Alternative title: All that glitters is not Marigold?
Because I am an itinerant student, I can’t have a home garden of my own. Happily enough, however, there is a huge community garden in my town. It consists of 96 garden plots (each 12 ft by 15 ft) and it is organic (no pesticides or chemical fertilizers).
I’ve had a plot in the garden since it was started in 2009, so I have had the opportunity to watch it evolve from a slip shod weed patch behind the baseball stadium, to a location of great beauty, complete with water tanks, a tool shed, and a picnic area.
I’ve also had the opportunity to watch the pests change from year to year. In the first year of the garden, we were all blissfully pest free. But by the second year, the potato beetles had found us, and the cucumber beetles in the next year. Soon it is a constant battle against the herbivores.
Because we cannot use chemical pesticides in the garden, there have been (what seems like) a million organic suggestions for how to manage the pests. From the highly dubious to the practical, all of the methods require a bit of faith.
But there was one idea in particular that piqued my interest: marigolds. The suggestion was that marigolds deter insect pests. But how? I asked immediately. The friendly gardener just shrugged, so I decided to do some research.
Indeed, the internet is full of suggestions that marigolds have anti-pest properties. Some claim that their “smell” deters insect pests, while others claim that they release toxins in the soil that kill harmful nematodes, and others still say that they attract aphids and whiteflies away from other plants. But not everyone on the internet agrees…there are many websites dispelling the marigold “myth”.
As a scientist, of course, I am dubious of anything that is not peer reviewed, so I turned to the literature. (Science!) This is a really brief lit scan, so of course you could find more refs if you wanted.
Trap Cropping: Non-crop plants are used to attract pests away from crop species. Marigolds have sometimes been successfully used to attract away the rape blossom beetle (Meligethes aeneus) (Hokkanen 1989, Hokkanen 1991) and the fruit borer (Helicoverpa armigera) on tomato (Srinivasan et al. 2008)
Natural Enemies: Plants can sometimes be used to support the natural enemies of pest species (such as parasitoids that lay their eggs in pests and ultimately kill them, or predators of pest species). Marigolds seem relatively undervisited in field trials (Colley and Luna 2000) and may even decrease longevity of adult Syrphid flies (Laubertie et al. 2012)
Nematodes: There is a diverse community of nematodes in the soil, and only some are harmful to crop species. Nonetheless, the ones that are crop pests are notoriously difficult to control. The marigold effect on nematodes depends strongly on a) the marigold cultivar b) the crop species and c) the nematode species. Compounds extracted from the marigold are only weakly nematicidal in the soil, and the effects it has in field trials may be common to many other Aster species (Chitwood 2002).
Deterrence: The marigold may also deter pest insects from crops. There is some evidence that the marigold deters Mexican beetles from beans, but there was a confounding negative effect of the marigold’s allelopathy (it produces chemicals in the soil that suppress competition from other plants) on the bean crop (Latheef and Irwin 1980). Otherwise, very little support for this claim.
Unknown mechanism: There is some evidence to suggest that intercropping with marigolds reduces pest attack rates (e.g. Jankowska et al. 2009), but the mechanisms are not clear. It may be that intercropping with any species acts to confuse the pests.
Conclusions: Marigolds may be of some use as an organic pest deterrent in agricultural fields where they can be planted along margins at high densities. However, their beneficial effects are likely to be a) pest specific and b) moderate. If planted in close proximity to the crops, they are likely to do more harm than good, due to the chemicals they produce in the soil to compete with other plants; this probably eliminates their capacity to assist with nematode control.
In your garden? Probably not that useful unless you have room for an isolated marigold patch. Also, you should probably research which variety is best for your particular pest situation.