Toxic Effects of Sublethal Pyriproxyfen on Baseline Hemolymph Biochemical Balance in Euschistus heros

The sublethal toxic effects of pyriproxyfen, an insect juvenile hormone analog (JHA) insecticide, on the circulating metabolite balance in the Neotropical brown stink bug, Euschistus heros, one of the main agronomic pests in South America, were investigated. Our objectives were to evaluate changes in the baseline levels of lipids and carbohydrates, along with three selected micro-elements—nickel (Ni), copper (Cu), and zinc (Zn)—in the hemolymph of adult E. heros, following the application of a predetermined LC30 of pyriproxyfen (single topical application) in fourth-instar nymphs (N4). Hemolymph was sampled using glass capillaries, with the resulting concentrations of lipids and carbohydrates determined through vanillin- and anthrone-positive reactions, respectively, and micro-element analysis was performed through total reflection X-ray fluorescence (TXRF) spectrometry. Lipids are the main and more stable energy metabolites for E. heros, with a proportion of 2:1 compared to carbohydrate levels. A remarkable sensitivity of carbohydrate levels to sublethal pyriproxyfen exposure, irrespective of sex, was observed. Baseline micro-element levels based on untreated control insects indicate sex-based differences in Ni and Zn, but not in Cu, concentrations. After insecticide exposure, the levels of these micro-elements were variable, with Ni and Zn generally decreasing and Cu decreasing in females but nearly doubling in males. The observed disproportion in lipids, carbohydrates, and inorganic micro-elements suggests potential physiological shifts triggered by pyriproxyfen activity in E. heros during late juvenile stages.