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We’ve been doing some research into plants that repel pests and have compiled a list of 10 plants that can be planted together with other plants as a simple form of insect control.
The idea of selecting plants for insect control is not a new one – and is part of the overall subject of companion planting. Companion planting has actually been in use since ancient times; for example the mosquito fern has been planted alongside rice in China for over 1000 years in order to assist with nitrogen fixing and the crowding out of other weeds. Native Americans also used companion planting before the Europeans arrived with the “three sisters” technique of planting corn, beans and squash together. 
As time passes by, it seems that more and more people are getting concerned (rightly!) about the hazards and damages of using pesticides / insecticides. Regardless of what corporate PR departments say, pesticides are not supposed to be ingested by the human body. They are not supposed to be consumed, but if they are utilized, ingesting them becomes inevitable. Even when we wash our fruits and vegetables, we can’t just get rid of them totally – as some has soaked in.
Another of the ironies of the use of insecticides is that not only do they kill the “bad insects” (the ones that eat crops), but they also wipe out “good insects” – the ones that feed on the bad insects! The insecticide approach fails to see nature as a complete system and compensates for lack of “understanding of the whole” by indiscriminate poisoning – which has been shown by many studies to have devastating further consequences. Pesticide chemicals are an obvious monstrosity and typically used out of “fear of nature” and a lack of understanding of better practices.
Companion planting for insect control can work in two ways a) plants that deter the pests and b) plants that attract the “good insects” that eat the ones that harm the plants.
Here is a list of 10 companion plants which are considered useful in repelling insect pests:
Marigold – the smell of marigolds is reported to be repellant to aphids. The flowers also attract hoverflies – which are considered beneficial as hoverflies are predators of aphids.
Borage – Borage attracts bees and predatory insects, and repels many insect pests. It also has lovely blue flowers. Considered one of the best companion plants – helpful for many plants, especially strawberries, tomatoes and cabbage.
Carrots – Carrots inter-planted with alliums (onions, leeks, garlic) confuse onion and carrot flies. If carrots are allowed to flower, they also attract assassin bug, lacewing, parasitic wasp, yellow jacket and other predatory wasps which will attack other predators.
Dill – Dill is considered helpful for cabbages, corn lettuce, onions and cucumbers. It attracts Tiger Swallowtail butterflies/caterpillars, Hoverflies, Wasps, Tomato Hornworm, honeybees and Ichneumonid Wasps. It repels Aphids, spider mites, squash bugs, cabbage looper. Dill should not be planted with cilantro as it will cross pollinate with it.
Garlic – garlic is thought to assist Apple trees, Pear trees, Roses, Cucumbers, Lettuce and Celery. It is repellant to aphids, ants, rabbits.
Radish – Radishes can be used as a trap crop  against flea beetles, to prevent them from attacking squash, eggplant, cucumber, lettuce. This means that they will prefer the radishes and leave the main crop alone.
Parsnip – Parsnips planted around fruit trees and left to flower will attract a variety of predatory insects to the garden – the predators attacking codling moth and light brown apple moth. Parsnip root can also be used to make a “natural pesticide: It contains Myristricin, which is toxic to fruit flies, house flies, red spider mite, pea aphids. Blend three parsnip roots with one litre of water through a food processor (not one for preparing food). Leave overnight, strain and use within a few days.
Tarragon – The scent of tarragon is disliked by most pests, and this plant is also thought to have “Nurse Plant” properties, enhancing the growth and flavor of crops grown with it.
Lovage – another herb considered to be one of the best companion plants for a wide variety of species. Will attract Ichneumonid Wasps, ground beetles (good). Thought to increase the health of plants around it.
Sage – deters cabbage flies, carrot fly, black flea beetle, cabbage looper, cabbage maggot. Attracts bees. Reported beneficial for rosemary, cabbage, beans, carrots.
FURTHER READING / references:
http://en.wikipedia.org/wiki/List_of_repellent_plants (giant list!)