Turn over an alder leaf at this time of year and you may find what look like small lumps of cotton wool stuck to the leaf blade and leaf stalk. Watch for a while and they move around. They’re insects – the nymphal stage of alder psyllids Psylla alni, which feed on the sap of the plant. The source of the ‘cotton wool’, which is really a mass of waxy filaments secreted by the insect, becomes apparent when these aphid-sized insects are examined under the microscope. The wax is produced by glands at the tail, so a psyllid that’s just begun to produce this material looks like its tail is on fire. The wax is a defence against water loss and predators. The nymphs have stubby wing primordia that eventually develop into fully-formed wings, by which time they will have shed their ‘cotton wool’ covering, ready to take flight and disperse. Psyllids are commonly known as jumping plant lice and different species tend to be associated with specific plant hosts. The alder psyllid is one of the larger species in the UK.