Onions and Slugs - Knowledgebase Question

Omaha, NE
Avatar for daisy00
Question by daisy00
February 17, 1998
Why are onions known to have detering effects on slugs and
why are onions and lettuce considered companion plants?
Please include any chemical information you may have on
onions, why they are so strong and if this may influence
their reaction on slugs and or lettuce.

Answer from NGA
February 17, 1998
Bulbs of onion and garlic contain a volatile oil consisting of predominantly allyl sulfide. This compound has been shown to have pest-detering qualities, as well as medicinal qualities.

As far as why these plants contain this compound, I can onlyspeculate that as they evolved, they incorporated this substance into their cells, perhaps as a deterrent to herbivores. Different types of onions contain various amounts of this substance--that's why some are stronger-tasting than others.

Onions are grown as companion plants for several vegetables, including onions, carrots, and broccoli. Peas, beans, and sage are reputed to be "bad neighbors" for members of the onion family. Onions are believed to deter Colorado potato beetles and carrot root maggots. The reference book I am consulting doesn't list onions as a traditional companion plant for lettuce. Certainly, if slugs don't like the smell or taste of onions, then interplanting onions with lettuce may well deter slugs.

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