The Insect Repellent Efficacy of Sweet Basil (Ocimum Basilicum) and Mexican Mint (Plectranthus Amboinicus) Extracts on Chilli Plants

Chua, Yi Lin (2026) The Insect Repellent Efficacy of Sweet Basil (Ocimum Basilicum) and Mexican Mint (Plectranthus Amboinicus) Extracts on Chilli Plants. Final Year Project (Bachelor), Tunku Abdul Rahman University of Management and Technology.

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Abstract

Insect pests are a major threat to chilli plant cultivation, leading to significant yield losses. The feeding activity of pests such as aphids, thrips and mites causes visible damage such as leaf curling and stunting. Intensive applications of chemical insecticides have also led to resistance development of these insect pests. This study was conducted to investigate the efficacy of Basil and Mexican Mint, as natural repellents for insect pests on chilli plants. Methanolic extractions were performed using freshly oven-dried plant samples for Sweet Basil and water-based for Mexican Mint as solvent with yield quantified. Crude extracts were applied via foliar spray method onto chilli seedlings at four-leaf stage with distilled water as control on a 10-day interval frequency. Observations were done on a weekly basis for the presence of insect pests. Plant growth parameters were collected including plant height, plant leaf number and chlorophyll content index. The findings revealed the control seedlings were infested with aphids and spider mites whereas treated plants were having less insect pests observed for all treatments; the Mexican mint extract at low concentrations induced the growth of chilli plants with highest seedlings height recorded at 12.70 ± 0.25 cm and the highest plant leaf number at 7.00 ± 1.00 leaves for 3000 ppm of Mexican mint extract. The chlorophyll content index recorded also revealed that the extracts promote growth thereby enhancing photosynthesis as the highest chlorophyll content value was at 10.2 CCI. In contrast, the basil extract at moderate concentrations 50,000 ppm showed phytotoxicities suppressing plant growth. The pest repellency activity is likely attributed to volatile organic compounds including phenolic compounds. Preliminary phytochemical analyses were conducted to quantify their total phenolic and flavonoid content. The results of these analyses confirmed the presence of these bioactive compounds in both extracts which are known for their repellent properties towards insect pests. The study’s findings suggested that the application of Mexican mint crude extract at 3000 ppm to 50,000 ppm offers a potential option for sustainable management of chilli pests, decreasing the usage of chemical insecticides; while the basil crude extracts may need to be further purified prior to application to reduce their phytotoxicity effects.