Plants affected
Horse chestnut, lime, elm, sycamore and other Acer species, magnolia, Cornus and some other ornamentals, including bay trees.
Symptoms
First found in the London area in the late 1960s, this scale
insect is now widespread and common in southern
England, and it has also spread to northern England. It is particularly noticeable on the branches and
trunks of its host plants in early summer when the distinctive
white egg masses are being produced (see right).
The heaviest infestations generally occur on plants growing in warm microclimates, such as trees planted in roadside pavements, car parks or growing on patios. It causes little damage to its host plants but heavy infestations look unsightly.
The fully developed scales are dark brown, flattish, round and about 4mm across. They perch on the edge of white egg masses which are deposited on the bark. The immature scales or nymphs on the foliage in mid-summer are pale yellow and oval in shape.
Biology
The eggs hatch in late June/July. The young nymphs move on to the foliage where they suck sap from the underside of the leaves. Before leaf fall they move to the branches and trunk where they overwinter as immature nymphs. They complete their development in the spring and begin depositing their eggs on the bark in late April to May. There are several other species of scale insect that also produce white egg masses.
Control
Because this scale has little effect on the health or vigour of plants, it is not something that has to be controlled. This is fortunate as many of its host plants are likely to be too large to be sprayed without access to sprayers more powerful than those normally used by home gardeners. It is also difficult to achieve good control of scale insects with the limited range of pesticides available to gardeners nowadays.
On small plants it is feasible to scrape or wipe the mature scales and their eggs from the stems in April to May in order to stop the next generation developing, but on large trees the scale has to be tolerated.
Plants that are small enough to be sprayed could be treated with imidacloprid + sunflower oil (Provado Ultimate Bug Killer concentrate), acetamiprid (Scotts Bug Clear Ultra concentrate) or thiacloprid (Provado Ultimate Bug Killer Ready To Use) in early July when the more vulnerable young nymphs are present.