Aspen, ash, cotoneaster, willow and lilac are among the most commonly damaged landscape plants. Damage and Diagnosis: Infestation of Oystershell Scale. Oystershell scale feeds the cells of trunks and branches. As it feeds it often kills the area of the feeding site.
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Sprays applied when the crawlers are present can be very effective in controlling oystershell scale. Oils are useful for control of oystershell scale. The oystershell scale, Lepidosaphes ulmi, is the most damaging scale insect present in Colorado. It develops on the bark of trunks and limbs of a wide range of commonly grown deciduous trees and shrubs, including aspen, ash, cotoneaster, poplars, willow and lilac.
Figure 1. Mature oystershell scales settled near the bud of an aspen. Figure 2. During outbreaks oystershell scales can encrust branches, which typically kills the affected limb. Figure 3. Oystershell scale injuries often weaken plants so they become susceptible to plant pathogens.
Cytospora fungi, indicated by the orange fruiting bodies, frequently develop in areas where oystershell scale infests aspen and poplar. Figure 4. Even if oystershell scale is controlled, affected areas of aspen bark later develop a fissured appearance, as indicated by the trunk on the left. Figure 5. Eggs of the oystershell scale, exposed from under the waxy cover of the mother. Figure 6. The crawler stage occurs after eggs hatch and is the only mobile stage of oystershell scale.
Figure 7. Sometimes holes are present in the scale cover, which indicated the insect has been attacked by a parasitic wasp. Figure 8. Gentle scrubbing can easily dislodge oystershell scale from bark. Figure 9. Poplar scales on aspen. The waxy cover of the poplar scale is round and often blends very well with the bark of aspen. Figure A bubbly appearance develops on the bark of aspen affected by poplar scale. The most commonly observed form of the oystershell scale is the covering of the full-grown female scale attached to the bark Figure 1.
On many plants the scale insect blends in well with the underlying bark and it is not uncommon for extensive crusts of scales, and injury symptoms, to be present before they are observed Figure 2. Developing oystershell scales feed by sucking the fluids of cells underlying the bark, often killing the cells at the feeding site.
In high populations, oystershell scales may cause limb dieback and, during heavy outbreaks, plant death can occur from outbreaks of oystershell scale. Oystershell scale damage can also weaken plants so that they become more susceptible to pathogens Figure 3. For example, oystershell scale often damages aspen in a manner that allows for development and spread of cankers produced by Cytospora fungi.
Bark cracking often occurs on areas of bark previously damaged by oystershell scale Figure 4. Life History and Habits There is one generation of oystershell scale produced per year in the region. Winter is spent in the egg stage under the old cover of the mother scale Figure 5. The pearly white eggs hatch on warm, calm days in late May or early June; Memorial Day is often the approximate time when eggs hatch in most years, although there are substantial differences in egg hatch timing related to weather.
Most eggs hatch over a fairly brief period of weeks. The newly emerged scales Instar I nymphs , known as crawlers, are pale yellow and they are active insects that move over the bark in search of sites where they may feed Figure 6. Crawlers are the only life stage of the oystershell scale that is mobile and it is during this brief period that they colonize plants.
A few crawlers may disperse more widely, carried by wind or on the bodies of animals birds, squirrels that move from tree-to-tree. Within a few days after egg hatch the crawlers have either successfully found a feeding site or perished.
They then molt, at which time they lose their legs and become immobile. Over the next couple of months they gradually increase in size and become full-grown in midsummer.
Eggs are laid in late summer and early fall and the mother scale dies at the end of the season. Males have not been observed in Colorado populations and regional oystershell scale populations appear to reproduce asexually. Eggs produced in late summer remain under the protective wax cover of the mother throughout winter. One generation is produced annually.
Management of Oystershell Scale Natural enemies. Relatively few natural enemies appear associated with oystershell scale. Parasitic wasps, common elsewhere in North America, are present but uncommonly associated with oystershell scale in Colorado Figure 7. Also seen feeding on some overwintering eggs of oystershell scale are predatory mites Hemisarcoptes sp.
Many maturing scales and overwintering eggs die if areas of bark beneath them are killed, as happens frequently when Cytospora invades damaged limbs.
Cultural controls. Vigorous plant growth, provided by proper siting and care, appears to help reduce oystershell scale infestations. A common evidence of this is that often only one or two trunks in an ornamental clump planting are seriously infested, the others apparently resisting the insect.
Hand removal. On smaller trees, old scale coverings and eggs can be destroyed by scrubbing the bark with a soft plastic pad. Very heavily infested branches may need to be pruned Figure 8. Dormant season oil sprays. Various kinds of horticultural oils are available that are sprayed on plant for the purpose of covering and smothering insects and mites on plants. See CSU Extension fact sheet 5. Dormant season oil sprays can kill many of the overwintering eggs of oystershell scale.
However, since they are well protected at this time, within the thick waxy cover of the now-dead mother, oils may not sufficiently penetrate to kill all the eggs.
Dormant oil applications should be considered as an important management tool for this insect, but during outbreaks it usually should be supplemented with other controls. Crawler sprays. The crawler period in the life history of the oystershell scale is highly vulnerable, as it lacks the waxy protective cover and is quite small.
Timing is critical as oystershell scale becomes much less vulnerable to most insecticides after the crawlers have settled, begun to feed, and molted to the next life stage during which the protective waxy cover begins to form. However, determining when the crawler period occurs does require some examination of the plants as timing can vary from season-to-season due to spring weather conditions.
Crawlers are very small, but can be seen with careful examination of plants. Alternately they can be shaken off scale infested limbs onto a sheet of paper for easily view or can be trapped on double-sided sticky tape attached to infested limbs. Examinations to determine the onset of the crawler period should generally be begun around mid-May.
Although most any insecticide and even a strong jet of water can kill crawlers exposed on bark, insecticides that have some persistence will be most effective. This is because eggs will hatch over an extended period. Insecticides that can remain active on the bark throughout the egg hatch period can provide effective control of crawlers with a single application. There are several insecticides that can be used as sprays to bracket the crawler period. Presently, most commonly available are various pyrethroid insecticides; pyrethroid insecticides that can be used on trees and shrubs include some products that contain active ingredients of bifenthrin, cyfluthrin, cyhalothrin, or permethrin.
Other insecticides useful for control of crawlers contain the active ingredients acetamiprid, carbaryl, and malathion. Summer season oil sprays. Most horticultural oils presently marketed are sufficiently refined to allow their use on plants when foliage is present. Young stages of oystershell scale, with minimally developed wax covers, can be effectively smothered with sprays of these oils.
Horticultural oils can also be combined with crawler treatments. The oils can provide improved coverage of the plant and will kill exposed stages of the insect. Insect growth regulators. Pyriproxifen is an insecticide that acts by affecting the development of certain insects, a type of insect growth regulator IGR and can be applied during the crawler period and post-crawler period. Pyriproxifen is particularly effective against scale insects and quite selective in its effects. Presently pyriproxifen is only sold for use by commercial applicators, under the trade name Distance.
Systemic insecticides. Several insecticides have the ability to move systemically within the plant and are very useful for control of many insects that affect trees and shrubs.
However, those most commonly available, products that contain imidacloprid or chlothianidin as the active ingredient, generally have little effect on oystershell scale and other armored scales.
This is because they fail to move in sufficient concentration to the feeding sites of these insects. One systemic insecticide, dinotefuran, is effective against armored scales as it is more water soluble and mobile within the plant.
Dinotefuran can be applied as a soil drench, for root uptake, or as a spray on the trunk, through which it is absorbed and subsequently translocated within the plant. Note: Old scales remain in place for several years after the scales have died.
In order to determine if controls have been effective, old scales should be cleared from at least some of the branch, so that reinfestation can be detected.
Also, when crushed, dead scales are dry and flake easily off the bark; scales covering eggs typically will produce some moisture when crushed. Notes on Poplar Scale: Another Common Armored Scale of Aspen In recent years, infestations of the armored scale Quadraspidiotus gigas—known variously as the poplar scale, willow scale or aspen scale—have been observed infesting the trunks of aspen.
It differs considerably in appearance from oystershell scale, having a circular scale cover, approximately 1. The cover is gray with an orange-yellow center, but this scale blends in very well on bark of many trees so that it may not be noticed unless there is significant injury to the tree.
Infestations on aspen typically occur on the trunk and larger branches, which produce a characteristic bubbling symptom on bark Figure
University of Illinois Extension
Biology[ edit ] The adult female oystershell scale is up to four millimetres long, elongated, tapering to a point at the posterior end and often slightly curved, somewhat resembling a mussel shell. The upper side is a banded, brown, waxy scale and the underside is cream coloured. There are no eyes or legs and the short antennae have only a single segment. The mandibles are lengthened into a stylet adapted for sucking sap. The female lays about one hundred oval white eggs, retaining them under her body, and then dies. Her scale darkens in colour and stays in place, protecting the eggs over the winter.
Lepidosaphes ulmi (Linnaeus, 1758)
Sprays applied when the crawlers are present can be very effective in controlling oystershell scale. Oils are useful for control of oystershell scale. The oystershell scale, Lepidosaphes ulmi, is the most damaging scale insect present in Colorado. It develops on the bark of trunks and limbs of a wide range of commonly grown deciduous trees and shrubs, including aspen, ash, cotoneaster, poplars, willow and lilac. Figure 1.
EPPO Global Database