https://www.networx.com/article/garden-p ... your-enemy
You may not want a companion with garlic breath, but plenty of plants are happy with garlic as a companion. The pungent smell keeps insects from finding the sweet-smell of roses, peaches and more. Cornell Cooperative Extension experts note that pest deterrence through companion planting is difficult to prove, but said there is substantial anecdotal evidence of success.
In his Giant Book of Garden Solutions, plant expert Jerry Baker recommends planting garlic in rose beds to ward off cane borers, aphids, rose chafers and Japanese beetles. To repel peach tree borers, Baker recommends planting a ring of garlic around a peach tree trunk, but the garlic and the tree must be planted at the same time.
In The Garden Pests and Diseases Specialist, David Squire suggests planting garlic around carrots to mask the carrot smell and keep carrot flies away. Leeks and onions also help.
North Carolina Cooperative Extension experts note that planting garlic around celery and lettuce deters aphids. Planting garlic between tomato plants can keep away red spider mites, according to Cornell Cooperative Extension. The Cornell experts also suggest planting garlic near cabbage to deter cabbage looper, cabbage maggot and imported cabbageworm. They also suggest garlic for keeping rabbits, slugs and snails away from the veggie patch.
This is not evidence, but it does come from University extensions, who presumably aren't going to be recommending nonsense.
There is scientific evidence, it just takes a lot of time to track down.
Here is a study of trap cropping:
Perimeter trap cropping (PTC) involves using a trap crop, and possibly other border defenses, to encircle and protect the main cash crop like fortress walls. Six growers in Connecticut used PTC to protect commercial summer squash plantings from cucumber beetles and bacterial wilt damage. Grower surveys were used to compare PTC program results to the conventional "multiple-full-field-spray" system formerly used on the farms. Most growers using PTC stated that this system improved and simplified pest control, reduced pesticide use (93%) and crop loss, and saved them time and money compared to their conventional program.
You could say nearly eliminated pesticide use. Study titled "Demonstrating a Perimeter Trap Crop Approach to Pest Management on Summer Squash in New England" from U. Conn published in Journal of Extension, October 2004 // Volume 42 // Number 5 // Research in Brief // 5RIB2.
Most plants produce defensive chemicals that help fend off insects and diseases. These chemicals may be insect poisons, feeding deterrents or have fungicidal properties. The roots of some French and African marigolds contain a substance which is toxic to certain types of nematodes. Nematodes are soil inhabiting microscopic roundworms that damage many species of plants. Certain nematodes can be eliminated from a site by growing a thick crop of marigolds for one season prior to planting the vegetable or fruit crop, or by interplanting marigolds between crop rows.
Destructive insects often locate their food by smell. Many plants, especially culinary herbs, produce strong scents which may confuse insect pests looking for a host to feed on. Garden vegetable plants such as garlic, onions, chives, and herbs such as catnip, horehound, wormwood, basil, tansy, and mints all produce scents which seem to repel insects or mask the scents which attract insects. A certain level of insect protection can be achieved by carefully interplanting some of these as companions to vegetables.
This is from Cornell University: https://www.gardening.cornell.edu/factsh ... plant.html
Again they don't cite their sources, but it is a University, I doubt they are making this stuff up. The article goes on to recommend trap cropping and use of flowers in the garden to attract beneficial insects and says: "Avoid monoculture in terms of space and time. A one-hundred foot long row of broccoli presents a large target for a cabbage moth that is flying by, but the same number of cabbage plants scattered over several thousand square feet, and interplanted with other crops, is less obvious and attractive to the insect. Pests which routinely plague large, commercial plantings of crops may never be a problem in the diversified home garden."
ATTRA (appropriate technology transfer for rural areas, the national sustainable agriculture information center funded by the USDAâ€™s Rural Business--Cooperative Service, put out a pdf on companion planting: https://www.asu.edu/fm/documents/arboret ... anting.pdf
there is general agreement today on the validity of several mechanisms that create beneficial plant associations.
They list among these trap cropping, symbiotic nitrogen fixation, biochemical pest suppression, physical spatial interactions, nurse cropping, beneficial habitats, security through diversity.
At different times when I have looked, I have seen controlled studies, showing cabbage plants were protected by having herbs growing around them, etc. But the nature of the internet is that this stuff doesn't stay up for ever. Often when you go looking for a study you found before, it is no longer available. Without access to scientific journals it is hard to have access to the data. But it is there!