Alan Rodger, of Twizel, asks :-

What is the odd growth on this Photinia hedge? It starts with a brown coloured scale which then bursts ejecting a very sticky white substance which is very elastic. Why would this suddenly appear, and will it spread to other photinia around town?

Paul Guy, a botanist at the University of Otago, responded.

The photinia is being attacked by an insect called white wax scale.

At early stages of its development when it is settling down to feed on host sap it has a scaly appearance but as it grows and feeds it secretes a white sticky covering which protects the insect from predators and environmental extremes. It is hard to control at this stage but you may have some success with conqueror oil or dishwashing detergent.

The white strings would have been produced (artificially) by touching a leaf to the sticky mess and letting it spring back to its natural position. White scale don’t fire off their protective coating.

The mature adults can produce hundreds of offspring which move from the protection of the white sticky mass to find fresh feeding sites; these are more susceptible to control measures. The scale insect is a member of the family Hemiptera which includes psyllids, aphids, planthoppers and leafhoppers. Many of these cause damage by feeding and reproducing on ornamental and crop plants and some also transmit virus diseases which cause millions of dollars damage to horticulture and agriculture each year.