Atratividade do parasitóide Lysiphlebus testaceipes (Cresson, 1880) (Hymenoptera: Braconidae, Aphidiinae) por plantas de trigo (Triticum aestivum L.) submetidas à adubação com silício e herbivoria por Rhopalosiphum padi (Linnaeus, 1758) (Hemiptera: Aphididae)

Abstract

The aphid parasitoid Lysiphlebus testaceipes (Cresson) is an important biological control agent of Rhopalosiphum padi (Linnaeus) and the use of silicon (Si) in plant nutrition induces resistance to aphids in wheat. The female parasitoids are attracted to chemical cues (volatiles) released by plants in the process of herbivory. However, the release of volatiles by plants can be changed also by abiotic factors, such as Si fertilization. In order to determine the compatibility of biological control with parasitoids and the induction of resistance by Si in the control of wheat aphids, this study evaluated the influence of Si fertilization and herbivory caused by R. padi in wheat plants on the olfactory response of L. testaceipes females. Silicon fertilization was done when the plants reached the Z07 growth stage, by applying 0.350 g silica gel (5.7% available silicon) to the soil, corresponding to 300 kg ha-1 of Si. Leaf Si content of plants that have not received and that were fertilized with this nutrient was determined. Herbivory induction was performed when plants reached the Z13 stage by infesting them with 40 R. padi aphids from different instars, which remained on the plant for 24 hours. After this period, aphids, as well as their exuviae, were removed from the plants. Bioassays of female attraction of the parasitoid L. testaceipes (24-48h old, mated and without oviposition experience) were performed in a \"Y\" olfactometer. Wheat plants were used with the following treatments: not fertilized with Si and undamaged by R. padi, not fertilized with Si and damaged by R. padi, fertilized with Si and undamaged by R. padi, and fertilized with Si and damaged by R. padi. Wheat plants fertilized with Si had 1.7 times (0.27%) more Si in the leaves than plants without Si fertilization (0.16%). Lysiphlebus testaceipes females did not respond to non-fertilized and undamaged plants. However, the parasitoids were attracted to plants not fertilized with Si and damaged by R. padi, to plants fertilized with Si and undamaged by R. padi and to plants that received the two stimuli: Si fertilization and herbivory. When treatments that had attracted parasitoids were compared, there was no preference of L. testaceipes by none of them. Positive responses were observed for plants fertilized with Si, regardless of herbivory, suggesting that volatiles released by these plants are similar to those of plants attacked by aphids. Silicon fertilization did not affect attraction of parasitoids by plants subjected to herbivory. Thus, induction of resistance by Si fertilization may be considered compatible with the biological control of aphids by parasitoids.