Browsing by Author "Njau, Marcelian Anton"

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    The neuroendocrine regulation of larval development in the maize-stem borer, BusseolaFusca (fuller) (Lepidoptera: Noctuidae)
    (University of Dar es Salaam, 1989) Njau, Marcelian Anton
    Busseolafusca {Lepidoptera:Noctuid 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, corpus allatum and prothoracic glands. Endocrine involvement in the regulation of development during the last instar larvae and in induction, maintenance and termination of diapause development were also investigated. Studies on the biology of the last larval instar showed that two types of development, namely, non-diapause and diapause occur in Busseolafusca. The non-diapause form of development is characterized by the feeding phase and post feeding phase and the diapause form of development is characterized by the feeding phase and post-feeding 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 neurosecretory 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 diapause larvae. The corpora allata in non-diapause development had cells which completely lacked glycogen-like deposits which were found allata of diapause larvae. The mitochondria of the cells of the corpora allata in non diapause development were not conspicuous and did not contain dense bodies. On the other hand, the mitochondria of the cells of the corpora 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 titres of juvenile hormone in non-diapause larva (as determined by Dysdercusbioassay}, was very low the average score was 0.8 out of 3.0, while that in the diapause larvae was high (average score of 2.8). The titres of the moulting hormone in non-diapause last instar larvae were 170, and 500ng/ml on days one, four and seven respectively; in diapause larvae the titre was generally low with a monthly mean titre 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 titres 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 observations indicated that diapause development was not due to deficiency of moulting hormone. Diapause can be initiated and maintained by high titres of juvenile hormone. Precocene showed no allatocidal effect on the corpus allatum of the diapause larvae. Fluoromevalonate treatment of the late diapause larvae at a dosage of l0ug 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 in the last larval instar is characterized by two distinctive behavioural phases whereas diapause development has only one behavioural phase. Also, diapause development in Busseolafusca lasts up to the pre-pupal stage of the last instar. In addition, type-A neurosecretory cells stain less 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 the storage of metabolites for use during the long period of diapause. Ecdysonetitres in the emolymph during non-diapause and diapause development were quite normal and so were the JH titres in non-diapause development. However, the persistent high titres of JH in the haemolymph during diapause development indicates that ease development in Busseolafusca 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 Busseolafusca.