Soft scales

The soft scales are the more important of the two groups of scales found in greenhouses. A wide variety of the flowering and foliage ornamentals, from orchids to ferns, are good hosts for soft scales.The brown soft scale attacks a broad range of hosts, while the black scale prefers woody plants.The hemispherical scale favors ferns, asparagus fern, schefflera, and many nonwoody evergreen plants. Plants in the family Acanthaceae, such as Crossandra and the zebra plant (Aphelandra squarrosa), are especially susceptible to the hemispherical scale. Another common greenhouse species is nigra scale.Woody plants such as weeping fig, citrus, ivy, and holly are common hosts.

Damage

A heavy infestation of soft scales will cause yellowed leaves, distorted foliage especially at the growing tips, twig dieback, or defoliation. But the main damage comes from the growth of sooty mold on the clear, sticky honeydew excreted by scales.

Description and life cycle

Soft scales are round to oval, dome-shaped on top, and V16-V4 inch (2-6 mm) long when mature. Immature scales start out light in color and darken at maturity. Brown soft scale females are pliable, oval, and somewhat flattened. They have a pale brown,yellow, or gray color that is mottled, shiny and crossed by a dark brown grid-like pattern.They produce live young, not eggs.The female hemispherical scale is hard, circular, steeply convex, smooth brown, and shiny. Numerous eggs pile up underneath their cup-like bodies.The black scale adult female is dark, oval, globular, and has ridges on the scale that form an "H"pattern.The nigra scale varies in size, shape, and color depending on its host. It tends to be more oval on leaves and more elongated on petioles or thin stems.

Female soft scales produce 50-2000 eggs or live young,depending on the species. Mobile, crawler-stage nymphs hatch from eggs after 1-3 weeks.They move to a suitable part of the plant, where they settle for the remainder of their lives.The nymphs go through three instars.A waxy covering envelops the female after she becomes an adult.The covering adheres tightly to the body of the female and cannot be separated from it. All three of these soft scales feed on the phloem and are often associated with stems or leaf veins.

Soft scales are most troublesome in greenhouses at temperatures around 68°F, and development does not usually occur above 86°F. In greenhouses there may be as many as six generations per year, with a new generation produced every 40-80 days, depending on temperature. All stages may be present simultaneously throughout the year.

Monitoring

Early detection will prevent many pest management problems.The best way to detect soft scales is to inspect plants visually, especially new shipments of plants. If scales are present, you will usually find them on the undersides of leaves and on stems, although some species may occur on upper leaf surfaces on some plants.The presence of ants, wasps, or bees may be a sign that soft scales are present.They are often attracted to the honeydew produced by scale insects.Yellowed foliage or sooty mold on leaves often indicates the presence of scales.

Natural enemies

Numerous parasitic wasps, predators, and several pathogenic fungi attack soft scales, but only a few have been investigated as candidates for use in greenhouses.

Parasites

The parasitic wasps that are important natural enemies of soft scales are all in the family Encyrtidae.

Coccophagus lycimnia. This cosmopolitan wasp will parasitize over 47 species of soft and armored scales, and has been shown to be effective against brown soft scales in citrus orchards and ornamental crops. Females oviposit in late first- to third-instar hosts but prefer second instars.The scale insect continues to develop after being parasitized but dies before maturity.This wasp is available commercially at least in Europe.

Metaphycus helvolus. This small encyrtid wasp from South Africa attacks young nymphal stages of several species of soft scales.The male wasp is dark brown.The females are orange-yellow and about 1/25 inch (1 mm) long. Each female lays up to five eggs per day under the bodies of late second and early third instar scales and kills up to 20 more nonparasitized scales of various ages by feeding on them.The females lay an average of 400 eggs over their relatively long lives.Wasp larvae develop singly inside the scale bodies. After about 2 weeks the adult wasp emerges by cutting a small hole in the scale. M. helvolus readily attacks black and hemispherical scales, as well as brown soft scale, nigra scale, citricola scale (Coccus pseudomagnoliarum), and European fruit lecanium (Parthenolecanium corni).This parasitic wasp is commercially available.

Metaphycus luteolus. This wasp, closely related to M. helvolus, has provided effective biological control of brown soft scale outdoors in California. Both the males and females of this species are lemon-yellow. Females lay 5-10 eggs per day in host scales over their 1-month life span.The larvae are gregarious internal parasites of all stages of the brown soft scale, but solitary parasites of black scales.This wasp will also parasitize early stages of other scale species. Larval development is very rapid; at 75°F adults emerge after 11 days.M. luteolus is not available commercially. Another species, M.zebratus, is commercially available in Europe.

The adult wasp Metaphycus helvolus is about V25 inch (1 mm) long.

Microterys flavus. This encyrtid is another internal parasite of brown soft scale, and of various other soft scales. It is available commercially for release in citrus groves.There are several strains with preferences for different host scales. For example, the California strain, probably of East Asian origin, prefers the brown soft scale and will not attack black scale.This wasp is solitary in small hosts but often gregarious in larger ones. Females readily feed on young scales.

Other parasitic wasps. Encyrtus infelix and Encyrtus lecaniorum have been used successfully alone and in combination with Metaphycus helvolus for scale control in French greenhouses. E. lecaniorum (commercially available in Europe) has parasitized hemispherical and soft brown scale more effectively than M. helvolus or C. lycimnia. E. lecaniorum mimics the ants that tend the scales, thereby preventing the other parasites from attacking the scales.These three wasps, used individually, have controlled hemispherical scale on ferns, peperomia, and dieffenbachia. A combination of M. helvolus and either E.lecaniorum or another wasp, Diversinervus elegans, gave satisfactory control of black scale on Aphelandra and Aralia.

Predators

Several species of lady beetles are well-known predators of soft scales outdoors in different parts of the world. Adults and the four larval instars all feed on scales. Females lay eggs on or under the scale. The elongated, flattened, spiny larvae that hatch consume large numbers of scales as they develop over a period of 2-4 weeks. Pupation normally occurs on the plant.Two to four generations are produced each year.

Chilocorus infernalis. This beetle from the foothills of the Himalayas is more cold tolerant than C. nigritis.It is not available commercially, but some closely related species (C. baileyii and C. circum-datus) are available as armored scale predators in other countries.

Chilocorus nigritis. This beetle, indigenous to India and East Asia, is a voracious predator with a wide host range including both soft and armored scales. The adult is a 5/32-inch (4-mm), black beetle with yellow edges on the thorax. A single female may lay over 350 eggs. One reason for its success in biological control programs is that adults and large larvae completely remove the adult female scale from the plant surface. Both beetle larvae and adults lift scales from the host plant with extremely sharp mandibles and then eat the soft parts below. C.nigritis is available in Europe, but not in North America.

Rhyzobius (=Lindorus) lophanthae.

This small black lady beetle is mainly a predator of armored scales (see "Armored Scales"for more information), although they will eat some soft scales, mealybugs, and smaller insects. Both adults and larvae feed on the scales. It is commercially available.

Other lady beetles. Many lady beetles will feed on soft scales if their normal prey is not available.The mealybug destroyer, Cryptolaemus montrouzieri, will feed on soft scales in greenhouses if mealybugs are not present.The alternate-host scales allow the mealybug destroyer to survive between mealybug outbreaks. Several species of Scymnus are known to attack soft scales.The convergent lady beetle, Hippodamia conver-gens,may provide limited control of soft scales if released in large numbers.

Green lacewing larvae. These will feed on immature scales, but because they also feed on a wide range of other insects, including many natural enemies, they are not recommended for use when other natural enemies are already present.

Pathogens

Verticillium lecanii. This fungus has been reported to infect and kill certain soft scales. Preliminary investigations indicate it is a promising biological control agent for the scale Philephedra tuberculosa on Annona squamosa in Florida. However, no research has been conducted on its ability to infect or control soft scales in greenhouses.

Predatory lady beetle larva feeding on soft scale.

Possibilities for effective biological control

The natural enemies described above can control some soft scales in certain situations. Parasitic encyrtid wasps, especially Metaphycus helvolus,have been particularly effective. M.helvolus works well against hemispherical scale and has provided control of black scale in several parts of the world, but it does not reduce brown soft scale populations on some plants. However, it has provided satisfactory control of brown soft scale on poinsettia and in combination with other wasps on Ficus,certain cacti, and cymbidiums, as well as other orchids.This wasp is most effective in semi-tropical conditions.

For soft brown scale control, Microterys flavus or Coccophagus lycimnia may be a better choice than M. helvolus.However, no research has been conducted on the use of either species in greenhouses. In the field, competition from other parasites limits the effectiveness of M. flavus.

To improve control with natural enemies, avoid the use of residual insecticides for a month. Reduce scale populations with insecticidal soap or horticultural oil, and wash scale honeydew from plants before releasing wasps. Some scales must be present, however, to

Soft scales

Enemies

Coccophagus establish the natural enemy population in the greenhouse.

Release live M. helvolus adults two or three times at 2-week intervals and at a rate of three per ft2 or up to 1,000 parasites per acre. Scatter the parasites on plants evenly throughout the infested area.These parasites can search and find the scales quite easily.The best time to release the wasps for control of hemispherical scale is just before the scale reaches the third instar—which is evident by the appearance of a ridged "H"on the scale surface.Wasp emergence holes should be visible after 2 weeks. Control should be achieved in 2-3 months. Periodic additional releases may be necessary to maintain control.

Predation by lady beetles, such as Rhyzobius lophanthae or Chilocorus nigritis, which feed on all developmental stages of scales, complements parasite activity, which tends to be more stage-or size-specific. Releasing a predator along with a parasite may offer the strongest potential for control. R.lophan-thae is most effective at cooler temperatures (59°-77°F) and relative humidity of 20-90%. Release adult beetles at a rate of three to six beetles per 10 ft2 for light infestations or five to seven beetles per 10 ft2 for heavy infestations.

nymph -

adult nymph -

Metaphycus helvolus

Metaphycus luteolus

Microterys flavus

Chilocorus

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