Pest 411 Gloomy maple trees?

Gloomy maple trees?

You may have gloomy scale infestation

by Shimat V. Joseph, Associate Professor, Department of Entomology, University of Georgia

John A. Davidson, Univ. Md, College Pk, Bugwood.org Fig. 1. Gloomy scale insect on the bark of red maple. Gloomy scale insect, Melanaspis tenebricosus, (Fig. 1) is a serious armored scale pest of maple trees in urban areas in Georgia. Infestation of gloomy scale insects can go easily undetected for years as they attack the bark and camouflage to the color and texture of tree bark. The affected trees show damage symptoms, such as branch dieback and canopy thinning, after about 6-10 years when the population of gloomy scale insects reaches extremely high densities. The waxy shield coverings on the bodies of the females protect them from certain predators and insecticide exposure. As an armored scale, the wax coverings produced on the bodies of gloomy scale insects can be detached using a knife. (This is not the case for soft scale insects, such as wax scales, where the waxy coverings are glued to the female bodies.) They have piercing-sucking mouthparts that appear as tubes and are inserted into the parenchyma cells located on the epidermal layer of the tree bark. As years go by, the black/gray dust that deposits on the tree bark, especially on trees planted along the right-ofways and parking lots, makes the detection even more challenging. Unlike soft scales, gloomy scales do not produce honeydew.

Description and biology

The nymphs go through three nymphal stages to become adult females, whereas they undergo five nymphal stages to become adult males. Females are round (with concentric rings/ridges) with a darker center, although their color can be variable (Fig. 2, black arrow). The adult females are sedentary, whereas males are winged. The adult female appears pink or cream-colored when the test (referred to for waxy covering) is removed (Fig. 2, blue arrow). Usually, females live up to 12 months, whereas males live up to four months. The armored coverings on females usually give a convex shape. The adult female is about 1.5 mm in diameter and has no legs and antennae (Fig. 2, black arrow). Oval-shaped males are smaller than females. Adult males fly to find their mate, and they lack mouthparts. Female scale insects produce 70 oval-shaped eggs during their lifetime, which are about 0.15 mm wide and 0.25 mm long. The eggs are produced in smaller clusters over eight weeks rather than laid in masses. The eggs develop and hatch within the females and emerge out as crawlers. The emerging crawlers seek feeding sites within 30 cm away from their mothers. Crawlers are the only nymphal stage with legs, eyes, and antennae. Once they settle on feeding sites, they feed and secrete the waxy covering on their body (Fig. 2, red arrow). They undergo one generation per year. Mated females overwinter, and crawlers emerge in spring. Unlike other scales insects, the crawlers of gloomy scales do not emerge synchronously; instead, they emerge over six weeks. Other scale insects typically emerge synchronously and have a distinct peak in emergence. Females reproduce both with and without mating. Reproduction without mating is believed to be more common than after mating. The crawler stage of gloomy scales can be dispersed by wind, foraging insects and animals (such as birds). The movement of infested trees can also contribute to dispersal but this is not confirmed.

Damage

When high densities of gloomy scales are observed, the surface of the bark becomes bumpy and has a grainy texture (Figs. 3 and 4). The young branches show dieback and thinning of the tree canopy. The dark-colored and bumpy bark surface as well as a thinned canopy of the infested tree give a dull or gloomy appearance. The infestation begins from the younger branches and then persistently progresses to the bark of older branches. The overall aesthetic value of the landscape deteriorates with infested gloomy trees. The affected trees eventually die in about a decade.

Host plants

Gloomy scale insects can infest more than 28 woody plant genera, but red maple (Acer rubrum L.), silver maple (Acer saccharinum L.) and Freeman maple (Acer × freemanii A.E. Murray) are by far the most common hosts. Other hosts include boxelder (Acer negundo L.), sweetgum (Liquidambar styraciflua L.), holly (Ilex spp.) and mulberry (Morus alba L.), sugarberry (Celtis laevigata Willd.), Osage orange [Maclura pomifera (Raf.) C.K. Schneid.], white ash (Fraxinus americana L.), tulip poplar (Liriodendron tulipifera L.), and lime prickly ash [Zanthoxylum fagara (L.) Sarg.].

John A. Davidson, Univ. Md, College Pk, Bugwood.org Fig. 2. Gloomy scale insect with adults (black arrows) and crawlers with cap (red arrows) on the bark of red maple. Pale cream or pink colored female (blue arrow) when the test or waxy covering is removed.

Monitoring

Management of gloomy scale insects is often challenging and expensive because scale infestation is diagnosed when it becomes severe and trees show damage symptoms. Any intervention after the high densities of armored scale on trees does not ensure a steady recovery and a clean and

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Eric Honeycutt, Bartlett Tree Experts, Bugwood.org Fig. 3. Gloomy scale insect infested bark surface is typically bumpy with waxy coverings of females (live and dead) pile over several generations.

Shimat V. Joseph, University of Georgia Fig. 4. Gloomy scale insect infested tree in a parking lot.

healthy tree. Thus, monitoring for early infestation is critical after planting a tree. Red maple trees are especially at risk for infestation. Double-sided sticky tapes can be attached around the branches to monitor crawlers. The trapped crawlers suggest an active population of gloomy scales. Because there is no threshold developed for management decisions, early intervention is warranted before the infestation becomes severe.

Management

For management, the crawler stage of the scale insect is often targeted with contact insecticides, such as horticultural oil or pyrethroids, before or during peak crawler emergence. However, this strategy may not be effective for gloomy scale as their crawler emergence is usually extended up to eight weeks. For those urban trees planted around impervious surfaces, such as concrete or pavement, high temperatures and lack of irrigation increase the risk of the rapid spread of gloomy scale infestation. This is because the reproduction of the gloomy scale increases with an increase in temperatures. Relative susceptibility of gloomy scale infestation among varieties of red maple was not evaluated to date.

Biological: Although some predators (such as lacewings, lady beetles), and parasitoid wasps have been observed attacking nymphs of gloomy scale, appreciable control has rarely been noticed on infested urban trees.

Mechanical: Power washing can mechanically remove gloomy scales, but effective control from this method is still inconsistent.

Chemical: Chemical management can be an important tool to suppress gloomy scale populations. Contact insecticides, such as horticultural oils and insecticidal soap, could suppress the crawler stage of gloomy scale. However, their effectiveness is questionable because of the prolonged emergence phase of crawlers in spring rather than a synchronized peak emergence. Thus, the timing of the contact insecticides with reduced residual activity (such as horticultural oils) will be challenging. Pyrethroids may offer better control on crawlers as their residues persist a bit longer than oils or soap. Systemic insecticides, such as neonicotinoids (dinotefuran, imidacloprid, thiamethoxam, acetamiprid, clothianidin) have shown effective against armored scale insects in general but have not yet been tested against gloomy scale insects. These systemic insecticides should be repeatedly applied as a drench at a higher dose for effective control. This is because gloomy scales feed on the parenchyma cells instead of the phloem tissue; thus, they are likely exposed to lower doses of systemic insecticides.

Also, many recent field studies suggest that neonicotinoids can be present at lethal levels in the floral parts and harmful to foraging pollinators and other beneficial arthropods. Thus, before using any insecticides, please carefully read the insecticide label, including precautionary statements.