Marigolds attract bees, as do nearly all flowering plants. Organically grown marigolds, or ones you started yourself from seed are best. Marigolds purchased in many commercial retail outlets are heavily dosed with an assorted array of insecticides and miscellaneous chemicals which at times can cause as much harm as good. In addition to attracting bees and other pollinators they are also known to attract the hover fly which is ugly little bugger that preys on aphids.
Marigolds have some alleopathic properties which are useful in repelling pests. All marigolds and French Mariggolds in particular produce and cumulatively leave deposits of a substance called alpha-terthienyl in the soil. Alpha-terthienyl will reduce root-knot nematode populations.
If you have an infestation of nematodes in your soil, don't expect marigolds to cure it like a shot of penicillin. The key word is cumulatively in the preceding sentence. The alleopathic compound alpha-terthienyl that Marigolds emit will help a tad in its first season, but needs to build up or accumulate for several subsequent seasons before being considered highly effective . You will need to continuously plant marigolds in the same areas each season.
Marigolds are also helpful in warding off fungi and bacteria that can be harmful to plants.
In addition to nematodes and assorted subterranean bacteria and fungi Marigolds are also useful above ground. Their strong aroma serves as a deterrent to several species of Beetles, slugs, leaf hopper and some worms, particularly the tomato horn worm.
Not all types of marigold will control all types of nematodes. There are a few species of nematode that actually benefit from the presence of marigolds.
Root-knot nematodes which are the most common problematic ones from a vegetable gardeners point of view are effectively controlled over time as are lesion nematodes. Some species such as spiral, sting, and awl nematodes are not hampered by Marigolds at all. In fact there is some indication that these later species may actually be nurtured by Marigold.
Calendula also known as Pot Marigold is a close relative of Marigolds and shares many of the same properties. Calendula however has beneficial herbal, culinary and medicinal attributes