Influence of food quantity, larval density and size of rearing container on intra-instar cannibalism in fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae)

The rearing of fall armyworm, Spodoptera frugiperda larvae in cohorts is usually associated with high levels of cannibalism-induced mortalities especially in the later instar larvae. To avoid this problem, rearing is done using an individual-based method that often requires relatively more man-hours, man power and laboratory space. To substantially reduce cannibalism-induced mortalities in cohort-based laboratory cultures of S. frugiperda , the primary factors influencing intra-instar cannibalism must be properly elucidated. In this study, the effect of food quantity, number of larvae reared (larval population density) and size of rearing container on cannibalism-induced mortalities was investigated at the fifth, early sixth and late sixth (fully mature) instar stages of S. frugiperda . Results showed that at each instar stage, larval population density rather than food quantity, significantly ( p 0.11) were observed between both factors. At the late sixth instar, the highest cannibalism-induced mortality (74.67%) occurred when 15-larvae fed only two grams of food were reared together. However, when food quantity increased to five grams, observed mortality was 52.00%. While higher mortalities were generally observed in small rearing containers, the size of larval-rearing container was not a significant ( p = 0.01) factor influencing cannibalism-induced mortalities at the three instar stages. The foregoing suggests that while food quantity may not be the most important factor influencing intra-instar cannibalism in S. frugiperda , it nonetheless plays a synergistic role with the number of larvae being reared, and is more important than the size of rearing container.