Development and determination of larval stages of fall armyworm on some staple crops.
Fall armyworm (FAW), Spodoptera frugiperda Smith (Lepidoptera: Noctuidae), is a serious emerging pest of maize (Zea mays L.) and many other alternative crop hosts in sub-Saharan Africa. Adequate knowledge on the development of S. frugiperda on maize and other alternative host crops is important in the development of integrated pest management programmes. The objective of this study was to determine the larval developmental stages of FAW using head capsule and other body morphometrics of FAW on maize and other alternative host crops in Nigeria. Food hosts (maize, cassava and cowpea) were replicated five times and arranged in a Completely Randomised Design. The results showed that mean growth ratio of larval development on maize, cassava and cowpea were 1.51, 1.54 and 1.50, respectively; and all conformed to Dyar's rule. Head capsule width of larval instars showed six frequency peaks, representing six larval instars. Mean width of head capsule from the first to sixth larval, in the three crops, were significantly different. The shortest (14 days) and longest (17 days) developmental periods were recorded on maize and cassava. Pupal weight and length were not significantly different among the crops. There was a linear and significant correlation (maize = 0.98, cassava = 0.98 and cowpea = 0.99) between the stages of larval development and head capsule width. The number of larval instars of FAW, duration of their developments and the weights of larva and pupa on maize, cassava and cowpea are useful information in determining the number of generations of FAW on each crop. This information could, therefore, be applied in decision making on the appropriate time and duration of application of control actions when these crops are infested.