As you probably know, marigolds are usually pest free. However the ever present spider mites do infect them from time to time. A simple but effective remedy for any insects attacking these plants is to mix about a teaspoon of common dish soap into a spray bottle with about a quart of water. After mixing thoroughly (stir it gently rather than shaking) spray the plants, soaking them from top to bottom. The soap won't hurt the plant but will kill any insects present. Best of luck!