Walnut Scale

If you have a walnut tree, you should know about walnut scale. Even if you don’t, this is still an interesting read.

​Walnut scale insects (Quadraspidiotus juglansregiae) have a unique behavior that makes them particularly fascinating. Like other armored scale insects, females protect themselves under round, dome-shaped covers. But walnut scale takes this concept to a whole new level.

Walnut scale lifecycle

Female walnut scale insects lay eggs in spring. These eggs hatch in only 2 or 3 days. Female crawlers move around a little bit, searching for a good spot to set up household. Once a spot is selected, they begin feeding and start building their protective cover. 

Male crawlers wander around, looking for a female. When they find one, they huddle up next to her, tucking themselves under the edge of her ‘skirt’, where they excrete their own elongated scale coverings. This often creates a daisy-shaped cluster of scales.

​After these groups mate, those females lay the year’s second batch of eggs. These eggs hatch, usually mid- to late summer, and stay in the crawler stage over winter. In spring, females claim real estate and males emerge with wings, which they use to find a female.

Walnut scale description

Walnut scale coverings start out white. This is called the white cap stage. Then they darken to gray or brown within a week or so. If you lift the covering off the central, round female walnut scale, you would see a yellowish body with indented margins. Other scale insects do not share those characteristics.

​Damage caused by walnut scale

Like other scale insects, walnut scales use piercing mouthparts to suck plant juices from the cambium layer of twigs and branches. This weakens the tree, leading to branch dieback, cracked bark, and reduced harvest. Walnut scale feeding also increases the likelihood of canker development and fungal diseases caused by Botryosphaeria.

How to control walnut scale

You can’t control them if you don’t know they are present. Make a point of inspecting your trees regularly for signs of infestation and infection. You can apply sticky barriers near walnut scale adults to capture crawlers, as they emerge. There are many beneficial predators that feed on scale insects. Parasitic wasps, twicestabbed lady beetles, and a tiny black beetle that goes by the name Cybocephalus californicus, in particular, love to feed on walnut scales. Commercial growers apply insecticides during dormancy or when crawlers emerge in spring.

Narrow range oils can also be used, but walnut trees are very sensitive to horticultural oils. Do not use oils on walnut during dormancy, or between bud break and shoot elongation. Oil use at these times can harm your tree. Horticultural oil can be used with caution as buds begin to swell and the tree enters the delayed dormant period. If your walnut tree is water stressed or suffering other forms of stress, do not apply oil. Oils should also never be used when temperatures are above 90°F.

Scale infestations are on the rise. This is believed to be the result of several different factors, including reduced numbers of beneficial insects. Keeping your trees healthy makes them less likely to be harmed by pests such as walnut scale.

Now you know.