There is little to no scientific evidence that any sort of companion planting works because no one will fund research into something that isn't going to make money for whoever is supplying the funding.

I do not believe tansy repels squash bugs. I have tansy all the time. I have squash bugs probably 8 years out of 10.

Tansy is a big rampant thug and I can imagine it would interfere in the growth of anything planted too close to it. My tansy sits along the garden fence line about 10' from where I'm growing squash. I cut tansy back hard several times a year or it would take over the entire garden. I also vigorously weed out tansy volunteers or that would be all I had growing in the garden. I have both regular tansy and silver tansy and never, ever have seen an aphid on them. I suspect if someone is growing them in nitrogen-rich soil, they might get aphids. My tansy is in poor soil along the fenceline and it doesn't get compost and it never gets fed any form of fertilizer. I'm afraid if I fed it, it might swallow up the house and acres and acres of land.

The kind of companion planting I do nowadays is not so much to match up this plant with that companion to attract or repel anything because I haven't seen that doing such a thing works with many things. I use companion planting mostly to attract beneficial insects and the beneficial insects help control the pest insects.

I do think that planting basil with tomatoes does keep tomato hornworms away in all but the worst hornworm years. I also think that using certain plants as trap plants can work. I like to use sunflowers planted 20-30' away from the garden to serve as trap plants for stink bugs. When they congretgate there I can kill them more easily than when they are in the veggie garden hiding on every plant in there. They seem to really prefer sunflowers to the other stuff I grow.

Also, I've never had nematodes on tansy and I do grow it in the sandy soil at the NW corner of the front garden.

I think companion planting works in a more general way----that the more diversity you have within your garden, the more beneficial insects and other garden helpers (birds, frogs, toads, lizards, etc.) you'll attract and that your desirable plants benefit from that in the long term. About the only thing I've found that works at repelling some pests is garlic. However, to me it is just as easy to mix up garlic-pepper tea to spray on plants as it is to plant garlic here and there throughout the garden.

In a good caterpillar year I barely see any at all, no matter what I planted or didn't plant. In a bad caterpillar year, I see them everywhere, no matter what I planted or didn't plant. So, I really don't think you repel them by planting much of anything as a companion. Instead, I focus on attracting beneficials like parisitic wasps that will kill them for me. I have more parisitic wasps than usual this year and I am sure it is because the caterpillar population is huge.