Feeding by nymphs causes leaves (especially of giant knotweed) to twist and curl, providing further protection from predators and the elements (Figure 3-7c). Psyllid feeding also causes Japanese knotweed to produce more, but smaller leaves, with an overall reduction in total leaf area ... While feeding, nymphs excrete lerp, crystallized honeydew that is conspicuous as white strings and
flakes on the plant surfaces.

- Grevstad, Fritzi S. et al.: (2018) Biology and Biological Control of Knotweeds^©