Damage

All scale insects have sucking mouthparts and feed on sap from the tree.

Soft scale, like Kermes scale, feed directly in the tissues that move sap (phloem). Scale can remove large quantities of sap, stressing the host tree.

Trees can usually tolerate small populations of scale. The extensive feeding by a larger population often leads to yellowing of leaves and dieback of twigs. Over time, an untreated population of scale may lead to the decline of the tree.

Soft scale excrete their waste as honeydew. Honeydew is sticky and will coat plant parts, and often drips onto surfaces under the tree. A black fungus called sooty mold will grow on the honeydew, but does little actual damage to the plant. The sticky honeydew and black sooty mold are often noticed before the insects are seen. The honeydew may also draw other insects like ants and wasps to the tree.

Kermes scale is not usually a severe problem among natural populations of oaks, but can cause significant aesthetic and economic damage to oaks in urban settings (e.g., landscapes, parkways) and production tree nurseries. They spread between trees by wind, by crawling across branches that are touching, or by animals and birds.

These insects damage their host oak trees when they pierce through the food-conducting tissue (phloem) under the bark, to feed on tree sap.

The reduction in the flow of sap can lead to slowed growth, branch dieback, and growth of sooty mold on the sticky honeydew produced by the scale. Dieback resulting from new infestations usually presents itself in mid-summer as twigs and leaves turn brown. The ends of the branches are weakened and may fall off of the tree. The dark, round female scales can be found along branches, and at the base of buds and leaves.

Life Cycle

Kermes scale has one generation per year in most states, except in Florida where the climate allows for two generations.

Where there is one generation, the insect overwinters as immature crawlers, which are less than 1/32-inch, light pink, and oval. They have antennae and legs, but no wings.

They become active again in the spring. At that time, the males move to bark crevices and females go to branches where they will become immobile. Females settle at tree wounds or new bud or leaf growth and form a waxy protective shell. Males settle further down the trunk and cover themselves with a white waxy pupal covering.

These immobile stages are when most of the feeding occurs, causing damage to the host. While feeding, the scales produce honeydew, which allows the sooty mold fungus to grow.

Males eventually stop feeding and pupate, while females become adults.

In early summer, adult males emerge and fly to the upper branches to mate with females.

Adult males and females look very different. Males are very tiny, reddish light brown flying insects with legs, antennae, and large wings. Females are about ¼-inch, yellow-brown with a marbled pattern, black dots, and a light stripe down the middle. They are rounded and shaped like small turtle shells. They have legs and antennae and produce a waxy shell to cover them while attached to the tree.

The adult males live for less than a week, just long enough to mate. From mid to late summer, females lay about 3,000 eggs underneath their bodies, their bodies shrink, and they die while the waxy shell remains.

The eggs hatch in September or October and the immature crawlers nestle into branch crevices to overwinter.

Management

Cultural Management

Scale can be removed by hand, and infested branches can be pruned out and destroyed to help keep infestations under control.

Biological Management

Kermes scale has natural enemies that can help control populations, but these beneficial insects may be unintentionally killed when insecticides are used to control other pests.

These natural enemies include ladybird beetles, lacewings, predacious moth larvae, and parasitoids. Ladybird beetles and lacewings prey on scale nymphs. Parasitoid wasps lay their eggs inside the nymph or adult scales, and the larvae then feed on and kill the scales.

Use of chemicals for any reason should be considered and planned with great thought to avoid creating conditions in which scale infestations can flourish.

Chemical Management

Insecticides are most effective when used against the newly hatched crawlers in the early spring or early fall. This is when the scales are moving around and their bodies are fully exposed before they settle and form their protective shell.

To reduce any unintended effects on other species from insecticide use, it is important to closely monitor life stages of scales so that insecticides can be applied at the appropriate time.

Horticultural oils can be effective at suffocating scale while allowing natural enemies to survive, and there are various insecticides with different active ingredients and levels of efficacy.

Products will vary in their method and ideal time of application from spraying, soil drenching, or injection.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.