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Shrub Insects

Bagworms


The bagworm, Thyridopteryx ephemeraeformis, is an interesting pest of ornamental shrubs and trees. The spindle-shaped bag that the larva carries around as it feeds characterizes this insect. The larva constructs the bag from silk and covers it with bits and pieces of leaves and twigs from the host plant. Therefore, the bag looks quite different when the worm is feeding on juniper than on rose. The larva is dark brown with a yellow head and yellow and black spots on the body. Adult females are wingless and lack functional legs, eyes, and antennae. They are almost maggot like, yellowish and seldom seen. Adult males are typical moths and are sooty black and densely hairy. The wings are nearly clear and have a span of 1 inch.


Bagworms often prefer juniper, arborvitae, and pines, but they are also found on many broadleaf shrubs and trees including rose, sycamore, maple, elm, and black locust.


Defoliation is the primary injury caused by the caterpillar. Heavy populations of bagworms kill many ornamental arborvitae and juniper.


Bagworm larvae hatch from overwintering eggs during May. The young larva spins a silken case that it carries about as the larva feeds. As the larva grows, it enlarges the bag and continues to add bits of foliage to the bag. When the larva is resting, it attaches the bag to a twig with silk. Pupation occurs in late summer. In 7 10 days the adult emerges. Males are free flying and leave the bag. They search out bags containing females, enter the bag, and mate. After mating, the female lays 500 1000 eggs in the bag and dies. Because the larval stage is the only mobile stage, one bush can have a high population and another one several feet away can be free of bagworms.


Bagworms are attacked by several parasites and birds and are killed by low winter temperatures. A very effective control is picking the bags off the host in winter or early spring to help prevent problems the coming season. The bags, and eggs inside can be destroyed or the bags can be placed in deep containers that allow any parasites to escape but keep the larvae in.


Young larvae are especially sensitive to the bacterium Bacillus thuringiensis. This bacterial insecticides are commercially available under the trade name DIPEL. The disease caused by the bacterium effects only the moth larvae and is safe to use. Always read and follow label directions before treatment. Best control is achieved when spraying is done in late afternoon or early evening. Small larvae are much easier to kill than mature larvae. Therefore, treating infestations in the spring when they are first noticed is important.


Japanese Beetle


The adult Japanese beetle is a brightly colored oval insect about ⅓ to ½ inch long. The body and legs are a bright metallic green while the elytra (wing covers) are coppery brown. The Japanese beetle is distinguished from any other beetles with similar coloration by the white spots or tufts, five on each side and a pair at the tip of the abdomen. Females are slightly larger than the males. The larva (grub) is about ¾ to 1 inch long when mature and usually takes on a crescent shape when exposed. It is grayish white with a yellowish brown head. The larvae can be distinguished from other “white grubs” by a V-shaped row of spines on the underside of the last segment of the body.

 

Adults are voracious feeders on the fruit and foliage of more than 275 kinds of trees, shrubs, flowers, and other plants. A ripe peach may have 100 beetles clustered on it. Leaves attacked by the beetle have a lacy appearance since everything but the veins is consumed. Beetles may also affect the development of sweet corn by eating the silks and preventing pollination. The larvae feed on the roots of grass. Severe damage can occur on lawns, golf courses, and pastures when populations of 10 or more larvae/square foot are present. Large areas of brown grass often show up during dry spells in September and October. Under heavy populations, large pieces of sod can easily be peeled back revealing the larvae.


The life cycle requires about one year. Adult beetles begin emerging in mid-May and early June. Peak emergence is reached by early July. Adults remain active for 4-6 weeks. Egg-laying takes place in July. Eggs are laid in small clusters in cells 2-4 inches below the soil surface. Each female lays 40 to 60 eggs during her life. In about 10 days the larvae hatch, move into the root zone, and begin feeding. As winter approaches, the grubs move deeper and form an earthen cell where they spend the winter. Early in the spring they move back to the root zone and resume feeding. Pupation takes place in late April and May. During this stage the insect transforms to the adult stage.


Numerous insecticides are available from local home stores to control Japanese beetles. Always READ THE LABEL before using to check for any special instructions and precautions. During peak beetle activity, treatment will have to be made at weekly intervals to provide adequate protection.


Grub control can be approached from two ways. They are highly susceptible to a disease called milky spore disease. It is caused by a bacterium and is highly specific to Japanese beetle larvae. Once the milky spore disease is established, it remains active for 20-30 years. The only drawback is that it takes 2-3 years to achieve complete control. It is available as a dust or spore powder under the trade name MILKY SPORE. Follow label directions for application.


Several chemicals are labeled for Japanese Beetle grub control. However, they only provide temporary control. Best results are obtained when the insecticide is washed into the soil immediately after application. Most products should be applied between August and mid-October.


Lace Bugs


Lace bugs are very small insects, about 1/8 inch long, that cause considerable damage to the foliage of ornamental shrubs and trees. Adults are characterized by sculptured wings and hood giving them a lacy appearance from which they derive their common name. Most adults have some dark markings on the wings and hood although some are nearly transparent. The immature stage or nymphs are spiny and generally black.


Symptoms of damage are chlorotic or yellowish blotches on the upper leaf surface. Severe damage can cause the entire leaf surface to take on a yellow or bronze coloration. The damage is similar to that caused by spider mites. A sure way to differentiate the two pests is to look at the underside of the leaves. Lace bugs always feed on the underside of the leaf. The lower leaf surface will be covered with shiny, black spots of excrement, egg fragments, and cast skins. These diagnostic signs will be present even if the insects have left the leaf. Heavy populations can severely weaken a tree or shrub making it more susceptible to other problems.


The Azalea and Rhododendron Lace Bugs overwinter as eggs inserted into leaf veins or cemented to the leaves with a brown, crusty material. The nymphs hatch during February and March and many adults are present by April and May. The adults lay eggs and a second generation occurs in July and August. A complete generation takes from 35-45 days. Sycamore and Hawthorn Lace Bugs differ in that they overwinter as adults on the host in bark crevices, branch crotches, or other protected areas. In the spring, adults lay eggs on the young leaves. Again there are a spring and a late summer generation. During the late summer generation, especially, both nymphs and adults may be found together.


Lace bugs have several natural enemies that feed on them. These include lacewings, assassin bugs, spiders, and predaceous mites. However, when lace bug populations get out of hand using chemical controls is necessary. Always READ THE LABEL CAREFULLY since some of these chemicals are not safe to use on all plants. Incorrect usage can severely damage the foliage or even kill the plant. Whenever spraying a shrub infested with lace bugs remember that they feed only on the underside of the leaves. This means that the spray must be directed to the underside of the leaves to achieve satisfactory control. A second application in 7-10 days may be needed to kill late hatching nymphs or adults that migrate to the shrub from adjacent areas.


Spider Mites


The most common spider mite is the two spotted spider mite, Tetranychus urticae, which is found on numerous host plants. Other common spider mites include the southern red mite, Oligonychus ilicis, which feeds on spruce, azalea, camellia, and related plants; the spruce spider mite, Oligonychus ununguis, the major mite pest of conifers worldwide; and the European red mite, Panonychus ulmi, a pest on many fruit trees, grapes, and roses. The clover mite, Bryobia praetiosa, feeds on a number of herbaceous plants and turfgrasses; the boxwood mite, Eurytetranychus buxi, is found primarily on boxwoods.



The two spotted spider mite female is typically larger than the male and has a rounded posterior. The slightly smaller male appears to have longer legs, and his posterior is more pointed. The male typically moves around faster than the female.


The first obvious sign of spider mite infestation is stippling on the upper surfaces of the leaves. This looks like the leaves have been pierced with many tiny needles. The stippling occurs when spider mites pierce individual cells of a leaf with their chelicerae (needle-like mouthparts), releasing the cell's cytoplasm. A heavy infestation of spider mites can reduce photosynthesis, cause leaves to fall off, and eventually cause the death of the plant. Large amounts of webbing around the terminal ends of upright branches is a sure sign of heavy spider mite infestation . A possible reason for webbing being attached to the terminals of plants is because this is the area where humidity is lowest. The webbing is spun everywhere the spider mites go. Individual silk strands can be seen on the underside of leaves with the aid of a magnifier.

 

Detection of spider mites for making treatment decisions can be accomplished by viewing the underside of leaves using at least a 10x lens. Another detection method is to beat leaves and stems over a white piece of paper. Dislodged spider mites can be seen as little "specks" running around on the paper. The size of the spider mite "specks" on the paper is about the same size as the period at the end of this sentence.


Chemical control is usually needed at some point. Spraying with a labeled miticides kill only the nymphs and adults; the eggs are unaffected by most miticides. Therefore, two treatments should normally be applied at an interval of approximately one week. Mite suppression can be obtained with horticultural oils and insecticidal soaps. These materials have no residual activity and must come in direct contact with the mites. However, they both will kill the egg stage. Sulfur will provide suppression of mites. Traditional miticides often contain the active ingredient hexakis.


Powdery Mildew


Powdery mildew is the name given to a group of diseases caused by several closely related fungi. Their common symptom is a grayish-white, powdery mat visible on the surface of leaves, stems, and flower petals. There are many hosts; and although this disease is not considered fatal, plant damage can occur when the infestation is severe.


In spring, as daytime temperatures rise above 60 °F, the fungi responsible for powdery mildew begin to produce spores (conidia) which are dispersed into the air. Infections occur when they contact a suitable host and environmental conditions are favorable. Initial symptoms are small, circular, powdery, white spots which expand and eventually join as infections progress. Infections spread as spores produced in these white patches move by wind and splashing rain to other locations on the plant or nearby plants.


The fungus survives the winter attached to plant parts and plant debris such as fallen leaves. As weather warms in spring, the process begins again.
Humidity is an important factor related to the onset and spread of powdery mildew. Unlike most fungi, these do not require free water to germinate; only high levels of relative humidity. High relative humidity favors spore formation, and low relative humidity favors spore dispersal, which explains why powdery mildew tends to be a problem when the days are cool and the nights are humid. Temperature is also a factor. Although powdery mildew can occur all season long, it is less common during the heat of the summer.


Hosts


Powdery mildew is caused by several different species of fungi, and they each have a limited host range. In other words, observing powdery mildew on oak leaves should not be cause for concern for nearby zinnias. Plants that commonly become infected with various powdery mildews include; azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, English ivy, photinia, blueberry, pecan, cucumber, and squash.


Abnormal growth, such as leaf curling, twisting, and discoloration may be noticed before the white signs of the fungus are visible. On dogwood, for example, leaves may take on a yellowish or reddish cast in summer or may develop reddish blotches or dead, scorched patches. The white powdery growth is not always apparent.

 

Powdery mildew damage on rose flower buds.

 

Powdery mildew creates other effects that are not readily visible. For example, a severely infected plant may have a reduced level of winter hardiness. Trees have also been observed to leaf out later in the spring after being infected the previous season.


Cultural Controls


As with all diseases, optimum plant health is the first line of defense. This begins with selection of healthy plants that are planted properly and in the proper location, giving attention to requirements for light, soil, and moisture. Space them so they are allowed to grow without being crowded and water thoroughly during establishment, and later during dry periods. Avoid overhead irrigation which raises the level of relative humidity within the plant canopy.


If powdery mildew is noticed on a few leaves, simply removing them will help with control. At the end of the growing season, prune out infected stems and remove fallen leaves which can serve as a source of further infection. Suckers are common on crape myrtle, dogwood and other plants. These should be pruned as they develop because they are especially susceptible and the disease will spread from them upwards to other plant parts.


Fertilize to optimize plant health, but avoid overfertilization with nitrogen as it stimulates young, succulent growth which is more susceptible to infection.
Plants with a severe infection should be monitored closely the following spring so that if infections reoccur, they can be treated early.


When possible, select plants that show resistance to the disease.


Chemical Control


Ornamental Plants: For fungicides to be effective, they must be applied as soon as symptoms are noticed. Product labels will provide information on how often to spray. When ranges are given, use the shorter interval during cool, damp weather. Be sure to cover both the upper and lower surfaces of the leaves.


Table 2 lists fungicides labeled for ornamental plants. Myclobutanil, propiconazole, thiophanate-methyl, and triadimefon have systemic properties and can be sprayed less often than sulfur or copper-based fungicides. When powdery mildew persists and sprays are repeated, it is recommended to rotate (alternate) fungicides to decrease the chance of fungi developing resistance.


When deciduous plants are infected, consider the season. Generally, foliar diseases occurring in late summer do little damage. The leaves have already produced food for the plant and are going to fall off soon anyway. Just be sure to rake and dispose of them as they fall.


As with any pesticide, read the label and heed all precautions. Sulfur, for example, can damage plants if applied when temperature and humidity are high.


Scale Insects


Scale insects are often mistaken for fungal disease because many cluster together, resembling a crusty or fuzzy mass that often oozes when rubbed. Scale gets its name from the protective fish scale-like covering produced by a tiny insect about the size of a pencil tip. Scale insects anchor to plant parts by their piercing-sucking mouthparts and feed on plant sap. Individual scales may look like oval or rod-shaped bumps, ranging in color from white, yellow, grey, brown to black.

These insects can be a major problem on many of our evergreen and deciduous trees and shrubs and can occur on leaves, twigs, branches or trunks.


Two Types Of Scale Insects


Soft Scales secrete a thin, waxy layer over themselves, which cannot be separated from the insect’s body. Because they consume large amounts of plant sap, soft scales excrete a sticky, sugary liquid called “honeydew” as a by-product of their feeding. Honeydew provides the perfect source for the growth of a black mold, called “sooty mold”. Large patches of sooty mold that blacken leaves and stems are often what draws attention to a scale problem. Soft scales range from 1/8 to 1/2 inch in size.


Armored Scales secrete a hard, lacquered covering over their bodies. This cover is not attached and can usually be separated from the scale’s body. They typically are smaller than soft scale, ranging from 1/16 to 1/8 inch. Armored scale insects do not excrete honeydew and so do not support sooty mold growth.


Life Cycle


Though the biology differs for various scale species, understanding some general characteristics is helpful in controlling this insect. Immobile, legless females lay eggs under their bodies then die. Eggs hatch into tiny pinkish to yellow mobile immatures called “crawlers” that move around the plant seeking suitable sites to feed, secrete their scale covering, and mature to adulthood. Some species overwinter as eggs beneath the dead female's cover and hatch in the spring; others overwinter as fertilized females and resume feeding in the spring, when they lay eggs and die. Adult male scale insects are tiny and winged; do not feed; and live only a few hours. Females of many soft scale species reproduce without mating. Armored scales are more likely to have several generations per year while most soft scales have only one.


Damage


Scale insects obtain food by sucking vital fluids from the host plant, causing yellowing and possibly stunted growth of the affected leaves or needles. A heavily infested plant will have extensive leaf yellowing, premature leaf drop, and possibly branch dieback. A plant weakened by a scale population is often more susceptible to damage by a secondary pest that may ultimately kill the plant.


Although sooty mold growth does not damage the plant, it looks unsightly and in large amounts can interfere with photosynthesis, slowing plant growth.


Prevention


The best defense against severe scale infestations is to monitor landscape plants weekly throughout the year, paying close attention to the undersides of leaves and stems for scale, and avoid over-fertilization. Insects often lay more eggs and survive better on plants that are lush from heavy doses of nitrogen.


Control of Sooty Mold


Sooty mold can be washed off, but not easily. It washes off and weathers away better following horticultural oil applications.


Chemical Control


Smothering scale insects by applications of horticultural oil is the easiest and often the most effective means of control. There are numerous types of oils, each with different temperature capabilities. There are some ultra light oils that can be used during the growing season, but it is critical to read the label carefully for guidelines on plant sensitivity and temperature restrictions. Most contact insecticides cannot penetrate the protective covering of the immobile scale nymphs and adults. Only the crawler stage is susceptible to contact insecticides. Systemic insecticides may provide control of soft scales, but is generally not effective for armored scales.