Influence of the nonsteroidal ecdysone agonist, tebufenozide, on certain biological and physiological parameters of the cotton leaf-worm, Spodoptera littoralis (Boisd.) (Noctuidae: Lepidoptera) in Egypt

Abstract

In Egypt, cotton represents the most economical and important field crop. Cotton plant is the target of different pest species from cultivation until harvest. The Egyptian cotton leaf-worm, Spodoptera littoralis (Boisd.) (Noctuidae: Lepidoptera), is an important polyphagous insect pest attacking cotton, several cultivated crops and ornamentals worldwide. The integrated pest management (IPM) programs create the need for novel insecticides having more selective modes of action. So, the no steroidal ecdysteroid mimic, tebufenozide (RH-5992) was studied on some parameters of Spodoptera littoralis. This compound exhibits, in spite of its nonsteroidal structure, a high affinity for the ecdysteroid receptors (EcRs) through a lock-and key principle just like the natural insect molting hormone 20, hydroxyecdysone (20E). The present investigation was performed to throw more light on the following objectives: (1) the toxicity of tebufenozide against the fifth larval instar of a laboratory strain of S. littoralis, (2) the effect of this compound on some biological parameters, i.e. weight of mature larvae, mortality and percentages of larval malformations, (3) the action on some haematological parameters such as blood volume, total haemocyte counts (THCs), absolute haemocyte counts, differential haemocyte counts, and (4) the characterization of haemolymph proteins in treated as well as in the untreated larvae, using the electrophoresis technique on acrylamide gel expressed in the electrophoretic mobility. The obtained results could be summarized as follows: (1) the treated larvae reached their maximum weight after 48 h from treatment with tebufenozide; while the untreated ones reached the maximum weight after 72 h (2) the highest mortality (73%) occurred by applying tebufenozide at the concentration of 18 ppm. In addition, the deduced percentage of larval malformations, indicated that the maximum rate of malformed larvae was attained using the concentration of 36 ppm (3) the blood volume of the 5th instar larvae reached its maximum after 72 h from treatment; while in the untreated ones blood volume increased gradually and reached the maximum rate after 96 h (4) the total haemocyte counts (THCs) decreased in comparison with the control. The maximum value of THCs occurred at 72 h, resembling a lower rate (50%) than that of the untreated ones. (5) The absolute counts reached their maximum rates after 72 h from treatment. It was lower (80.7%) than the untreated larvae. (6) Tebufenozide cause the induction of (10.7 & 14.3 Kda) polypeptide in the haemolymph after 24 h of application. These bands were absent in that time in the untreated larvae (control). Later on, these bands were formed in the control larvae after 72 h of treatment. Induced synthesis of these transcript resulted in precocious molting.