Insecticidal Efficacy of <i>Syzygium aromaticum</i>, <i>Tephrosia vogelii</i> and <i>Croton dichogamus</i> Extracts against <i>Plutella xylostella</i> and <i>Trichoplusia</i> <i>ni</i> on <i>Brassica oleracea</i> crop in Northern Tanzania

Abstract

The insecticidal efficacy of 10%, 5% and 1% w/v of Tephrosia vogelii, Croton dichogamus and Syzygium aromaticum aqueous plant extracts were assessed against larvae of Plutella xylostella and Trichoplusia ni on Brassica oleracea var. capitata crop field. Synthetic organophosphate pesticide (Chlorpyrifos) was used as a positive control and the negative controls were water and water plus soap. It was revealed that, aqueous plant extracts significantly (P ≤ 0.05) controlled the number of P. xylostella and T. ni larvae compared to negative controls. The 10% concentration of aqueous plant extracts showed significant higher efficacy in terms of reducing the insect population and their damage than 1% and 5 % concentration. The population of P. xylostella larvae per B. oleracea in five weeks of treatment applications at 10% w/v of T. vogelii, C. dichogamus and S. aromaticum aqueous plant extracts were 0.08, 0.15, 0.13, 0.05 and 0.08; 0.08, 0.20, 0.15, 0.13 and 0.18; 0.03, 0.05, 0.15, 0.18 and 0.13, respectively which was significantly (P ≤ 0.05) lower than in water (1.13, 1.68, 2.28, 2.20 and 3.28) and water plus soap (0.75, 1.60, 2.58, 1.83 and 3.30) negative controls respectively. The number of T. ni larvae per B. oleracea in five weeks of treatment applications at 10% w/v of T. vogelii, C. dichogamus and S. aromaticum of aqueous plant extracts were 0.00, 0.03, 0.05, 0.03 and 0.00; 0.03, 0.08, 0.05, 0.08 and 0.08; 0.05, 0.03, 0.00, 0.05 and 0.03, respectively which was significantly (P ≤ 0.05) lower than in water (0.50, 0.63, 0.60, 0.48 and 0.78) and water plus soap (0.30, 0.48, 68, 0.65 and 0.80) negative controls. The percentage damage of B. oleracea in five weeks of treatment applications at 10% w/v of T. vogelii, C. dichogamus and S. aromaticum aqueous plant extracts were 10.0, 6.3, 7.5, 7.5 and 5.6; 11.3, 11.3, 11.3, 11.3 and 12.5; 8.8, 8.1, 6.9, 8.1 and 9.4, respectively compared with water (33.8, 33.1, 38.8, 45.0 and 70.6) and water plus soap (30.0, 31.9, 41.3, 41.3 and 56.3). These pesticidal plants can be recommended for smallholder farmers to significantly control P. xylostella and T. ni larvae in B. oleracea crop.

Keywords

• cabbage insect pests
• phytochemicals
• rotenone
• terpenoids and alkaloids