Plant surfaces with cuticular folds are slippery for beetles

Abstract

Plant surfaces covered with three-dimensional (3D) waxes are known to strongly reduce insect adhesion, leading to slippery surfaces. Besides 3D epicuticular waxes, cuticular folds are a common microstructure found on plant surfaces, which have not been quantitatively investigated with regard to their influence on insect adhesion. We performed traction experiments with Colorado potato beetles on five plant surfaces with cuticular folds of different magnitude. For comparison, we also tested (i) smooth plant surfaces and (ii) plant surfaces possessing 3D epicuticular waxes. Traction forces on surfaces with medium cuticular folds, of about 0.5 mm in both height and thickness and a spacing of 0.5 -1.5 mm, were reduced by an average of 88 per cent in comparison to smooth plant surfaces. Traction forces were reduced by the same order of magnitude as on plant surfaces covered with 3D epicuticular waxes. For surface characterization, we performed static contact angle measurements, which proved a strong effect of cuticular folds also on surface wettability. Surfaces possessing cuticular folds of greater magnitude showed higher contact angles up to superhydrophobicity. We hypothesize that cuticular folds reduce insect adhesion mainly due to a critical roughness, reducing the real contact area between the surface and the insect's adhesive devices.

Leaf beetle attachment on wrinkles: isotropic friction on anisotropic surfaces

“…The effect has been explained by the reduction of setal contact area between small surface irregularities and the characteristic size and shape of setal tips . Similarly, males of Colorado potato beetles generated about 88% lower traction forces on hydrophobic plant surfaces with cuticle folds in comparison to smooth surfaces (Prüm et al, 2012). Folds were about 0.5-14m in height, 0.5-2m in width, had a distance of 0.5-15m, and roughly corresponded to the wrinkled PDMS surfaces that resulted in a decrease of traction forces of the beetles in our study.…”

“…The structure and performance of tarsal attachment devices in both beetle species have previously been well studied (e.g. Peressadko and Gorb, 2004;Schuppert et al, 2005;Voigt et al, 2008;Voigt et al, 2010;Gorb and Gorb, 2009;Bullock and Federle, 2009;Bullock and Federle, 2011;Clemente et al, 2010;Hosoda and Gorb, 2011;Prüm et al, 2012). Both species bear spatula-shaped lanceolate and filament-shaped tarsal adhesive setae in males and females, and additionally setae with discoid terminal tips only in males (Stork, 1980a;Pelletier and Smilowitz, 1987).…”

“…In addition, (4) in hierarchically, anisotropically structured surfaces of carnivorous plant traps and kettle trap flowers, cuticular folds in combination with downward pointing papillae or ideoblasts are assumed to interfere with insect attachment, probably by preventing claw hooking, and creating a micro-roughness, thus reducing the contact area of insect adhesive pads (Pohl, 1927;Ehler, 1974;Poppinga et al, 2010). In traction experiments with different plant species, traction forces of male Colorado potato beetles were reduced by an average of 88% on hydrophobic surfaces with isotropic folds of about 0.5-14m in height, 0.5-2m in width and a distance of 0.5-15m, compared with smooth plant surfaces (Prüm et al, 2012).…”

“…Such regular structures are present in the natural terrain of insects as cuticle folds on the surfaces of plant flowers, leaves, stems, fruits and seeds ( Fig.1) (Kurer, 1917;Barthlott and Ehler, 1977;Knapp, 1993;Koch et al, 2009;Prüm et al, 2012). There is a large diversity of folding patterns in plants (Bringmann and Kühn, 1955).…”

Surface roughness rather than surface chemistry essentially affects insect adhesion

“…Certain of these natural surfaces can effectively prevent wetting by water, while simultaneously protecting against attachment by insects by taking advantage of the same or very similar surface features [16–23]. Unfortunately, these natural anti-attachment properties have received relatively little attention from researchers working on surface science and engineering [24–25].…”