Abstract
The defoliator, Xanthodes graellsii (Feisth) (Lepidoptera: Noctuidae) is a serious pest of okra [Abelmoschus esculentus (L.) Moench] in Ghana. The larvae are very destructive resulting in reduced leaf surface area and photosynthesis. Studies were conducted to assess the efficacy and residual toxicity of some selected chemical and biological insecticides for control. Chlorpyrifos at 1200 mL ha-1, Lambda-cyhalothrin at 800 mL ha-1, Bacillus thuringiensis (16 000 International Units mg-1) at 400 g ha-1, and water as control at 100 000 mL ha-1, were fortnightly applied in the field at 3, 5, and 7 weeks after seedling emergence (WASE) on okra cv. Boto in the 1995 and 1996 early season crop. Pre-spraying assessment of leaf damage and larval numbers of X. graellsii were carried out prior to every treatment. Chlorpyrifos gave the most effective protection to okra leaves, followed by B. thuringiensis. The 14th WASE recorded the highest fruit yield with chlorpyrifos-sprayed plots recording the highest yield, followed by B. thuringiensis-, cyhalothrin-, and water-sprayed (control) plots in decreasing order of yield. Residual effectiveness of the three insecticides against the 4th instar larvae of X. graellsii was evaluated by comparing the percentage mortality of larvae on treated leaves at 1, 4, 9, and 14 days after the 7th WASE application. Chlorpyrifos had the highest residual effect in terms of larval mortality, followed by B. thuringiensis. Irrespective of the insecticides used, larval mortality decreased after initial insecticide application. One day after insecticide application, chlorpyrifos and cyhalothrin gave a good kill (>95%), but did not differ significantly (P > 0.05) from that of B. thuringiensis. Chlorpyrifos, however, maintained a better residual effect in terms of larval mortality.
