Fertilização com silício altera a defesa direta e indireta da planta de milho (Zea mays L.) contra a lagarta-do-cartucho Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae)

Abstract

Silicon (Si) has been used in agriculture as a tatic to enhance plant resistance against abiotic and biotic stress, such as water deficit, pathogen and insect pests. At first, researchers believed that deposit of Si on leaf epidermal cells provided a mechanical barrier against herbivory. Nevertheless, Si amendment has been shown to activate biochemical chain events involved in the synthesis of plant defensive compounds against herbivory, including herbivore-induced plant volatiles, which recruit pest natural enemies. This novel piece of information indicates that Si amendment is a potential tactic for integrated pest management given that deposit of Si in the plant not only increases direct defenses, but can also be used together with biological control. In this context, the goal of this work was to test Si as an elicitor of direct and indirect plant resistance in corn against the main pest Spodoptera frugiperda. Si was given to plants through drench method as Silicic Acid at 1% and dosage of 0.93 ton Si ha -1 (2 ton SiO2 ha -1 ). To assess plant direct defenses, parameters of the early stages of developmental cycle were determined, such as host preference for oviposition, mortality and leaf consumption of S. frugiperda neonate caterpillars, as well as weight gain by third instar caterpillars. Indirect plant defenses were assessed by examining olfactory response of the minute pirate bug Orius insidiosus, a predator of S. frugiperda neonates, towards odors emitted by corn plants either infested, or not, by first instar caterpillars. In dual choice assays, S. frugiperda female adults preferred non-fertilized plants (Si-) over Si-fertilized plants (Si+) as hosts for oviposition. Despite S. frugiperda neonate equally consumed leaves from Si- and Si+, larval mortality was higher in Si+. Third-instar caterpillars that fed on Si+ plants gained less weight than those fed on Si-. The minute pirate bug did not orient to constitutive corn plant volatiles, and fertilization with Si did not trigger in uninfested plants the emission of attractive volatiles. Expectedly, O. insidiosus was attracted to volatiles of neonate-infested plants over uninfested ones, irrespective of Si fertilization. Nonetheless, O. insidiosus exposed to volatiles of Si-and Si+ plants, both infested, preferred the herbivore-induced volatile blend emitted by Si+. Therefore, this work indicates that Si amendment in corn plants shows a great potential to reduce crop colonization by the fall ar myworm as females prefer to deposit eggs on non-fertilized plants, and neonate mortality is high due to direct and indirect (recruitment of minute pirate bug) induced by Si.