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The African bollworm, Helicoverpa armigera (Hbn.) (Lepidoptera: Noctuidae), is the most important cotton pest in all the cotton growing areas of the Sudan and most of the protection measures are directed for its management. The previous action threshold level for chemical control interference was increased in 1993 from 10 larvae and/or eggs /100 plants to 30 eggs or 10 larvae/100 plants and no spray to be conducted before flowering advances. This level was being subject to revision since 1999. Studies conducted at the Gezira Research Farm during 2001/2002, 2002/2003, 2003/2004, 2004/2005 and 2012/2013 were carried out on this regard at growth stages of cotton plants, i.e. Pre-flowering, flowering and bolls formation stages and the consequence on yield. Barac. (67) B cultivar, Gossypium hirsutum, was used. The action threshold levels were 10, 15, 20 eggs and/or larvae/100 plants and 30 eggs or 10 larvae/100 plants, in addition to untreated and Helicoverpa infestation free as control treatments. The results of damage on fruiting bodies (squares, flowers and bolls) and seed cotton yield showed that there were no significant differences between the tested action threshold levels in the pre-flowering and the flowering stages of cotton growth as compared to the control treatments. From the yields and number of sprays in the treatments it can be concluded that the already recommended action threshold 30 eggs or 10 larvae/100 plants is still be applied when flowering advances and bolls formed. Moreover, the standing action threshold seems lower than the actual level. Therefore, it is suggested to investigate higher action threshold than that in current use.

ACTION THRESHOLD LEVELS OF HELICOVERPA ARMIGERA (HBN.) ON COTTON AT DIFFERENT STAGES OF PLANT GROWTH