Our vegetable patch is more and more looking like a marigold (Tagetes patula) plantation, although it looks very nice and colourful, some of them are getting a bit out of hand. Half a meter wide and about the same height. There won’t be any roundworm alive in the earth after this attack!
Serious, the reason for the Marigolds in the garden is roundworms. Roundworm, nematodes for scientists, are little worms that live everywhere. Soil, water, roots, animals and humans; they are all over the place. You can’t see them with your own eyes but under a microscope you will find up to 50 in a cubic centimetre of soil. In a square meter of soil there are up to 10 million of them.
According to wikipedia there are 80.000 known species of roundworms and there are and estimated 500.000 different species. One group of this half a million, targets the roots of plants, specially perennials but also potatoes, eggplants and tomatoes. Against these roundworms you can use the marigolds, it is not that the worms are scared away by them. Although everything looks peaceful there is an underground war going on.
Some science for the other geeks: the inner skin of the roots (endodermis) of the Marigolds contains chemicals called thiophenes. When a roundworm enters the cells of this inner skin, it forms peroxidase. The combination of this peroxidase and the thiophenes create O3 (ozone), this aggressive form of oxygen burns the roundworm. So far for the peaceful vegetable patch.
The bee doesn’t care about any of this and comes for very different reasons to the Marigolds.