Spent Coffee Grounds Applied as a Top-Dressing or Incorporated into the Soil Can Improve Plant Growth While Reducing Slug Herbivory

Between 8 and 15 million tons of spent coffee grounds (SCG) are produced as global waste each year. To reduce to landfill, SCG proposed a carbon nutrient source for degraded soils. contain caffeine other toxins that inhibit plant growth. However, they also repel slugs snails. We examined whether partial decomposition can neutralize promote growth while maintaining anti-herbivore properties. aged <1 14 months SCG-derived Black Soldier Fly (Hermetia illucens) frass. The frass were applied, either incorporated into soil or 1 cm top-dressing, pots with radish tomato seedlings. treatments direct (repellent) indirect (plant-mediated) effects on four slug species (Arion ater, Deroceras laeve, Derocerus reticulatum Lehmannia marginata). ≤7 inhibited development reduced herbivory when soil, whereas 14-month-old promoted but had no effect herbivory. When applied at 7 through repellent host quality effects—including possible systemic effects. Our results indicate the benefits be achieved simultaneously by applying partially decomposed (aged up months) top-dressing.