reported reduction in cabbage-worm damage; mechanism unresolved
Mechanism: Unresolved, and the rule says so rather than picking one. Volatile masking of host-finding cues and disrupted landing are both proposed. The trials that report an effect measured DAMAGE, which cannot separate fewer eggs laid from fewer larvae surviving or more predators arriving - so no mechanism here is established, only candidates.
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
Univ. of Minnesota Extension, 'Companion planting in home gardens', extension.umn.edu/planting-and-growing-guides/companion-planting-home-gardens (read 2026-07-25): 'In an Iowa study, thyme, onion and nasturtium helped to reduce cabbage looper and imported cabbageworm damage in broccoli' - citing Riesselman, Leah B. (2006), 'Companion Planting: A Method for Sustainable Pest Control', Iowa State Research Farm Progress Reports. Thyme is this guild's canonical masking species, so the one trial found is on the plant the corpus actually prescribes.
Same page, on the wider claim base: 'While gardening charts online will provide you with long lists of plants that repel insects, these are not always accurate or backed by research', and of marigolds specifically, 'Sometimes yes and sometimes no.' That is the mixed picture this rule's effect_size asserts, now cited rather than claimed.
PRIMARY READ: the maintainer read Riesselman (2006) itself on 2026-07-25, at dr.lib.iastate.edu/farms_reports/295, and promoted this rule on it - not on Minnesota's summary alone. The trial's own measurements are not transcribed into this pointer; what the corpus asserts is still the DAMAGE claim the summary reports, and the read did not overturn it.
GAP that survives the read: the effect is measured as damage, and damage cannot separate fewer eggs laid from fewer larvae surviving or more predators arriving. That is why this claim does not mention egg-laying and why the mechanism is stated as unresolved. Verified means the source was read, not that the mechanism is known.
NEGATIVE RESULT, recorded so it is not cited by its title: Morkeliune et al. (2024), 'Effect of White Cabbage Intercropping with Aromatic Plant on Yield, Mineral and Biochemical Composition', Plants 13(13):1870, intercrops cabbage with calendula, French marigold, thyme and sage across 2017-2019 and measures NO pests, eggs or oviposition at all - yield and biochemistry only. It cannot support this rule. Nor can the '42% fewer egg clusters under sage' figure circulating in search results, which traces to commercial content sites and to no study.