The phytochemical ptumbagin, isolated from Plumbago zeylanica L., was applied in 2 and 5ul doses of 0.1 % concentration on the surface of Dysdercus koenigii F. adults and its effect on the haemocytes of the bug, studied by transmission electron microscopy. All the five haemocyte types viz. the prohaemocyte, plasma tocy te, granular haemocyte, oenocytoid, and the adipohaemocyte are affected within a period of 24-48 hr.The effect begins with an acute state of vacuolization of the cells and a gradual destruction of the organelles, followed by dissolution of the plasma membrane and passing out of the internal organelles, like the mitochondria and the endoplasmic reticulum. This phenomenon is responsible for a consistent elimination of the haemocytes from the blood, leading to lowering of the resistance and mortality of the insects.Resume-La substance phytochimique plumbagin, extraite de Plumbago zeylanica L. a ete appliquee a des doses de 2 et 5 microlitres dans une concentration de 0.1% sur des adultes de Dysdercus koenigii P.; et ses effets sur les hemocytes de la punaise sont observes au microscope electronlque a transmission. Tous les cinq types, hemocytes, prohemocytes, plasmatocytes, granulocytes, oenocytoides et adipohemocytes sont affected en I'espace de 24-48 heures. L'effet commence par un etat de vacuolisation severe des cellules et une destruction graduelles des organelles suivies par la dissolution de la membrane plasmique et celle des organelles internes teiles que les mitochondries et le reticulum endoplasmique. Ce phenomene est responsable de l'hemolyse avec pour resultats l'abaissement de la resistance de I'insecte suivi de sa mort.
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