Behavior and developmental consequences of cannibalism in Spodoptera frugiperda

The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a highly destructive agricultural pest. The ability of S. frugiperda to survive and reproduce is widely recognized, but it is also important to consider the role of larval cannibalism as a significant biological trait. Understanding the cannibalistic habits of pests helps to explore the dynamics and evolution of their populations and facilitates the monitoring of insect outbreaks. Therefore, the stages, densities, and intensity of cannibalism in S. frugiperda larvae, as well as the consequences for their development, were investigated under laboratory conditions. Spodoptera frugiperda larvae showed obvious cannibalism after the third instar even in the presence of maize (Zea mays L., Poaceae) leaves; cannibalism was not observed in first instars. In the case of only one cannibal, the percentage of prey consumed by cannibalism decreased with higher prey density, and the predatory functional responses of fourth- to sixth-instar predators to first- to third-instar preys were fitted using the Holling II model. Compared with larvae feeding on maize only, larvae feeding on a diet of maize supplemented with third-instar conspecifics had a longer developmental duration and a higher body weight, with no significant differences in larval survival, pupation, or eclosion rates. Larvae supplied with conspecifics only, in the absence of maize leaves, generally had a lower body weight than larvae supplied with maize only, although developmental duration was still longer, and longevity and fecundity rates were lower for these larvae. Overall, consuming a small quantity of conspecifics in addition to maize leaves prolonged the developmental duration of larvae and pupae to some extent, increased the body weight, and did not impact the longevity or reproduction of S. frugiperda. However, cannibalism only, without access to maize leaves, had a detrimental effect on adult longevity and reproduction. These findings could serve as a valuable reference for studying the life history of insects that consume agricultural crops and may have potential applications for biological control.