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Feasibility of chemical control of the African bollworm on sunflower in South Africa Du Plessis, H. Abstract The efficacy of methomyl and fenvalerate as a tank mixture for the control of African bollworm (Helicoverpa armigera Hubner) was compared by means of aerial application from two directions in relation to row direction and two plant reproductive stages (budding R-3, and during flowering and pollination, R-5) of sunflower (Helianthus annuus L.). Chemical control was effective during both reproductive stages, reductions in pest numbers ranged from 55.3% during R-3, to 86.5% during R-5, irrespective of the direction of application. Cost of control constituted 14% of the net profit margin. To determine the effect on yield and the ability of sunflower to compensate, insect damage was simulated on florets before pollination as well as on young achenes in the milky stage. A reduction in yield, although not significantly different from the undamaged control heads, was observed at damage level greater than 20%. Due to the cost of aerial insecticide application and the ability of sunflower to compensate for losses, chemical control of H. armigera should not be applied at first signs of eggs or larvae. Based on observations in this study, it is proposed that insecticide application be done when at least 20% damage per head occurs. Keywords Helicoverpa armigera, Lepidoptera, compensation, Helianthus annuus

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Du Plessis, H. Résumé L'efficacite de methomyl et de fenvalerate pour controler le ver du grain Africain (Helicoverpa armigera Hubner) etait comparee par une application aerienne en deux directions en relation avec la direction en rangees, et a deux phases reproductives, (phase bouton, R-3 et pendant la floraison, et phase pollinisation, R-5) du tournesol (Helianthus annuus L.). Le controle chimique etait efficace pendant les deux phases reproductives et la reduction du nombre d'insectes variait entre 55.3% en R-3, a 86.5% en R-5, independamment de la direction d'application. Les couts du controle constituaient 14% de la marge beneficiaire nette. Pour determiner l'effet sur la recolte et la possibilite de compensation du tournesol, l'attaque des insectes etait simulee sur des fleurons avant pollinisation et aussi sur les akenes a la phase laiteuse. Une reduction de la recolte, mais pas significativement differente des temoins, etait observee a un niveau d'attaque de plus que 20%. Comme les insecticides sont chers et le tournesol possede la capacite de compenser des pertes, un controle chimique n'est pas recommande des qu'on observe des oeufs ou des larves. Se basant sur les observations de cette etude, on propose une application d'insecticide quand 20% d'attaque par tete de fleur apparait. Mots Clés le ver du grain Africain, controle chimique, compensation, Helianthus annuus

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