Disruptive Effects of Certain Chitin Synthesis Inhibitors on Some Haemogram Parameters in the Egyptian Cotton Leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae)

Abstract

The Egyptian cotton leafworm, Spodoptera littoralis, is a key pest attacking several economically important crops in Egypt and several parts in the world. The objective of the present study was to investigate the effects of novaluron and cyromazine, chitin synthesis inhibitors, on the larval haemogram of this pest. Five main types of the circulating hemocytes, viz., plasmatocytes (PLs), granulocytes (GRs), prohemocytes (PRs), oenocytoides (OEs), and spherulocytes (SPs) had been categorized in last instar larvae. The most important differentiating characters of each type were described. After treatment of the freshly moulted penultimate instar larvae with LC50 of novaluron or cyromazine (2.71 and 74.44 ppm, respectively), the successfully moulted last (6th) instar larvae were used to assess the haemogram responses. Novaluron remarkably induced the total hemocyte counts (THCs) at two limits of the larval instar. In contrast, cyromazine exhibited a general inhibitory effect on THC during the majority of larval instar. As a response to novaluron treatment, PRs and PLs counts had been slightly induced during the first half of instar but slightly reduced during the second half. Also, GRs population decreased but SPs population increased along most larval period. With respect to the effect of cyromazine, count of PRs and GRs had been remarkably inhibited but PLs was significantly enhanced along the larval life. The same compound exhibited diverse effects on SPs, depending on the larval age. In addition, OEs count was enhanced by both compounds during the second half of larval instar. Both compounds exhibited serious cytopathological effects on all types of hemocytes, except cyromazine against OEs. INTRODUCTION In insects, the circulating haemocytes perform primary functions in the body, such as phagocytosis, coagulation to prevent loss of blood, encapsulation of foreign bodies, nodule formation, detoxification of metabolites and biological active materials, as well as storage and distribution of nutritive materials to various tissues (for detail, see: