Juniper scale insect Carulaspis juniperi Hydrangea scale Pulvinaria hydrangae on lime (Tilia sp)
Scale insects suck sap from foliage, usually on the underside, and from plant stems. Some species, such as cushion scale and soft scale, excrete large quantities of a sugary excrement called honeydew. This makes the upper surface of leaves sticky and allows the growth of sooty mould. Newly hatched scales will crawl around to find suitable feeding places but then remain stationary for most of the rest of their lives.
Reproduction and life cycle
In many species of scale insects, males are rare or non-existent, with the females reproducing asexually
. Where both males and females occur, as in euonymus scale, the two sexes often differ in size, shape and colour. When mature, adult males are small winged insects that emerge from under their scales to mate with the wingless females. The males often only live for a few days as adults.
Most scale insects deposit their eggs underneath their bodies and the covering scale. Pulvinaria species, however, place their eggs in a white waxy secretion that can form a distinctive mound or band behind the scale. Some species have several generations a year but most scales on outdoor plants have a single generation, with eggs laid in late spring and hatching in early summer. The newly hatched nymphs crawl around for a while before settling down to feed. Most scale insects overwinter as young nymphs but some overwinter as adults.
Role of scale insects in gardens
Plants can generally cope with light scale insect loads however heavy infestations of scale insects can weaken plants to the extent of causing dieback. Those species that produce honeydew
spoil the appearance of plants, especially if sooty mould develops on the foliage. Fortunately, in most years infestations are at a relatively low level and do not have a damaging impact. Some ladybirds specialise in feeding on scale insects and their numbers are also reduced by various parasitic wasps.
Other sources of information
Malumphy, Chris & Badmin, John. (2012)
. Scale insects and whiteflies (Hemiptera: Coccoidea and Aleyrodoidea) of Watsonian Kent; with a discussion on the impact of naturalised non-native species. British Journal of Entomology and Natural History. 25. 15-49.
Page drafted by Andrew Halstead, reviewed by Andrew Salisbury, edited by Steve Head