Landscape & Ornamentals

SCALE INSECTS ON SHADE TREES AND SHRUBS

Clifford S. Sadof, Extension Entomologist

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Scale insects are common pests of shade trees and shrubs. More than 60 different kinds occur in Indiana, yet they are often overlooked or ignored until tree or shrub branches “mysteriously” start to die. Upon closer examination, these branches are likely to be covered with small bumps that are actually scale insects. They damage plants by sucking out plant juices.

From a damage standpoint, there are two types of scales, those that excrete a sugary liquid (honeydew), and those which do not. Honeydew is both a nuisance and a threat to plant health. Parked cars, walks, and benches beneath infested trees often become a sticky mess. The sugary liquid attracts ants, flies and wasps. Plants become unsightly when this liquid becomes a food for a black fungus called sooty mold. This mold can shade leaves and reduce plant growth.

Soft (Lecanium), kermes, and bark scales produce honeydew. These scales feed directly on plant parts that transport fluid and nutrients. Armored scales and pit scales do not produce honeydew. The armored scale's straw-like mouth moves like a plumber’s snake to burst plant cells and feed on their contents. Pit scales are likely to do the same to the raised plant tissue that surrounds them.

GENERAL SCALE LIFE CYCLE

Scales spend most of their lives feeding on the same spot of a plant, and unable to walk. After the eggs hatch beneath females the young scales are called crawlers because they can walk at this time. Crawlers are small (<1/32”) and flattened, looking like dust on the plant surface. Scale infestations spread when crawlers walk or are blown by the wind to nearby plants or plant parts.

After an armored scale crawler begins to feed, it becomes very flat and covered with a clear wax shell. As it continues to grow, it remains beneath its waxy armor. This armor is difficult to penetrate with insecticides. Winged males crawl out from beneath their cover and mate with covered females who produce eggs. Females can produce about 100 eggs each.

Soft scales, are not covered by a waxy shell. Crawlers that hatch from eggs in mid-summer will usually crawl directly to leaves. They spend most of the summer feeding on leaves and excreting honeydew. They return to the twigs and bark where they spend the winter as settled second stage scales. They continue to grow on twigs in the spring until winged males mate with wingless females, who swell with up to 1,000 eggs.

CONTROL MEASURES

Inspection

Check plants for live scale infestations. Flip over suspicious looking bumps on twigs and branches with a thumbnail. Bark is usually intact beneath a scale. When a soft body is beneath a cover, the plant is likely to have live armored scales. When the bump itself can be squashed it is likely to be some other type of scale. When honeydew falls from a tree, leaves should be inspected for live soft scales or mealybugs.

Cultural Control

Scales will thrive on trees that are under stress. Plant trees that are correctly suited to your landscape site. Slower growing plants with variegated leaves can require more care. Keep them watered. Carefully inspect newly purchased plants for scales. If a twig is unusually bumpy and leaves are somewhat yellowed it may have scales.

If a plant is normally a rapid grower, such as red-osier dogwood, or wintercreeper euonymus, consider cutting out heavily infested branches with a pruning shears to foster growth of uninfested shoots.

Biological Control

The stationary life of scales makes them easy targets for many natural enemies including lady beetles and microscopic wasps. These beneficial insects can keep the numbers of scales quite low in a natural woodland setting.

Chemical Control

Conventional pesticides cannot penetrate a scale’s tough skin or waxy cover. Scale crawlers are killed by these pesticides when they are covered during foliar application, or as they walk along treated surfaces. To achieve maximum kill, pesticides in this group should be sprayed at the begining of the crawler period. Several obstacles make conventional materials undesirable for managing scales. Thorough coverage on tall trees is difficult and these materials do not kill scales after they settle. More importantly, these materials kill the scale’s natural enemies responsible for lasting control in the landscape.

Conserve natural enemies and kill armored scales on infested trees by using a biorational material like horticultural oil. This material works by smothering scales. Unlike other conventional pesticides, this material can kill armored scales after they have settled while the scale body is still somewhat clear. After it dries, it is not toxic to natural enemies that can fly back to an infested plant and feed on the remaining scales. When applied in winter at the dormant rate, horticultural oil kills scales that do not winter as eggs beneath the scale cover (See Table 1 for winter stage).

For persistent armored scale problems or those scales that winter in the egg stage (pine needle or oystershell scle) insect growth regulators can be a promising biorational alternative. Apply when scales are crawling or when the scales are still clear. These materials (e.g., Pyriproxifen) kill insects as they molt. Soil applied systemic insecticides are effective rescue treatments.

Control of soft scales an be achieved by targetting egg laying females with bifenthrin in the spring. A second spray of pyriproxifen targetting crawlers on leaves. The next year apply pyriproxifen sprays to leaves, if needed. To get the most from both natural enemies and pesticides, do the following:

1. Identify the scale. Use picture sheet and Table 1.
2. Inspect plants for live scales in early spring and for active crawlers in summer
3. Use Table 2 to make decisions about pesticide use, and Table 3 to select a pesticide.

Photo of Insect	Kind and Description of a Scale	Plants Most Seriously Affected	Approx. Crawler Activity (Winter Stage)
SCALES THAT DO NOT PRODUCE HONEYDEW
	Armored scales with dark covers:
	Obscure Scale (Melanaspis obscura): Small (1/16"), round gray scales. Twigs appear covered with silver shells when rubbed. Black central nipple.	Pin-oaks, and red oaks, especially in urban areas	July (immature)
	Oystershell Scale (Lepidosaphes ulmi): Small (1/8" long) gray or brown scales shaped like oyster shells. May completely encrust branches.	Lilac, birch, dogwood, ash, elm, poplar, soft maple, privet, willow, walnut, hemlock	May and July (eggs)
	San Jose Scale (Quadraspidiatus perniciousus): Tiny (1/16") gray circular scales about the size of a pinhead and having a yellow central nipple.	Flowering ornamental fruit trees, rose, quince, mountain ash, pyrancantha, and others	Mid-June to mid-July (immature)
	Winged Euonymus Scale (Lepidosaphes yanangicola): Small oystershell shaped (1/16") covers found along ridges of winged euonymus branches (Burning bush). Much thinner than oystershell scale.	Burning bush (Euonymus alatus)	May, July, September (eggs)
	Armored scales with white covers:
	Euonymus Scale (Unaspis euonymi): Elongated (1/16") white ridged scale covers of males on leaves. Females on stems resemble oystershells but are more flattened.	Euonymus, pachysandra, bittersweet	Late May to early June, late July to August (adult female)
	Pine Needle Scale (Chionaspis pinifoliae and C. heterophyllae): Small elongated (1/8") white scales attached to needles of overgreens. C. heterophyllae feed on pine only, while C. pinifolae feed on other conifers as well. Can winter as egg, or adult female.	Pines, fir, spruce	May, July (eggs) June (adult female)
	Scurfy Scale (Chionaspis furfura): Small elongate (1/10"), dirty white, pear shaped scales. Lie flat on bark. Purplishred crawlers.	Young elms, apple, willow, dogwood	May, July (adult female)
	Juniper Scale (Carulaspis juniperi): Tiny (1/16") circular grayish-white scales with a yellow center. Packed between leaf scales of Juniper and arborvitae.	Juniper and arborvitae	May, July (adult female)
	Pit Scales (Pit shaped bumps on twigs): Golden Oak Scale (Asterolecanium variolosum): This scale makes a circular pit (1/8") around its gold colored body. Edge of body is surrounded by a waxy fringe.	Oaks, especially pyramidal English oak	June (adult)
SCALES THAT PRODUCE HONEYDEW
	Calico Scale (Eulecanium cerasorym) (soft scale): Small (1/4”) black scales with white tufts attached to branches in spring.	Crabapple, dogwood, firethron elm, hackberry, honeylocust	May (immature)
	Cottony Maple Scale (Pulvinaria innumerabilis) (soft scale): Large (3/8”) scales attached to undersides of branches. In spring, when depositing eggs, scales on twigs resemble strings of popcorn.	Soft maple, boxelder, linden	June (adult)
	European Elm Scale (Gossyparia spuria) (bark scale): Oval shaped (1/4"), reddish brown scales surrounded by a white waxy fringe. Found on bark, often in the crotch of small banches.	Elms of all ages	Mid-May to mid-June (immature)
	Fletcher's Scale (Parthenolecanium fletcheri) - Formerly Taxus Lecanium (soft scale). Round (3/8") deep brown scales found on yews.	Yew (Taxus)	Mid-June to mid-July (immature)
	Magnolia Scale (Neolecanium cornuparvum) (soft scale): Large female up to 1/2 inch long. Skin is covered with white waxy powder.	Magnolia only	September (crawler)
	Pine Tortoise Scale (Toumeyella parvicornus) (soft scale): Deep brown to black scales (3/8") with light colored spots in a pattern that gives them a turtle like appearance. If cream colored stripes are present, then it is the Striped Pine Scale (T. pini). Life cycle of T.pini is poorly understood.	Pines only	Mid-June to mid-July (adult female)
	Spruce Bud Scale (Physokermes hemicryphus): Brown adult females (1/4") closely resemble spruce buds late in the season. Black crawlers.	Spruce	June (immatures on underside of needles)
	Tuliptree Scale (Toumeyella liriodendri) (soft scale): Females up to 3/8” long with orange ridges on a brown body. Black crawlers.	Tuliptree, poplar, magnolia, walnut, and linden	September (crawler)
SCALES RELATIVES
	Hawthorn Mealybug (Phenacoccus dearnessi): All stages covered with white waxy filaments. Adults up to 3/16" and all stages are mobile. Black crawlers.	Hawthorn	May-June (immature)
	Oak Kermes Pubescent Leaf Kermes (Nanokermes pubescens) Pin Oak Kermes (Allokermes galliformis): Adults of both species are up to 3/8" long and are often present near terminal buds	Oaks	September (immature)

READ AND FOLLOW ALL LABEL INSTRUCTIONS. THIS INCLUDES DIRECTIONS FOR USE, PRECAUTIONARY STATEMENTS (HAZARDS TO HUMANS, DOMESTIC ANIMALS, AND ENDANGERED SPECIES), ENVIRONMENTAL HAZARDS, RATES OF APPLICATION, NUMBER OF APPLICATIONS, REENTRY INTERVALS, HARVEST RESTRICTIONS, STORAGE AND DISPOSAL, AND ANY SPECIFIC WARNINGS AND/OR PRECAUTIONS FOR SAFE HANDLING OF THE PESTICIDE.

January 2017

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This work is supported in part by Extension Implementation Grant 2017-70006-27140/ IND011460G4-1013877 from the USDA National Institute of Food and Agriculture.

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