Insecticidal activity of aqueous extract of Thevetia peruviana (Apocynaceae) on caterpillars of Lepidoptera pests of maize in Dschang-Cameroon

Abstract

Maize cultivation has been severely threatened in Cameroon since the invasion of Spodoptera frugiperda in 2016. Before its introduction, Spodoptera exempta, Spodoptera littoralis, Spodoptera litura, Busseola fusca, Sesamia calamistis, Ostrinia nubilalis and Helicoverpa spp. were the main Lepidoptera pests of maize in Africa. The damage caused by this insect is increasing, making it a serious threat to food security. The aim of this study was to assess the insecticidal activity of aqueous extract of Thevetia peruviana seeds on Lepidoptera caterpillar pest of maize in Dschang. Phytochemical analysis of the seed paste showed the presence of alkaloids, phenolic compound, triterpenes, flavonoids, saponosides, gall tannins, coumarins and anthraquinones. On 5 maize varieties (CHC201, CHC202, CMS8704, CHH101, Kaba), three doses of 25 and 30g.l-1, 40g.l-1 (30g.l-1+10g rock salt powder) of aqueous extract were applied every fortnight. The results showed that after 7 to 24 hours of treatment on infested maize stalks, the caterpillars were killed in maize whorls, followed by eradication of damage. T3 dose (40g.l-1) and T+ positive control (Emacot at 1.88g.l-1) completely destroyed the caterpillars on the organs of the maize plants (100% mortality). Screening tests on caterpillar corpses showed the presence of phenolic compound, triterpenes, coumarins and anthraquinones. Aqueous extract of T. peruviana seeds at 40g.l-1 is recommended for biological control by ingestion against Lepidoptera caterpillar on maize.