Diamondback moth egg adhesion to cabbage plants: structural, chemical, and mechanical aspects

Abstract

The diamondback moth, Plutella xylostella, has developed strategies to overcome the challenging waxy surfaces of plants. Females can lay their eggs on pruinose Brassicaceae plants using a secretion from their colleterial glands, which acts as an egg adhesive. The present microscopic analyses, along with contact angle and force measurements, show that this secretion wets hydrophilic glass surfaces significantly better than hydrophobic ones, forming superthin layers with limited volume. Consequently, the pull-off forces required to remove the eggs are significantly greater on hydrophilic glass (23 mN) compared to hydrophobic glass (2 mN), indicating adhesive strengths of 198 and 29 kPa, respectively. The safety factors, which indicate how many times the weight of the egg (23 µg) corresponds to the pull-off force, are remarkably high: 101,689 for hydrophilic surfaces and 8517 for hydrophobic ones. Egg adhesion to plants varies depending on plant surface structures. Pull-off forces significantly decrease with the increasing number of plant epicuticular wax crystals. For example, safety factors measure 1795 on young adaxial white cabbage leaves and reach as high as 25,461 on the petioles of older Chinese cabbage leaves. This attachment ability is facilitated by the predominantly protein and lipid composition of the egg adhesive, alongside the structural matrices created by plant wax crystals and trichomes embedded within the adhesive. Raman spectroscopy of the untreated solidified egg adhesive reveals characteristic amide I and III bands, a β-sheet structural motif, and the presence of aromatic amino acids phenylalanine and tyrosine, as well as saturated fatty acids. Based on a comprehensive discussion with previous findings, we propose that there is a trade-off between secure egg adhesion and the selection of oviposition sites that match the offspring’s preferences and provide enemy-free spaces. Understanding P. xylostella’s egg adhesion mechanisms and the characteristics of the adhesive substance may contribute to the improvement of pest control strategies, including physical measures, and the advancement of bioinspired adhesives. Moreover, our study should stimulate future integrative and multidisciplinary research on insect egg adhesives, promoting a comprehensive understanding from various perspectives.