This insect causes the leaves to become cup-shaped and there may also be splashes of a waxy white material apparent on the foliage. In most cases the damage can be tolerated.
This moth has become established in south east England, and its caterpillars can completely defoliate plants.
Box leaf-mining gall midge
The box leaf-mining gall midge, Monarthropalpus flavus is an uncommon problem in the UK but sometimes heavy infestations occur. This tiny fly deposits its eggs in the new leaves during late April - May. The larvae feed inside the foliage, causing a yellowish discoloration on the upper leaf surface. The lower leaf surface develops a slight swelling in the area affected by the larva's feeding. The yellow larvae are up to 3mm long and they feed inside the leaves during summer – winter before pupating within the mines in spring. Several mines can develop in a leaf and heavily damaged ones often drop off.
Whilst the damage caused can be unsightly it rarely affects the vigour of plants and light infestations can be tolerated and control is not always necessary. It is not easy to control the older larvae and pupae as they are protected within the foliage. A systemic neonicotinoid insecticide, such as acetamiprid (e.g. Bug Clear Ultra), applied in early May may prevent the young larvae developing.
Mussel scale, Lepidosaphes ulmi, is a sap-sucking insect that attacks a wide range of woody plants, including box, apple, hawthorn, ceanothus, cornus and cotoneaster. The blackish-brown shells or scales are shaped like mussels, up to 3mm in length, and are attached mainly to the bark but sometimes they spread to the foliage. Heavy infestations can result in plants dying back.
Box red spider mite
A fine whitish mottling on the foliage of box plants can be caused by the box red spider mite, Eurytetranychus buxi, which is a specific pest of box. This tiny creature feeds by sucking sap from the undersides of the leaves, particularly during spring and early summer. By late summer infestations die out and the mite overwinters as eggs, which are laid on the stems and underside of leaves.
Box tree red spider mite is difficult to control. Fortunately, although the mottling may be considered unsightly, this pest does not cause serious damage to the plants and so it can be tolerated. If treatment is considered necessary mite numbers can be reduced by spraying the undersides of the foliage thoroughly with plant oil sprays (organic e.g. Vitax Organic Pest & Disease Control or BugClear for Fruit & Veg Gun!), or fatty acids (organic e.g. Bayer Bug Free or Doff Greenfly and Blackfly Killer). Applications on three or four occasions at about five-day intervals when signs of renewed feeding damage are seen on the new foliage may be necessary to control newly hatched mites and break the life cycle.
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When the leaves take on distinctive yellow tips or an orange or bronze coloration, it suggests the plants are under environmental stress.
Many shrubs can suffer brown leaves. Trying to diagnose the problem as soon as possible may help save the plant.