Although this sounds good in theory, Goodspeed says he has seen ravaged vegetable plants surrounded by marigolds and other so called, repellant plants. In short, most of the research done at universities on companion planting indicates it is ineffective, or the results are inconclusive at best, he adds. This does not mean it will not work in your garden. But, it should make you stop and consider whether it is worth the time and effort. One problem with companion planting is selecting the plants, since no one plant repels all the insects, Goodspeed says.
Marigolds are a good example.
They are often used to repel whiteflies and some aphids from vegetable and other plants. Although it is true that whiteflies do not like marigolds, mites and slugs will search high and low for a good marigold patch to enjoy. So, while repelling whiteflies, you may invite all the local slugs and mites into the area. Another concern with companion planting is the room it requires, he adds. Weeds are enough.