Alliums interplanted with carrots reduce carrot rust fly damage.
A good hunch · suboptimal if ignored → advise
How it works
The carrot rust fly finds its host by the smell of carrot foliage. Onion and other allium volatiles growing among the carrots mask that scent, so fewer flies locate the row to lay eggs. Because the protection is the smell, it lasts only while the alliums are in the ground — once they are pulled, the masking goes with them.
How much it matters: Modest; protection collapses once alliums are harvested.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
New England Vegetable Management Guide (regional land-grant extension), 'Carrot and Parsnip - Insect Control' (Carrot Rust Fly, Psila rosae), nevegetable.org/crops/carrot-and-parsnip/insect-control (read 2026-07-11): 'Intercropping with onion has been shown to reduce damage by carrot rust flies.' Listed among partial/suppression measures (with row covers and staggered planting), consistent with the 'modest' effect_size. The Guide does not state the post-harvest collapse; that is the masking mechanism, not a claim from this source.
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- alliums vs carrot rust fly — fill the stub mechanism
- promote to verified — alliums interplanted with carrots reduce carrot rust fly
- cite New England Vegetable Guide for onion-carrot rust fly (rule confirmed)
Cite this page
Milpa Gardens. "Alliums interplanted with carrots reduce carrot rust fly damage." Evidence grade: A good hunch. https://milpa.garden/rules/alliums-interplanted-carrots-reduce-carrot-rust-fly/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.