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Management of scale insects which infest bromeliads and other plants is often difficult. Scales are small and difficult to detect especially as eggs and in the crawler (early larval) stage. Infestations can get started and become widespread in a short time. Scales may spread particularly rapidly in the greenhouse environment where natural enemies are absent, whereas outdoors, they are often controlled naturally by beneficial organisms such as parasitic wasps, predatory lady beetles, and pathogens such as fungal and bacterial infections.
Parasitic wasps may only attack a few scale species, but lady beetles (adults and larvae) that attack scales usually eat many different species. Be very cautious if you are considering the use of a harsh chemical pesticide because these substances will often kill the beneficial, natural enemies of insect pests and may lead to even larger infestations.
Eggs of scale insects resemble fine grains of sand and are found under the bodies of the adults. The larvae are mobile with six legs (crawler). The adult scale is heavily armored and it is necessary to look under this armor in order to see the living scale body. Individual scale insects undergo a single cycle from egg to adult but populations consist of many generations of individuals and these separate generations may overlap. Adult scales attach themselves tightly to the plant and often remain on the plant long after death. Adult males emerge, mate and die, so it is generally the adult females that are observed on the plant. The adult females also die, soon after producing eggs, so it is most productive to target scale insects during the more vulnerable crawler stage.
Careful, periodic inspection of plants is critical to the prevention of major infestations of scale insects. Once scales are identified the surroundings should also be examined for the presence of natural enemies. Lady beetle larvae and adults are easily seen feeding on the scales. Parasitic wasps can be detected by looking for scales which have small holes in the armor where the wasps have emerged after the scales death.
A Chilocorus lady beetle, a species that specializes in preying upon scale insects.
Scale insect showing emergence hole
of a parasitic wasp.
Early detection of a few scales may allow them to be successfully removed from plants by carefully using a finger or tool to mash and/or remove the insects. Successful mechanical control requires thorough destruction of all stages including the eggs. This may be aided by washing the plants with water under moderate pressure following removal of the adults. At this level of presence there may be no sign of natural enemies since it takes some time for predators to locate them.
The suppression of an unwanted pest species by its natural enemies; that is, pathogens, parasites, or predators, is termed biological control when it is caused to happen by human action, and is called natural regulation when it occurs without deliberate human action. Biological control and natural regulation are just two components of integrated pest management (IPM). IPM is a pest control philosophy which seeks to manage pests in an economically, environmentally and socially acceptable manner. Using appropriate growing and management practices helps plants remain vigorous and resistant to pest attack. Periodic inspection and monitoring of pest populations detects outbreaks before they reach destructive levels. If the pest population is not contained by natural enemies then other more aggressive methods of control may be required. IPM seeks to use the least toxic method, but if these fail, chemical pesticides may be used as a last resort. Finally, an evaluation follows to ascertain what, if anything, can be done to prevent future outbreaks.
When scale populations reach numbers so large that they cannot be controlled mechanically or biologically, they can be controlled with chemical insecticides labeled for ornamentals or greenhouses. However, the grower is again cautioned to read the label for information on use rates and potential phytotoxicity (toxicity to the host plant). Most conventional insecticides will not kill the adult scales so they should be applied when scales are in the most vulnerable crawler stage. Usually, 2 applications of insecticide about 7-14 days apart are needed to manage scales.
When plants are heavily infested, the best course of action may be to remove and destroy the plants.
Diazinon, Malathion, Orthene, and Cygon are the most effective available scalicides. These chemicals are sold by several distributors under different product names and are available wherever gardening and plant materials are sold. If the risk of phytotoxicity is unknown, a few plants should be treated and observed for 3-5 days for browning or burning of leaves before treatment of large numbers of plants is initiated.
Biorational insecticides like insecticidal soaps and oils act on scale by desiccation or smothering. They are safer to use but they have no residual toxicity. Mortality is caused by direct contact to the scale's soft body tissue (under the armor). These substances will kill adult scale but, like conventional insecticides, they are also more effective against the crawlers. Soaps and oils will usually require 2-4 applications 3-7 days apart to control an infestation. Soaps and oils can be found wherever conventional chemical pesticides are sold.
Soaps and oils can also cause phytotoxicity, especially to sensitive plants and when the temperatures are very high or very low. Testing on a few plants first is suggested before widespread use.
Enstar II is an insect growth regulator that controls scales, whiteflies, mealybugs, aphids and fungus gnats. It kills these specific pests by interfering with their development and does not affect other non-target organisms.
The cautions about phytotoxicity should also be followed for Enstar II.