Later instar nymphs are purplish-brown or reddish-brown, with white longitudinal stripes and black patches; the developing wing-pads each bear a single knobbed bristle.

[4] It was introduced accidentally into North America, being observed in Connecticut in 1832 and arriving in Washington State by 1939, soon becoming a serious pest of pears in the Pacific Northwest.

[8] Psylla pyri damages pear trees by sucking the plant sap; leaves are yellowed and distorted and flower buds and fruitlets are shed.

The excess honeydew produced by the insects coats the leaves, covering up the stomata, and encourages the growth of sooty mould.

Failure of nutrients to be translocated downwards can cause root starvation, with trees either declining slowly or suffering from sudden collapse.