White spots on palm leaves are usually indicative of a scale infestation. These tiny insect pests attach themselves to the leaves and stems of palms and other plants, sucking the sap and damaging or killing the plant. Sago palm scale (Aulacaspis yasumatsui) is the most deadly. Also called cycad aulacaspis scale, it spreads rapidly and kills quickly.
Infected palms show tiny white spots on the undersides of the leaves. In time, the spots merge and the fronds look like they've been "whitewashed" or painted white. Some fronds may turn brown on top and still be white on the bottom, or they may turn brown entirely and curl up or even drop from the plant.
The white spots are actually tiny insects called scale. The insects have a white, waxy covering on their bodies that protect them from some insecticides. As infestations increase, the scale move to the upper sides of the leaves and onto the stems and petioles of the palms. As they suck the sap from the fronds, the fronds turn brown and eventually die. In time, the entire plant dies. This is particularly a problem in sago palms.
Scale are difficult to get rid of entirely, although insecticidal oils are effective, according to the Florida Department of Agriculture and Consumer Services website. Repeated applications of insecticide are usually needed for all but the most minor of infestations. For stubborn cycad aulacaspis scale, insecticidal oils combined with natural predators is the most effective treatment. Two natural predactors of scale are the beetle Cybocephalus binotatus and a wasp called Coccobius fulvus.
Unfortunately, there is no surefire way to prevent scale infestations on your palm leaves. Monitor new plants closely for signs of scale before introducing them to your landscape or even bringing them into your home. If one plant is infected but others are not, remove the infected plant so that the scale will not spread.