Walnut Scale – Quadraspidiotus juglansregiae

Walnut Scale – Quadraspidiotus juglansregiae

Common Name: Walnut Scale

Latin Name: Quadraspidiotus juglansregiae


  • Quadraspidiotus juglansregiae adults and young females are virtually round, with the insect buried under a tough flattish scale made up of exuviae from the first two nymphal instars. Between the prothorax, mesothorax, and metathorax are tiny marginal notches.
  • The scale’s upper surface is pale grey with a reddish-brown patch, while the underside is yellowish. The mature female is around 3 mm (0.12 in) in length, whereas the nymphs are smaller.
  • Male nymphs are identical to females, except that the scale is oval rather than spherical; following the last moult, the adult male emerges as a tiny, winged insect that resembles a fly.
  • Male nymphs occasionally crawl beneath a female scale’s rim, producing a daisy-shaped clump.
  • Host plant:
  • Walnut scale insects infest many deciduous trees and shrubs, including boxwood, camellia, cherry, dogwood, hollies, peach, plum, red maple, sweetgum, tuliptree, and walnut.


The first generation completes development by mid-July, and females deposit eggs in mid-August. These eggs hatch and the crawlers settle and moult once before winter.

Damages caused by Walnut Scale:

Walnut Scale absorbs plant fluids from the inner bark by putting their mouthparts into twigs and branches. Infested trees appear water-starved when coated with scale insects, and inner fruiting wood on lateral-bearing cultivars may die back. Excessive numbers might cause the bark to fracture. The fact that scale insects can promote Botryosphaeria infection and canker growth is of more economic consequence.

Life history and habits:

The walnut scale has two generations every year in the Central Valley. It survives the winter as second instar females and males. Both sexes resume development and maturation in the spring. Adult males emerge as small, winged insects from the scale covering to mate with adult females, who stay hidden behind the scale covering. Females deposit eggs mid-May after mating; eggs hatch in 2 to 3 days. Female crawlers travel around the branches briefly before settling down, feeding, and secreting the scale cover. Male crawlers perch on the edges of a female cover. The scale cover is initially white (white cap stage), but after approximately a week, it transforms to grey or brown. The first generation is complete by mid-July, and females deposit eggs mid-August. The crawlers settle and moult once before winter when these eggs hatch.