Uptake, metabolism, and excretion of injected tritiated 22,33-dihydroazadirachtin in last instar larvae of Heliothis virescens

To evaluate its uptake, metabolism, and excretion, we studied the fate of tritiated 22,23-dihydroazadirachtin, when injected either orally or directly into the hemocoel of last instar larvae of Heliothis virescens (Fabr.). An oral injection of 1.0 μg of tritiated 22,23-dihydroazadirachtin, the minimum oral dose necessary to disrupt pupation, was rapidly absorbed through the midgut such that > 50% of the radioactivity was detected in the body 1 h post injection. Excretion of radioactivity following an oral injection was slow; 24 to 72 h elapsed before approximately 50% of the radioactivity was detected in the feces. In contrast, excretion of radioactivity following a hemocoel injection of 0.3 μg of tritiated 22,23-dihydroazadirachtin, the minimum hemocoel dose necessary to disrupt pupation, was rapid; almost 50% of the radioactivity was detected in the feces only 1 h post injection. Tritiated 22,23-dihydroazadirachtin was metabolized to a more polar compound(s) by H. virescens larvae, as indicated by thin layer chromatography. Metabolism to a more polar form(s) occurred probably following absorption through the midgut, since hemocoel-injected larvae exhibited a more rapid rate of conversion than oral-injected larvae. Even though metabolism to a more polar form(s) should enhance its excretion, approximately 25% of the radioactivity remained in the body 72 h following either oral or hemocoel injection, and this was sufficient to disrupt pupation. Resume Incorporation, metabolisation et excretion de 22,23-dihydroazadirachtine tritiee chez les chenilles de Heliothis virescens du dernier stade L'etude a ete realisee avec de la 22,23-dihydroazadirachtine tritiee injectee soit oralement, soit directement dans l'hemocoele des chenilles de H. virescens Fabr. du dernier stade. La dose orale minimale de 1,0 μg, necessaire pour perturber la nymphose, a ete rapidement absorbee a travers l'intestin moyen, d'une facon telle que plus de 50% de la radioactivite a ete retrouvee dans le corps une heure apres l'injection. L'excretion de la radioactivite a la suite d'une injection orale a ete lente; 24 a 72 heures se sont ecoulees avant que 50% de la radioactivite ait ete decelee dans les excrements. A l'oppose, l'excretion de la radioactivite a la suite d'une injection hemocoelique de 0,3 μg, – dose hemocoelique minimale pour perturber la nymphose –, a ete rapide; presque 50% de la radioactivite a ete decelee dans les excrements au bout d'une heure seulement apres l'injection. La chromatographic en couche mince a montre que la 22,23-dihydroazadirachtin a ete metabolisee en substance(s) plus polaire(s) par les chenilles de H. virencens. La metabolisation en forme(s) plus polaire(s) se produit vraisemblablement apres l'absorption a travers l'intestin moyen, puisque les chenilles injectees directement dans l'hemocoele ont presente un taux de conversion plus rapide que les chenilles injectees oralement. Bien que la metabolisation sous une (des) forme(s) plus polaire(s) ait augmente l'excretion, approximativement 25% de la radioactivite subsistait dans le corps 72 heures apres une injection orale ou hemocoelique, ce qui est suffisant pour perturber la nymphose.