Efficacy of African Marigold (Tagetes erecta L.) in Managing Cabbage Insect Pests in Tropical Vegetable Farming Systems

Cabbage farming in Sri Lanka relies heavily on synthetic insecticides, but their high costs reduce overall profitability. Additionally, synthetic insecticides pose considerable environmental risks. Therefore, it is important to explore alternative strategies for insect pest management. This study investigated African marigold (Tagetes erecta L.)-based insect pest management strategies for controlling major leaf-eating caterpillar pests in cabbage under field conditions. A Randomized Complete Block Design with three blocks was used to compare five treatments: untreated cabbage sole crop as the control (T₁), cabbage treated with synthetic insecticides containing Thiamethoxam 25% (w/w) and Emamectin Benzoate 5% (w/w) (T₂), cabbage treated with marigold water extract (50 g fresh plant matter/1 L; T₃), cabbage intercropped with marigold as an additive (T₄) and cabbage intercropped with marigold in replacement (T₅). In T₄, marigold was intercropped with cabbage without reducing cabbage density, while in T₅, some cabbage plants were replaced by marigold. Treatments T₂, T₄ and T₅ significantly reduced (P<0.05) leaf-eating caterpillar abundance by 94%, 94% and 83%, respectively, compared to the control. Both intercropping methods (T₄ and T₅) were equally effective in suppressing caterpillar populations, while T₃ showed no significant difference (P<0.05) from the control, indicating ineffectiveness of the treatment. Ladybird beetle (Coccinella spp.) abundance significantly increased (P<0.05) under T₄ and T₅ (by 100 % and 95 %, respectively), but declined sharply under T₂ (-95 %), revealing the negative impact of T₂ on non-target organisms. Cabbage plant diameter remained consistent across all treatments, confirming that marigold intercropping did not hinder cabbage growth. Economically, additive intercropping (T₄) was the most favorable strategy, maintaining full cabbage plant density and eliminating pesticide costs. These results suggest that additive intercropping with African marigold is a promising agronomic strategy for managing leaf-eating caterpillar pests in cabbage production in Sri Lanka.