Responses of Spodoptera frugiperda and Trichogramma pretiosum to Rice Plants Exposed to Herbivory and Phytohormones.

Abstract

Damage caused by herbivorous insects and application of phytohormones can activate signaling pathways, which result in greater production of secondary metabolites, increasing plant defenses. This study aimed to evaluate the induced direct resistance (local and systemic) of rice plants caused by herbivorous insects and exogenous application of methyl jasmonate (MJ) and salicylic acid (SA) in the development of fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae). Moreover, indirect defenses through chemotactic responses of Trichogramma pretiosum (Riley) (Hymenoptera: Trichogrammatidae) submitted to the same treatments were investigated. Direct defense was evaluated by measuring weight gain of fall armyworm larvae fed with leaves of plants previously exposed to herbivores or treated with MJ (2 and 5mM), SA (8 and 16mM), or control. Indirect defense was verified by chemotactic behavior of wasps in two-choice olfactometer tests to plants exposed to herbivores and evaluated after 24, 48, and 72h in comparison with undamaged ones, as well as plants treated with the same phytohormones contrasted with the control. The gain of weight was reduced in immature developmental stage of S. frugiperda fed in leaves previously damaged by fall armyworm and in newly formed leaves after damage to the plants, comparing with control. Leaves treated with MJ (2mM and 5mM) and SA (8mM) were less eaten than those not treated. Parasitoids triggered a positive chemotactic behavior in rice plants that had been sprayed with same concentrations. This study showed that rice plants can activate direct and indirect defenses through an exogenous application of phytohormones.