o How do I rid my plants of scale and mealybugs?

Scale and mealybugs are related insects, and since their life cycles closely parallel each other, they will be considered as one in this FAQ. Both are in the order Homoptera (one set of wings). Size is from 1/12 inch to 1/5 inch (2 to 5 mm). Although more than 160 species of scale and over 30 species of mealybug are recognized, only a few are serious pests of cacti and succulents. Some species are host specific, that is, one kind of scale/mealy will feed on only one family of plant. An example is the Golden Mealybug, Pseudococcus aurilanatus, found in California, which feeds on the Araucaria family (Norfolk Is. Pines, etc.) and few other plants. Others, like the Grape Mealy, feed on hosts as varied as potatoes, date palms, apples, Agaves, ivy, English walnut, orange trees and of course grapes, among a long list of likely hosts. One major difference between scale and mealybugs is that mealybugs remain motile, while scale females usually lose their legs as they shed, becoming immobile when maturity is reached. Males of both kinds of insects mature to small, winged midges that fly but, having no mouthparts, cannot eat. Each female lays between 300 and 600 eggs that can take up to 10 days to hatch (a few species are live bearers, the eggs are incubated within the female and the young are born live). The young move about, inserting their mouthparts into plant tissue and excreting "honeydew". This honeydew forms the major part of the diet of other insects, some of whom actively transport the scale to uninfested areas. (Paradoxically, honeydew forms a large part of the diet of adult lacewing insects, whose juvenile form is a major predator of scale.) Besides honeydew, scale insects provide shellac, cochineal red dye, and jewelry in the form of beads (from Ground scale, the family Margarodidae esp. M. meridionalis, and M. rileyi).
Outbreaks of scale can be triggered by plants being under stress (overwatering/underwatering), too much new growth resulting from an overabundance of nutrients (overfertilizing), overcrowding (unavoidable in some collections) and other conditions specific to the infestation. These other conditions can include injudicious spraying that kills the predators but leaves the scale eggs alive, introduction of a new, more favorable host plant, etc. Major predators of scale include lacewings, ladybugs, parasitic wasps, and animals such as woodpeckers and chickadees. Many remedies are available for control. Organic controls include alcohol spray (isopropyl alcohol, straight or diluted), soapy emulsion (can be mixed w/alcohol), horticultural oil (read the directions, taking the plant out of the sun is a must) and pyrethrum spray. Soapy water/alcohol should be reapplied every 2-3 days for 2 weeks. This is because little residual action can be expected, and newly hatched insects are occuring. Chemical sprays with proven results include Sevin and malathion. WARNING. DO NOT use sprays containing malathion on the Crassulaceae family of plants. These plants include the common Jade plants, as well as Aeoniums, Adromischus, Echeverias, Sedums, Kalanchoes and other genera. Some species in these genera are very sensitive to malathion, and damage or death may result. Cacti, however, exhibit tolerance to malathion. For root mealy control (generally Rhizoecus falcifer or R. cacticans), soil drenches are commonly available. Alcohol in large quantities in the root zone is generally to be avoided since it can dehydrate some tissues. Soapy water/alcohol mix should not be allowed to dry in the root zone since damage can occur. If it is used, after a short period it can be flushed by pouring clean water through the root zone.

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