A study of the effects of the juvenile hormone analogue methoprene on the intermediary metabolism of the African migratory locust
Insect juvenile hormone (JH) regulates the growth and development of insects. Synthetic analogues of JH (JHAs) have been used as agents of pest control, disrupting the metamorphosis of insects. The purpose of the present study was to determine physiological and biochemical effects of methoprene, a commercial JHA, on certain organs and tissues of the African migratory locust (Locusta mi- gratoria migratorioides, phase gregaria). Methoprene was topically applied to newly moulted, fifth (final) instar larvae and the subsequent development of the animals was followed. Cytological development of fat body and dorsal longitudinal flight muscle was studied by light and electron microscopy. Fat body cells of control insects were synthetically active early in the fifth instar, and stored lipid and glycogen in the latter half of the instar. Fat bodies of 8-day old adults were sexually dimorphic, female cells showing high levels of RNA and protein synthesis while male cells were filled with lipid and glycogen stores. Methoprene treatment stimulated the synthetic activity of the cells in fifth instar and adult stadia, especially in female tissue. Cell nuclei were abnormally enlarged, suggesting increased ploidy levels. Levels of lipid and carbohydrate were measured in fat body and haemolymph but methoprene had no obvious effect on them, nor on glycogen phosphorylase activity. However, the JHA affected rates of incorporation of [(^14)C]glucose into fat body lipids during the first four days of the fifth instar. Separation of haemolymph proteins by gel electrophoresis revealed an extra protein band in the blood of treated female locusts from the middle of the fifth instar onwards. The same band appeared in the blood of control females only when they reached sexual maturity. Methoprene treatment disrupted normal development of dorsal longitudinal flight muscles during the fifth instar and early adult life. The JHA reduced muscle fibre growth but seemed to accelerate myofibril and mitochondrial differentiation in the fifth instar. Treatment also inhibited formation of interfibrillar tracheoles and caused disruption of the myofilaments and sarcoplasmic reticulum in adult muscle. Mitochondria were isolated from flight muscles of mature adults and their respiratory metabolism was measured using an oxygen electrode. Mitochondria from control animals showed high rates of oxygen consumption and good respiratory control. Mitochondria from treated locusts had poor respiratory control and low respiratory rates. Similar results were obtained by in vitro applications of methoprene or juvenile hormone to mitochondria.