ARMOURED SCALE INSECTS -
REDUCTIO AD ABSURDUM?

We, together with other members of the consortium (White Admiral 62), are trying to find out about the relationships between kidney spot ladybirds and their prey, armoured scale insects, probably Euonymus scale (Unaspis euonymi), which have colonised some spindle bushes in Westleton.

In the case of the scale insect we have embarked on a steep learning curve. It is a member of the Homoptera (Insecta > Hemiptera > Homoptera > Coccoidea > Diaspididae) and there is very little written about them; the only British book was published more than 100 years ago (Newstead, 1901). Eggs are laid under the scales, and only the newly hatched young scale insects, known as crawlers, move actively over plants and spread infestations. The crawler establishes its feeding site, inserts a feeding tube and never moves again. As it grows it forms a waxy scale that completely covers the body. It develops through three instars, the third of which is the adult, looking to all intents and purposes like a larva. At present we are studying the over-wintering stage, the adult female, a very strange insect, which is without legs, eyes, antennae or wings and, unlike a typical adult insect, is not clearly demarcated into head, thorax and abdomen.

With so few features it is difficult and time-consuming to identify; indeed neither the Natural History Museum nor the Central Science Laboratory could find time in their busy schedules to provide a positive identification. It is probably the Euonymus scale (Unaspis euonymi), which is spreading northwards, but the spindle bush may also be a host to the willow scale (Chionaspis salicis), which is the most common prey of the kidney spot ladybird.

Identification depends on the arrangements of small pores and bristles on the exoskeleton of the adult female insect. To show these up involves a ‘time-consuming and tedious’ staining method …but tenax propositi!

An intriguing feature of the insect is its manner of feeding. When carefully prized from the plant it is sometimes possible to see the very fine feeding tube, about 5 micrometres in diameter and up to 1.2 millimetres long, although since it is so fragile and breaks easily that may not be full length. The insects colonise not only young fresh twigs but also five-year-old stems with mature bark. We have an ambition to find out which plant issues are penetrated and to what depth, but like everything else to do with the insect, this may not be easy.

Reference
Newstead, R. (1901) Coccidae of the British Isles, Volume 1.

Michael Kirby & David Walker