The signals are substrate-borne vibrations, generated by the abdomen and transmitted through the woody stem rather than the air. Recorded with a laser vibrometer or a contact microphone, they are elaborate and species-specific, and they are inaudible to a person standing beside the plant.
In Umbonia crassicornis the nymphs feed in a group on a twig with the mother nearby. A nymph attacked by a wasp signals, neighbouring nymphs join, and the combined signal is graded, becoming stronger as more nymphs contribute. The female orients along the branch to the vibrating group and defends it by fanning her wings and pushing at the predator.
One widely repeated claim about treehoppers deserves care. A 2011 study proposed that the ornate pronotal helmet is a modified third pair of wings. That interpretation was contested within a year on developmental and anatomical grounds and is not accepted.