Ficus benjamina needs careful attention to water needs. Use room-temperature water to avoid shocking the plant. Keep the soil just barely moist during the winter months. According to Royal Horticultural Society website, ficus require medium to high humidity, so mist plants to keep humidity high, especially in indoor heating.
According to North Dakota State University's website, scale infestation causes curling of leaves on ficus. Scale is an insect that produces a hard shell that sticks to stems and leaves and feeds on the plant. It can cause significant reduction in vigor and often death. Horticultural oils and insecticidal soaps help to control scale infestations in plants, but do not always cure the problem. You can attempt to scrape off the hard scale from plant surfaces to remove the insect. If the scale returns, remove affected branches and destroy them. You may need to destroy the plant to avoid infesting other houseplants.
Thrips are tiny yellow or black insects that often infest ficus benjamina plants. These insects can infest trees in plant nurseries, which are then carried into home environments. They possess sucking mouth parts that feed on the leaves of plants. Immature groups of thrips feed on the leaves of ficus, causing the leaves to curl and distort, according to the University of Florida website. Females will lay eggs on the upper sides of curled leaves. They recommend controlling thrips with insecticides such as carbaryl, malathion or pyrethins. Monitor thrip population on ficus plants with blue, white or yellow sticky traps to avoid problems with the health of your ficus benjamina plants. These are available at garden centers and nurseries.
Ficus benjamina displays curling leaves when insufficient amounts of magnesium are in the soil. You may see orangeish-brown veins running through the veins at first, which then progresses to leaf withering and curling. Provide additional magnesium to the plant by mixing one tablespoon of Epson salts in two gallons of water, according to the National Gardening Association website. Gently mist the plant two times each week until you see improvement.