Effect of Host Plant on Moulting in the African Armyworm Spodoptera Exempta (Walk.) (Lepidoptera: Noctuidae) At Constant Temperature and Humidity Conditions
There are five caterpillar instars in the life cycle of Spodoptera Exempta (Walk.) when they feed upon star grass, Cynodon daciylon (L.) Pers., maize, Zea mais L. and Kikuyu grass, Pennisetum eland. Jtinum Chiov. at 25–30°C. When the temperature is 18°C, however, there was an additional instar in the cycle. Similarly extra instars are formed on Guinea grass, Panicum maximum Jacq., Setaria plicatilis (Höchst.) Hack, and a sedge, Cyperus maranguensis K. Schm. even when the temperature appears to be sufficiently high for normal development. Occasionally the additional sixth instar is also too small to pupate, in which case a seventh instar is formed. It appears that caterpillars on nutritionally inadequate host plants or those at low temperatures are being partially starved and must continue moulting until the appropriate size is attained or exceeded. Measurements of the head capsule widths or distances between the frontal clypeal setae suggest that only individuals reaching or exceeding a species specific threshold size pupate. Extra moults appear to be an adaptation to environmental fluctuations obtaining within the range of occurrence of S. exempta.
Key WordsAfrican armyworm host plants head capsule widths distance between frontal setae variation in the number of instars
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