Mechanical properties, polymerization, and humidity effects on the egg glue of the Southern green stink bug, Nezara viridula L. (Hemiptera: Pentatomidae)

The Southern green stink bug Nezara viridula, a significant pest affecting various plants, secretes an egg glue that secures eggs together to diverse substrates. This study investigates the mechanical properties of egg glue, its polymerization time, the development of adhesion strength during polymerization, and the effect of humidity. Fourier transform infrared spectroscopy (FT-IR) of the polymerized glue indicated a proteinaceous nature, consistent with a β-sheet structure and C-O related vibrations associated with carbohydrates. Brillouin micro-spectrometry revealed rapid polymerization within minutes of air exposure, accompanied by a time-dependent increase in adhesion strength as measured with a force sensor. Once polymerized, the glue was analyzed to obtain mechanical maps, which revealed a micrometric-scale mechanical heterogeneity, with ∼2% variations in Brillouin frequency shift between thicker and thinner regions, accompanied by corresponding Raman spectral differences indicating variations in chemical composition. Nanoindentation measurements showed that the polymerized glue exhibits high stiffness, approximately twice than that of polymerized chicken egg albumen. Furthermore, when the glue was subjected to various humidity levels during and after polymerization, no significant differences in adhesion strength were observed. Collectively, these findings shed light on the mechanical design of this natural adhesive and lay the groundwork for further research on biomimetic adhesive development and insect egg adhesion mechanisms. STATEMENT OF SIGNIFICANCE: The natural egg glue of the Southern green stink bug Nezara viridula is a biological adhesive that firmly attaches eggs to various surfaces. This study investigated its polymerization time, development of adhesion strength, elasticity, hardness, humidity response, and chemical structure using FT-IR spectroscopy. The polymerized glue exhibits high stiffness and microscale mechanical heterogeneity and a proteinaceous network with β-sheet structural features. These findings provide a fundamental insight into the structure-function relationship of this natural adhesive, contribute to understanding insect adhesive systems, and provide a basis for future studies on biomimetic adhesives and insect egg adhesion mechanisms.