A pesticide used on a specific plant won’t necessarily stay there. It can easily wash off the intended victim and the runoff can wind up in sensitive watersheds where fish, algae, and, through a nasty ripple effect, whole ecosystems can suffer. Even if the pesticide stays inside the garden gates it can have a detrimental effect on the garden itself by discouraging (and possibly killing) natural enemies of the very pests you are trying to ward off.
Instead, encourage ladybugs—–who are predators of leaf-destroying aphids—–into your garden. Attract them by planting small, sweet-smelling plants like carrots, fennel, and dill and letting them flower. Equally powerful "good bugs," like the lacewing, will show up and pitch in as well.
To kill aphids outright you can also mix up an organic, lower-impact pesticide of your own. Mix two teaspoons of liquid castile soap (Dr. Bronner’s, for example) into a liter of water, then spray the mixture onto the affected leaves and stem of the plant. This soap mixture will kill the aphids, but it can also kill the good bugs, so use it sparingly to maintain a diversity of insects in your garden.
When battling pests in the garden, it’s helpful to take a step back and think about managing rather than controlling them. Your garden does not exist in a vacuum. Choose plants that are generally problem-free and that invite beneficial insects into the garden. Finally, practice patience and remember that your garden is a vibrant member of a much larger ecosystem.