''Quadraspidiotus juglansregiae'', commonly known as the walnut scale, is a species of armoured scale insect in the family

Diaspididae Diaspididae is the largest family of scale insects with over 2650 described species in around 400 genera. As with all scale insects, the female produces a waxy protective scale beneath which it feeds on its host plant. Diaspidid scales are far mo ...

. It is native to North America where it feeds on a wide range of ornamental and forest trees and bushes.

Description

Adult and young female ''Quadraspidiotus juglansregiae'' are nearly circular, the insect being hidden under a tough flattish scale composed of the

exuviae In biology, exuviae are the remains of an exoskeleton and related structures that are left after ecdysozoans (including insects, crustaceans and arachnids) have moulted. The exuviae of an animal can be important to biologists as they can often b ...

from the first two nymphal

instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ...

s. There are small marginal notches between the prothorax, mesothorax and metathorax. The upper surface of the scale is pale grey with a reddish-brown spot, and the underside is yellowish. The adult female is about long while the nymphs are smaller. Male nymphs are similar, except that the scale is oval rather than round; after the last moult, the adult male emerges as a small, winged insects similar to a fly in appearance. Male nymphs sometimes burrow under the rim of a female scale forming a daisy-shaped group.

Distribution and habitat

''Quadraspidiotus juglansregiae'' is native to North America, where it occurs in Canada, the United States and Mexico. It has been found on at least forty genera of ornamental and forest trees and bushes, including

walnut A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''. Although culinarily considered a "nut" and used as such, it is not a true ...

maple ''Acer'' () is a genus of trees and shrubs commonly known as maples. The genus is placed in the family Sapindaceae.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 nd more or less continuously updated since http ...

liquidambar ''Liquidambar'', commonly called sweetgum (star gum in the UK), gum, redgum, satin-walnut, or American storax, is the only genus in the flowering plant family Altingiaceae and has 15 species. They were formerly often treated in Hamamelidaceae ...

tuliptree ''Liriodendron'' () is a genus of two species of characteristically large trees, deciduous over most of their populations, in the magnolia family (Magnoliaceae). These trees are widely known by the common name tulip tree or tuliptree for their ...

boxwood ''Buxus'' is a genus of about seventy species in the family Buxaceae. Common names include box or boxwood. The boxes are native to western and southern Europe, southwest, southern and eastern Asia, Africa, Madagascar, northernmost South ...

camellia ''Camellia'' (pronounced or ) is a genus of flowering plants in the family Theaceae. They are found in eastern and southern Asia, from the Himalayas east to Japan and Indonesia. There are more than 220 described species, with some controv ...

dogwood ''Cornus'' is a genus of about 30–60 species of woody plants in the family Cornaceae, commonly known as dogwoods, which can generally be distinguished by their blossoms, berries, and distinctive bark. Most are deciduous trees or shru ...

holly ''Ilex'' (), or holly, is a genus of over 570 species of flowering plants in the family Aquifoliaceae, and the only living genus in that family. ''Ilex'' has the most species of any woody dioecious angiosperm genus. The species are evergreen o ...

, cherry,

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and cultivated in Zhejiang province of Eastern China. It bears edible juicy fruits with various characteristics, most called peaches and others (the glossy-skinned, non-f ...

and plum.

Ecology

Second instar nymphs and adult female ''Quadraspidiotus juglansregiae'' pierce twigs and branches with their mouthparts and suck sap from the inner bark of the host tree. If there are large numbers of scales, the tree may become water stressed; soft shoots may droop, inner tissues may die back, and the bark may crack. A more serious consequence of infestation by the scale is that it may allow entry of pathogenic funguses of the genus ''

Botryosphaeria ''Botryosphaeria'' is a genus of pathogenic fungi in the family Botryosphaeriaceae. There are 193 species, many of which are important disease-causing agents of various important agricultural crops. Species *'' Botryosphaeria abietina'' *'' Bot ...

'', which can cause cankers, dieback and tree death. Natural

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

of this scale insect include the tiny beetle '' Cybocephalus californicus'', and the

twice-stabbed lady beetle ''Chilocorus orbus'' is a species of lady beetle in the family Coccinellidae. It is native to North America. It is a black, domed beetle with two large red spots and is commonly called the twice-stabbed lady beetle. Both adults and larvae feed on ...

(''Chilocorus orbus''). Additionally, two small parasitic wasps, an ''

Aphytis Aphytis ( grc, Ἄφυτις), also Aphyte (Ἀφύτη) and Aphytus or Aphytos (Ἄφυτος), was an ancient Greek city in Pallene, the westernmost headland of Chalcidice. Around the middle of the 8th century BC colonists from Euboea arrived. ...

'' and an ''

Encarsia ''Encarsia'' is a large genus of minute parasitic wasps of the family Aphelinidae. The genus is very diverse with currently about 400 described species and worldwide distribution. Noyes, J. S. 2003Universal Chalcidoidea database/ref> The number ...

'' species, lay their eggs inside the scale nymphs. In Maryland there is one generation per year. Both sexes overwinter as second instars, winged males appearing in April or May, and adult females laying a batch of twenty to forty eggs in June and July, and single eggs sporadically thereafter. The first

nymphs, known as crawlers, emerged from under the female's scale in late June and moved to new locations, most settling by the end of July, but a few still on the move in September. In Ohio however, there are two generations each year, with both males and females overwintering as second instar nymphs, followed by a prepupal stage in April. In Missouri there may be three generations in the year.

References

{{Taxonbar, from=Q10470807 Scale insects Agricultural pest insects Insects described in 1881