The effects of azadirachtin and analogues upon feeding and development in locusts
The antifeedant and insect growth regulatory (KGR) activities of azadirachtin, its natural analogue 3-tigloylazadirachtol and its synthetic analogues 22,23-dihydroazadirachtin and 22-alpha-bromo-22,23-dihydro-23-alpha,beta-ethoxyazadirachtin were investigated using different bioassays on Schistocera gregaria (F.) and Locusta migratoria (R &38 F). The effects of azadirachtin on midgut histology were studied at the light microscopical and electron microscopical level. Light microscopy revealed that azadirachtin affected midgut histology in a dose and time dependent manner. Increasing effects were seen at doses of 5-15 mug g-1 body weight. With a dose of 1 mug g -1, effects were apparent but recovery occurred after three days. Azadirachtin caused the normal elongated midgut epithelial cells to become cuboidal; thickening of the connective tissue with invading regenerative cells. The gut musculature was swollen. Electron microscopy showed the midgut cells to undergo necrosis with swelling of the mitochondria and endoplasmic reticulum, reduction in glycogen granules and increaesd autophagic vacuoles. Bursting and disruption of the cell contents was sometimes seen. Such effects were not seen in starved insects. Feeding activities, measured in terms of antifeedant indices (AI) and consumption rates (CR) in binary choice bioassays on fifth instar nymphs demonstrated dose-dependent variations for all the compounds in both species of insects. S.gregaria was more discriminatory and more sensitive to azadirachtin and its analogues than was L.migratoria. ED_50's were measured for all compounds tested. Ingestion rates of the compounds showed a dose-dependent decrease with increasing dose. The highest intake of the compounds was observed mostly at an AI of 50-60&'37. The short- and long-term effects of topical application and low doses (0.1 and 1.0 mug g-1 body weight) of azadirachtin, the analogues and an additional unidentified compound X (related to bromeothoxyazadirachtin) were measured in S.gregaria. The effects on growth were reduced body weight and reduced feeding as measured by faecal pellet production. Azadirachtin induced the highest mortality and most deaths occurred within the first few days of treatment; the analogues were less acutely toxic. Mortalities at the moults were also dose related. In all the insects which survived, none of the compounds exerted any significant effects on dry body or ovary weights, egg size or fecundity. Thus once toxic effects were overcome, the insects matured and reproduced normally; none of the compounds had any long term accumulative effects.