Efficacy of certain Plant oils on the American Bollworm Helicoverpa armigera Hubner

Three plant oils viz., Artemisia annua Linn., Ageratum conyzoides Linn, and Azadirachta indica A. Juss, were evaluated against the larval instar of the American bollworm, Helicoverpa armigera Hubner, when applied topically on the dorsal side of the mesothoracic region of final instar larvae. ED₅₍₎ values for 50% normal adult emergence, were 1763.39, 3280.30 and 753.80 ug g⁻¹ body weight for A. annua, A. conyzoides and A. indica, respectively. The treated larvae showed constriction at the site of application. Few larvae died just before pupation, some others attempting to pupate, ended as larval-pupal intermediates. The percentage of larval-pupal intermediates was higher in Azadirachta oil treatment, being more than 80% at the highest dose tested. Higher doses of A. annua were toxic causing larval mortality and failure of normal adult emergence.