Crushed eggshells deter slugs.
Crushed eggshells haven't been shown to deter slugs. Use beer traps, hand-pick at night, or put down iron-phosphate bait, which is safer around pets and wildlife. Put the shells in the compost for the organic matter. Their calcium releases too slowly to matter.
What is actually known
The claim is that sharp shell edges form a barrier slugs won't cross, but crushed eggshells have not been shown to deter slugs. Nor are the shells much use for their calcium, because they break down too slowly to supply it, and many soils already have plenty. Their real value is as organic matter in the compost. For slugs, what works is trapping, hand-picking at night and iron- phosphate bait, and bait alone will not control them in the long term.
The evidence
Evidence: checked against 3 sources in the research-based literature.
The 3 sources
Crushed egg shells or coffee grounds have not been shown to be effective deterrents.
Even if you did need the calcium in your soil, eggshells generally decompose too slowly to be effective.
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Milpa Gardens. “Crushed eggshells deter slugs.” Evidence grade: Contested. https://milpa.garden/beliefs/eggshells-slugs/
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