Larval cannibalism among conspecifics of Fall armyworm, Spodoptera frugiperda (Lepidoptera, Noctuidea) in Sri Lanka

Abstract

Cannibalism is a prevalent phenomenon with an ecological and evolutionary significance, seen in many non-carnivorous larval Lepidopterans. This intraspecific predation between the cannibal and the victim occur in conditions of low food quality and availability, confined space, large size disparity and high population density as well as due to significant changes in temperature and humidity of its living environment. Cannibalistic behaviour has been reported for larval Fall Armyworm (Spodoptera frugiperda)(FAW); an economically important pest worldwide. Therefore, 73 laboratory trials with not less than 5 replicates for each stage were conducted to determine cannibalistic behaviour among conspecifics of 1-6 FAW instar stages. Each laboratory reared larval stage was provided with their conspecifics along with sufficient food. Direct observations were carried out to record the missing larvae in 24 hours period and missing ones were presumed as cannibalized. Non-consumed potential victims were removed. Under laboratory conditions, larvae displayed cannibalism in different proportions even when adequate food was present. No cannibalism was observed in 1st and 2nd instar stages and the highest mean cannibalism proportion was recorded among 5th larval instars (0.22 ± 0.05). An increasing trend from 3rd (0.16±0.04) to 5th instar stages was evident in significance to the higher energetic requirement in later stages. But the trend dropped at the 6th instar stage (0.15 ± 0.10) which can be attributed to decreased feeding rate of larvae upon transition to pre-pupal stage. This study concludes the general cannibalistic behaviour of FAW larvae under laboratory conditions. Although, cannibalism is seemed as a costly process in terms of fitness of adult females who lay eggs in large clutch size, this can have a considerable impact on reducing the larval numbers in natural populations if viewed as a self-regulating measure of populations. Therefore, further studies on FAW larval cannibalism under field conditions are much required.