Question
Dear John,
i love in rome (italy) & have  cottony cushion scale on my acer.
what is the best way to treat this pest - speaking on the phone with my dad, before knowing what creature this was, suggestid maybe removal & rubbing with something like nail-varnish remover...so far i have done nothing.
thank you,
emma.

Answer
Hi Emma,

The one thing that you didn't mention was whether this is an ornamental indoor species, or an outdoor species in the ground.  It can make a big difference in treatment.  If the plant is indoors, then the natural predators will usually control the insect populations once placed outdoors.

Although adult females with their white, ridged egg sacs are the most obvious stage, adults are not well-controlled by insecticides. The females and their eggs are protected both by the cottony egg sac and by their position inside the canopy of the tree, making them difficult to treat. If scales cannot be tolerated, apply narrow range oil to deciduous hosts during the dormant season or spray foliage with insecticides when the females are dead and the tiny reddish scale crawlers and younger stages are out on the leaves.  I don't know what horticultural oils are available in Italy, but a horticulture center (garden center) should be able to help.  These oils are best used during a dormant period.

You can use traps made of double-sided sticky tape to determine when crawlers are hatching. Before crawlers begin to emerge in spring, tightly encircle several twigs or branches near adult female scale with transparent tape that is sticky on both sides (available at stationary stores). Change the tapes at regular intervals, about weekly, and examine them with a hand lens to identify the crawlers. Once eggs begin hatching, scale crawlers get stuck on the tapes and appear as red or orange specks. Spray after you observe a sharp increase in crawler production.

Natural enemies are the best method for controlling cottony cushion scale, so look carefully for their presence and avoid insecticides if you find evidence of them attacking cottony cushion scale. If natural enemies are absent, and the population is in a treatable stage, the organophosphates malathion (in US) or acephate (in US, brandname - Orthene) can be effective.

In any application of chemical insecticides, it must be done with care as the beneficial insect predators may also be killed in the process.

If the plant is relatively small, indoors, you may attempt to dislodge the scales from the plant by using an old toothbrush and soapy water.  You may be able to dislodge them with a bit of effort.  The small juvenile "crawlers" can usually be controlled by using rubbing (isopropyl) alcohol on a cotton swab.  

Good Luck.