Asked to write something up about it so here so here I go.
Tagetes minuta is a relative of the marigold, and it is the variety that most articles claiming "marigolds" stop nematoes and insect attack are referencing.
It is not the much smaller and more ornamental relatives like Tagetes erecta, Tagetes patula etc etc etc that they generally use for a picture. That is why those pretty ones still get eaten, it's bait a switch with the consumer.
The real deal, as proven by science to be effective one looks like in the picture.
It doesn't have pretty flowers, they are about 1/3 the size of your little finger nail, or about 2 match heads.
It is often huge, 2meters no dramas at all in good conditions, tall thin and straight, kinda sort of looking like lanky hemp from a distance.
The smell is strong and I like it, funky mint muskiness. Some folks say it smell like old piss.
The long straight stems make great stakes for trellising, 3 tied teepee style is ideal for a tomato cage, and they are quite effective at repelling cut worms, and very mildly repellent for cabbage moth, no where near 100% though.
The whole plant can be boiled to a black soupy sludge then filtered for use as an insect spray.
I use it as my number 1 go to whenever I have issues. It kills most soft bodied critters, and stops things like grasshoppers from feeding and unlike pyrethrum and oil or soap based insecticides, it does not burn or damage the plants even in full summer Queensland sun. Never lost a plant to it, where as I have lost heaps to every other "organic" solution on the market, especially the commercially packaged ones.
As a preventative it is really effective too, just be aware that diversity is the best way to go.
Only kill the bad guys, and only when you have to.
It is very effective, I would say on par with tobacco, but unlike tobacco it is legal to grow in every state of Australia, and it is also edible and really quite safe.(great in a salad or mixed with mashed potatoes.)
**Contact dermatitis or allergy is possible, but that's the same with most herbal insecticides.
I had a major nematode problem when we came, couldn't grow potatoes or root crops at all really, and our tomatoes really struggled, but a couple seasons of mass seeding with this fella and they are not an issue at all.
Known by a heap of names, Huacatay, stinking roger, black mint, Aztec marigold, chinchilla, dwarf marigold, false marigold, khaki bush, khaki-bush, and considered a weed in some places, a valuable food source in others.
I love it and very highly recommend it!
P.S. As always, if you have something cool I would be very happy to swap some seeds.