Alliums interplanted with carrots reduce carrot rust fly damage.
A good hunch · suboptimal if ignored → advise
How it works
Volatile masking of host odour cues.
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.