The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: I have 'inherited' an eight-foot, beautiful weeping fig (Ficus benjamina) from a friend. The plant is located in my solarium and did not suffer any leaf loss despite moving it to its new location in the back of a van during a cold, damp day in January. It has plenty of new growth emerging and overall adjusted to the new environment. However, in the last few days, I noticed that leaves are yellowing and falling off. I looked under the leaves and discovered a small, maybe the size of the tip of a pencil, wax-like, white speck with lots of sticky ooze, on the midvein where the leaf stalk attaches to the leaf. The white speck is easily dislodged from the leaf with my finger. A lot of the leaves have this white speck. It appears that this is killing the leaves because the leaves with the white speck are slowly turning yellow.
Answer: The leaves of houseplants can fall off for a variety of reasons. However, the problem you describe sounds like mealybugs. Mealy bugs are oval, white insects up to 1/4 inch long. Mealybugs can cluster in white, cottony masses on leaves, stems, and in the crotches where leaves are attached. A sticky material may coat the leaves. When the condition is severe, leaves and plants may wither and die.
Mealybugs damage plants by sucking sap. The adult female mealybug may produce live young, or may lay eggs in a white fluffy mass of wax. The immature mealybugs, called nymphs, crawl all over the plant and onto nearby plants. Soon after they begin to feed, they produce white waxy filaments that cover their bodies, giving them a cottony appearance. As they mature, they become less mobile. Mealybugs cannot digest all the sugar in the sap and excrete the excess in a fluid called honeydew, which coats the leaves and may drop onto furniture below the plant.
If you choose to use an insecticide and the description above sounds like the problem your fig has, use an insecticide that is labeled for use indoors. You can also wipe mealybugs off of the plant with a damp cloth soaked in soapy water or with cotton swabs dipped in alcohol. Inspect the pot, including the bottom, for mealybug egg masses. Wipe off any egg masses that you find.