Many animals and plants have evolved elaborate water-repellent microstructures on their surface, which often play important roles in their ecological adaptation. Here we report a unique type of water-repellent structure on plant surface, which develops as an insect-induced plant morphology in a social context. Some social aphids form galls on their host plant, in which they produce large amount of hydrophobic wax. Excreted honeydew is coated by the powdery wax to form “honeydew balls”, which are actively disposed by soldier nymphs through an opening on their gall. These activities are enabled by a highly water-repellent inner gall surface, and we discovered that this surface is covered with dense trichomes that are not found on normal plant surface. The trichomes are coated by fine particles of the insect-produced wax, thereby realizing a high water repellency with a cooperative interaction between aphids and plants. The plant leaves on which the gall is formed often exhibit patchy areas with dense trichomes, representing an ectopic expression of the insect-induced plant morphology. In the pouch-shaped closed galls of a related social aphid species, by contrast, the inner surface was not covered with trichomes. Our findings provide a convincing example of how the extended phenotype of an animal, expressed in a plant, plays a pivotal role in maintaining sociality.