Fracture behaviour of plant epicuticular wax crystals and its role in preventing insect attachment: a theoretical approach

It is known that attachment ability of insects depends among others on the texture of the substrate. On plant surfaces, covered with microscopic epicuticular wax crystals, insect attachment was found to be highly reduced for many insect species. In some plants, this effect may be explained by the contamination of insect adhesive organs (pads) by wax crystals. In the present study, mechanics of the wax crystals fracture during contact formation between insect adhesive pads and plant surface is examined, in order to explain the observed contamination of pads by wax. It is shown that mechanisms of the wax crystal fracture may be rather different, depending on the slenderness ratio of the crystals. Crystals with high values of the ratio may buckle elastically or in an elastic-plastic way. For five plant species under consideration, the critical value of the slenderness ratio for elastic buckling is 26.5, while for elastic-plastic buckling it is 18.7. If the values of the slenderness ratio are lower than the critical values, one has to consider bending of the crystals under the weight of insects. Although this study considered only crystals of a tubular shape, the general approach is valid also for crystals of other shapes