Evaluation of the efficacy of three insecticides against pests of cabbage (Brassica oleracea var cap it ata)

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An experiment to evaluate field and laboratory efficacy of Kombat (Pyrethroid), Rimon (Benzoylphenyl urea) and Dipel OF (Biological) against cabbage pests was carried out at the Kwame Nkrumah University of Science and technology, Kumasi. A field survey was undertaken in the Kumasi Metropolis to locate cabbage farmers and ascertain some views on their production practices. The field layout was a 3x3 factorial arranged in a Randomised Complete Block Design (RCBD). The first factor was the three insecticides namely: Rimon, Dipel DF and Kombat. The concentrations of 0, 2 and 5rn1/l for Kombat and Rirnon and 0,2, and 5g/l for Dipel DF served as the second factor. These gave nine treatment combinations which were replicated three times. The treatments were applied fortnightly using three hand atomiser sprayers, one for each insecticide. Cultural practices were carried out as and when appropriate. In the laboratory experiment, 30 larvae of Plutella xylostella were exposed to filter papers impregnated with 100%, 80%, 60%, 40% and 20% of the active ingredients for the three insecticides. Water was used as a control in the experiment. Data were collected on all relevant parameters. Results from the field survey showed that chemical insecticides appear to be the most important agents for controlling pests of cabbage. 1-lowever, the insecticides are applied at irregular intervals and dosages are usually higher than the recommended rates. The use and choice of insecticides depend largely on the retail chemical sellers who help farmers in the selection of the insecticides and mixing of insecticides for spraying. Results from the field experiment showed no statistical differences in plant height, canopy spread, number of leaves, total marketable weight and number of multiple heads for all the treatments. However, total marketable weight increased with increasing concentration of insecticides and decreased the number of cabbage plants with multiple heads. The extent of head damage by leaf perforations was statistically different among insecticide and dosage treatments. Rimon recorded the lowest percentage head damage compared to Dipel DF and Kombat. Applying 2ml and 5m1 per litre of the insecticides did not result in significant difference in head damage of cabbage but each of the dosages was more effective in reducing head damage compared to the control plants. The number of diamondback moth and spiders was statistically different among dosage and insecticide treatments. Rimon recorded the lowest number of diamondback moths. Applying 2 and 5m1 was effective in reducing the number of diamondback moths compared to the control plants. Kombat was found to reduce the number of spiders compared to Rirnon and Dipel DF. The laboratory experiment indicated that Kombat was highly toxic to Plutella xiuvtella compared to Rimon and Dipel DF.
A thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfilment of the requirements for the degree of Master of Science,(Environmental Science, 2005