Marigolds, along with chrysanthemums and some daisies, have a distinctive odor that seems to be effective in warding off some bugs. This is the smell of pyrethrum, a compound used in commercial insect repellents to ward off mosquitoes.
Marigolds are effective at warding off mosquitoes, though they must be arranged in a barrier at doors and windows and thus might be troublesome to humans as well. Because they are also good for warding off tomato bugs, a few marigolds growing near tomato plants will help keep them healthy. Different types of marigolds ward off other pests. For instance, Mexican marigolds may keep away rabbits and several other insects, while French marigolds repel nematodes. For proper effectiveness, the marigold must be fragrant, and the more aromatic, the better.