Interspecific predation of a native herbivore facilitates colonization by fall armyworms, Spodoptera frugiperda

Spodoptera frugiperda is a newly invasive pest in China, often sharing the same feeding niche with native S. exigua. To clarify the competitive displacement, colonization and potential threat to crops by S. frugiperda, the cannibalism and predation behavior, feeding preferences, development and reproduction of both species under laboratory conditions, as well as the population dynamics and damage to maize plants in the field, were studied. Results found that food scarcity intensified interspecific predation between S. frugiperda and S. exigua, but the survival rate of 4th instar S. frugiperda was not significantly influenced (> 93%; P > 0.05). Although S. frugiperda exhibited less aggressive behaviors, its cannibalism coefficient and attack intensity were significantly higher than those of S. exigua (P < 0.05). Moreover, S. frugiperda showed a higher feeding preference for mechanically lethal insects when the maize leaf supply was insufficient. Additionally, a combined diet of maize leaves and lethal insects significantly increased their pupal weight and fecundity (P < 0.05). Field trials showed that when S. frugiperda and S. exigua co-occurred, the population of S. frugiperda at mature stage was similar to that after the intraspecific treatment, and seriously damaged maize plants. These results suggested that interspecific predation by S. frugiperda on S. exigua not only conferred an obvious advantage, but may also promote its development and reproduction, and facilitated its colonization in the invaded area. Our results provide an understanding of the rapid colonization mechanisms of S. frugiperda and will assist development of integrated management strategies.