Oxidative stress in herbivorous insects is caused by reactive oxygen species (ROS), such as superoxide anion radical (O_2­­^─), hydrogen peroxide (H­­₂O­­­­­₂) and hydroxyl radical (×OH) that are produced by endogenous sources (autoxidizable molecules, oxydoreductases, electron transport) and exogenous ones (pro-oxidant compounds, photodynamic reactions). Most of the exogenous factors belong to pro-oxidant allelochemicals produced by plants, including o-dihydroxyphenols, that may be activated by one-electron oxidation to a very reactive semiquinone radicals, which in turn react with O₂ to generate reactive oxygen species. All the activated forms of oxygen react with macromolecules such as DNA, RNA, proteins and lipids causing alterations within their structures. In response to pro-oxidant properties of the plant allelochemicals, phytophagous insects have evolved specific defence mechanisms, especially towards the reactive oxygen species. Among them an important role play antioxidant enzymes system composed of superoxide dismutase (SOD), catalase (CAT), Se-independant glutathione peroxidase (GSTPX) and glutathione reductase (GR). The aim of this study was to compare level of the GSTPX and GR activies within tissues of two species of cereal aphids - monophagous grain aphid, Sitobion avenae (F.) and oligophagous bird cherry-oat aphid, Rhopalosiphum padi (L.). In addition, an influence one of the most important group of plant pro-oxidants, o-dihydroxyphenols on the aphid enzymes activity was studied.

Among the studied aphid morphs the highest activity of GSTPX and GR was noted for winged adults (alatae) and the lowest for wingless females (apterae). Generally, higher activity of the Se-independant glutathione peroxidase and glutathione reductase was observed within oligophagous species R. padi that alternate between woody host-plants and grasses. Exposition of the aphids to plant o-dihydroxyphenols caused decrease in activity of the both glutathione enzymes. Among the studied plant allelochemicals caffeic acid showed the strongest inhibitory effect on activity of the aphid antioxidant enzymes. The influence of the o-dihydroxyphenols on activity of the GSTPX and GR within the cereal aphid species is disscused.

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