Efficacy of organic and synthetic insecticides on the control of cotton pests: The bollworm complex, Helicoverpa armigera, Diparopsis castanea, Earias insulana (Noctuidae) and the leafhopper, Jacobiell

摘要

A research was conducted on cotton to test different plant extracts with potential insecticidal properties against bollworms (Noctuidae) and leafhoppers (Cicadellidae) for the use by small-scale, cotton farmers. The study was carried out during the 2006/07 and 2007/08 seasons at ARC Institute for Industrial Crops, Rustenburg (25°39.0 S, 27°14.4 E) in the North West Province of South Africa. Four organic insecticides, tobacco (Nicotiana tabacium), khaki weed (Tagetes minuta), thorn apple (Datura stramonium) and garlic (Allium sativum) were compared with two chemical treatments, Mospilan® (acetamiprid) and Decis® (deltamethrin), and an untreated control. The cultivar, DeltaOPAL, was planted and the trial conducted using standard practices. Overall, Decis® and tobacco treatments exhibited significantly less bollworm larvae than the other treatments. Tobacco was the most promising biological pesticide against American bollworm (Helicoverpa armigera) and spiny bollworm (Earias insulana) larvae. All the treatments were significantly effective against the red bollworm (Diparopsis castanea) larvae, but Decis® and Mospilan® provided the best control. Although khaki weed, garlic and thorn apple were promising biological pesticides against the bollworm complex, tobacco was the most effective organic treatment. Mospilan® seemed to be more effective on the control of leafhoppers (Jacobiella fascialis), followed by Decis® and tobacco. Among the plant extracts, tobacco and garlic gave higher seed cotton yields compared to the khaki weed and thorn apple during both study seasons. In 2006/07, none of the treatments reached the corresponding economic threshold levels (ETLs). In contrast, in 2007/08, only rarely were the treatments (mostly Decis® and Mospilan®) below the corresponding ETLs. These differences were attributed to the higher rainfall recorded during the 2007/08 season, which reduced the effectiveness of the treatments by partly washing away the pesticidal applications. In addition, a high weed infestation also created competition for nutrients in the soil, thus resulting in lower (0.50-1.25 ton/ha in 2007/08 vs 2.50-5.00 ton/ha in 2006/07), but acceptable cotton seed yields. In conclusion, it is suggested that some plant extracts (particularly from tobacco and garlic) can be used as a cheaper and more environment-friendly alternative to chemical insecticides for the control of bollworms and leafhoppers, although it has been demonstrated that their efficacy do not reach the one of chemical treatments. Further research in the near future is recommended.