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Scale refers to an insect infestation that occurs almost exclusively on perennial shrubs and trees. Numerous species of scale insects exist and when a specific species attacks a host that is vulnerable to it, this unfortunate event can quickly kill the plant. That said, other species of scale will cause no damage, and most scale species are innocuous in moderate numbers. Adult male scales are almost never seen. Female and immature -- properly called nymphal -- scale of both sexes appear as round or oval lumps, are immobile, have no identifiable head or body parts and are wingless. Adult males do not feed; juvenile males and both juvenile and adult females suck sap from the host plant through tiny strawlike mouth parts. An infestation of scale effects all parts of a plant, including bark, leaf and fruit, but in palm trees is most prevalent on the leaves.
Identify the species of scale infesting the host plant. This is vital because insecticides balanced to eradicate one species of scale may be entirely ineffective on another. Consult the tables provided by the University of California to identify the specific form encountered. Armored scale live under a platelike, nippled cover that is approximately 1/8 inch in diameter. Euonymus, oystershell, California red and San Jose are some pernicious armored scales. Soft scales reach a maximum length of approximately 1/4 inch, and have a smooth and waxy surface. Black, brown and European fruit scales are some pernicious soft scales. Although less frequently encountered, European elm scale, cottony cushion scale, sycamore scale and oak pit scale can all cause problems.
Use methods of cultural control to discourage infestation; methods include monitoring and providing accurate irrigation and prompt, appropriate pruning. Palms maintained in over-wet conditions and in soils with poor drainage are more susceptible to dangerous or unattractive scale. Conversely, washing the leaves of infested trees to remove dust will encourage natural scale predators. Prune off any limbs or fronds that have a heavy occupation. Open up the canopy by rigorously cutting back upper foliage; scale is naturally held back by high levels of uninterrupted sun exposure.
Encourage organic resistance to scale by monitoring your plants' environment and allowing natural balance to eradicate -- or at least moderate -- the parasites. The larvae of small parasitic wasps such as Aphytis, Coccophagus, Encarsia and Metaphycus are voracious scale predators, as are many beetles, bugs, lacewings and mites. Predatory Chilocorus, Hyperaspis, Rhyzobius and Asian lady beetles -- almost universally colloquially called ladybugs -- also act as natural control agencies. Eggs and larvae of species that do well in specific locales are often available from special local landscaping suppliers.
Prevent honeydew ants from occupying the area around a plant. Scale can flourish when their natural enemies, such as the vedalia beetle Rodolia cardinalis, are prevented from working by ants tending what are effectively farmed scale herds. Wrap the palm's trunk with a collar of fabric tree wrap and fit wedges into uneven areas, then coat the collar with a sticky substance to arrest the movement of ants up the tree to the areas where they are tending the scale. Move the collars twice a year to avoid damage to the bark.
Spray the infested plant with narrow-range horticultural oil, insecticidal soap or a mixture of oil and soap, in the palm's least active season, usually late winter. The plant must be completely sprayed; trunk, branches and even the underside of fronds. This measure should be taken only when atmospheric moisture, such as fog and rain, is not expected for at least 24 hours. Horticultural oil is most effective on woody, deciduous trees, but may have a positive impact on some palm infestations.
Apply systemic insecticides, which are absorbed into the tree then transferred to the parasite in lethal doses, where spraying and environmental control are not practical options. Use systemic insecticides according to the manufacturer's instructions; some are applied to the tree's roots where they are absorbed, others are injected or implanted into the trunk.
Use contact insecticides such as carbamates, organophosphates and pyrethroids only as a last resort in scale control. Although they are effective in killing the insects, they also cause great disruption in the life cycles of other "friendly" insects and mites. Further, their residues remain present on the trees for extended periods, and this persistence continues to disrupt the presence of the "good bugs."
Honeydew is a substance excreted by certain species of scale as a byproduct of their ingesting massive quantities of sap. Honeydew is sweet and sticky, and is very attractive to certain types of ant. A mold that looks like soot flourishes on honeydew, which can spoil the appearance of the plant even if the scale is relatively harmless.
Do not use dormant oil or dormant oil emulsions on scale-infested palms. These products will injure the host tree.
John Cagney Nash
John Cagney Nash began composing press releases and event reviews for British nightclubs in 1982. His material was first published in the "Eastern Daily Press." Nash's work focuses on American life, travel and the music industry. In 1998 he earned an OxBridge doctorate in philosophy and immediately emigrated to America.