The Neuroendocrine regulation or larval development in the Maize - stem borer, Busseola Fusca (Fuller) (Lepidoptera:Noctuidae)

Abstract

Busseala fusca (Lepidoptera:Noctuidae) is one of the most important insect pests of maize and sorghum in Africa, south of the Sahara. Last instar larvae of this species survive the dry season in the stalks and stubbles of their host plants by undergoing diapause.The present study was done with the main objective of investigating the involvement of the neuroendocrine system during the development of the last instar larvae with respect to non-diapause and diapause development. The main aspects studied were the biology of last larval instar, histology, ultrastructure and physiology of the components of the larval neuroendocrine system, particularly the type-A neurosecretory cells, corpus cardiacum, ::orpus allatum and prothoracic glands. Endocrine involvemen·~ in the regulation of development during the last instar larvae and in induction, maintenance and -+.:,ermination of diapau,se development were also investigated. Studies on the biology of the last instar showed that two typesof development, namely, non-diapause and diapauae occur in Buseeala fl1sca. The non-diapause form of development is characterized by the feeding phase and postfeeding phase and the diapause form of development is characterized by only a long, non-feeding phase. The structure of the components of the endocrine system revealed that both in non-diapause and diapause larvae, the type-A neuroeecretory cells in the brain and the prothoracic glands were structurally comparable with insignificant differences. However, the corpora allata were structurally different in non-diapause and diapauee larvae. The corporaallata in non-diapause development had cells which completely lacked glycogen-like deposits which were found abundantly in the corpora allata of diapause larvae. The mitochondria of the cells of the corpora allata in nondiapause development ~~ere not conspicuous and did not contain dense bodies. On the other hand, the mitochondria of the cells of the corI>ora allata in diapause larvae were large and pleomorphic and contained bodies within the mattrix. The corpora allata in non-diapause larvae were slightly smaller in size than those of diapause larvae.However, the prothoracic glands in non-diapause larvae were larger than those in diapause larvae .The titers of juvenile hormone in non-diapause larvae were very low (as determined by dysdercus bioassay, were of an average score of 0.8 out of 3.0), while those in the diapause larvae were high (average score of 2.8). The titers of the moulting hormone in non-diapause last instar larvaewere 10, 170, and 500ng/ml on days one, four and seven respectively; in diapause larvae the titer was generally low with a monthly mean titer of less than 200ng/ml. Injection of the juvenile hormone analogue resulted in induction of diapause in non-diapause larvae. Elevation of the moulting hormone titers by experimental injection of ecdysone into the diapause larvae triggered moulting but it was usually a larval-larval (stationary) moult. Injection of the moulting hormone in isolated abdomens of diapause larvae induced progressive moult. These observations indicated that diapause development was not due to deficiency of moulting hormone. Diapause can be initiated and maintained by high titers of juvenile hormone. Thus the corpora a"llata of the diapause larvae were active. Precocene showed no allatocidal effect on the corpus allatum of the diapause larvae.Fluoromevalonate treatment of the late diapausi· larvae at a dosage of lOμg per individual prevented larval moult and.(delayed pupation relative to the non-treated larvae. At a dose of 5ug per individual, larval moults occurred and pupation was not delayed but the resultant adults were deformed.In conclusion, this study has shown that non-diapause development is characterized by a last inetar period of two distinctive behavioural phases whereas diapause development has only one behavioural phase. Also, diapause developmentin . Buseeala fusca lasts up to the pre-pupal stage of the last instar. In addition, type-A neurosecretory cells stain lesser with advance of time in non-diapause and diapause development during the last instar while the presence of glycogen in the corpora allata of the diapause larvae signified storage of metabolites to be used in during the long period of diapause. Ecdysone titers in the haemolymph during non-diapause and diapause development were quite normal and so were the JH titers in non-diapause development. However, the persistent high titers of JH in the haemolymph during diapause development indicates that diapause development in Busseola fnsca is primarily controlled by the juvenile hormone. Thus, both ecdysone and juvenile hormone are involved in the regulation of the type of development which occur during the last larval instar of Busseola fusca.