''Diaspidiotus juglansregiae'', commonly known as the walnut scale, is a species of armoured

scale insect Scale insects are small insects of the Order (biology), order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient g ...

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 females of this species 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 molted. The exuviae of an animal can be important to biologists as they can often be ...

from the first two nymphal

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

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

''Diaspidiotus 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 Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

of ornamental and forest trees and bushes, including

walnut A walnut is the edible seed of any tree of the genus '' Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''. They are accessory fruit because the outer covering of the fruit is technically an i ...

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

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

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 and boxwood. The boxes are native to western and southern Europe, southwest, southern and eastern Asia, Africa, Madagascar, northernmost So ...

camellia ''Camellia'' (pronounced or ) is a genus of flowering plants in the family Theaceae. They are found in tropical and subtropical areas in East Asia, eastern and South Asia, southern Asia, from the Himalayas east to Japan and Indonesia. There are ...

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

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 A cherry is the fruit of many plants of the genus ''Prunus'', and is a fleshy drupe (stone fruit). Commercial cherries are obtained from cultivars of several species, such as the sweet '' Prunus avium'' and the sour '' Prunus cerasus''. The na ...

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and Agriculture, cultivated in China. It bears edible juicy fruits with various characteristics, most called peaches and the glossy-skinned, non-fuzzy varieties called necta ...

and

plum A plum is a fruit of some species in Prunus subg. Prunus, ''Prunus'' subg. ''Prunus'.'' Dried plums are often called prunes, though in the United States they may be labeled as 'dried plums', especially during the 21st century. Plums are ...

Ecology

Second instar nymphs and adult females of this species 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'', which can cause

canker A plant canker is a small area of dead tissue, which grows slowly, often over years. Some cankers are of only minor consequence, but others are ultimately lethal and therefore can have major economic implications for agriculture and horticultur ...

s, dieback and tree death. Natural

predators Predation is a biological interaction in which 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 ...

of this scale insect include the tiny beetle ''

Cybocephalus californicus ''Cybocephalus californicus'' is a species of beetle Beetles are insects that form the Taxonomic rank, order Coleoptera (), in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing the ...

'', and the twice-stabbed lady beetle (''Chilocorus orbus''). Additionally, two small parasitic wasps, an ''

Aphytis Aphytis (), 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. The city became ...

'' and an '' Encarsia'' 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 Hemiptera of North America Diaspididae Agricultural pest insects Insects described in 1881