Effect of tannic acid on the development and resistance of the Gypsy moth Lymantria dispar L. to viral infection

Abstract

It is known that crop plants’ induced resistance to phyllophages, which is due to accumulation of secondary metabolites, decreases the viability of phyllophages and changes the sensitivity to entomopathogens [1]. One of the most important classes of compounds that are involved in defensive responses of deciduous plants is phenols, in particular, flavonoids and polymeric tannic compounds (tannins). Results of experiments show that correlation between the content of tannins and the viability of phyllophage depends primarily on the specific characteristics of the insect. Today, data on the effect of tannic acid on the sensitivity of insects to viral infections are almost absent, although it is known that viruses can markedly influence the populational dynamics of many species of phyllophagous insects, including the gypsy moth Lymantria dispar L. (Lepidoptera, Lymantriidae) [2]. The latter can regularly form foci of mass-scale reproduction nearly everywhere in Russia, except for the northern regions. It is known that tannins, including tannic acid, can form complexes with proteins; when oxidized to quinones in the midgut of insect, they can produce semiquinone radicals and other forms of reactive oxygen species [3]. In the course of evolution, phytophages, in turn, developed a number of adaptations with respect to the secondary metabolites (allelochemicals) of plants that attenuate the deleterious effect of allelochemicals on insect’s organism. These adaptations include behavioral adaptations, physiological isolation, and metabolic transformation [4]. A key role in detoxication and elimination of many xenobiotics belongs to the antioxidant system of insects, which include both enzymatic and nonenzymatic antioxidants. The second group comprises high- and low-molecular-weight thiol-containing compounds [5]. The proportion of these compounds in the midgut of insects provides information on the processes of free radical peroxidation in insect organs and is an important criterion of the so-called oxidative stress [6]. In view of this, the goal of this study was to investigate the effect of a phenolic compound, tannic acid, on the development of L. dispar and its sensitivity to nuclear polyhedrosis. The ratio between the concentrations of oxidized and reduced thiol-containing compounds in insect midguts—an index reflecting the antioxidant status of the insect—was used as a criterion of the effect of this allelochemical on insect’s organism.