things.

I would at least water the flowers - that's necessary.

Assuming that there really is a problem that needs chemical intervention, I would say insecticide first - get rid of the pests before feeding up the plant. But there is an equal argument that says a healthy well-fed plant can fight off infestation better than a weak plant.
But as Loosestrife says, determine whether there is a need for either - photos of the affected plants (and critters) would help.

I feed my plants with watering can at ground level also put slow release food on the garden -- And spray for pest as and when needed but a strong blast from the watering hose onto the pests normally does the trick -- then you don't have to worry what you do 1st