I have been battling this white growth on several plants this fall. It seems to be getting worse and spreading from plant to plant. There is now sticky stuff on the leaves and some leaves are turning yellow/dying. I’ve tried spraying with soap and water, but that doesn’t help.
The insects in the photo you provided are mealybugs. These small soft-bodied, wingless sucking insects cover their bodies with a white, cottony substance which is what makes them easy to identify. Mealybugs are a type of scale insect that feed by inserting long sucking mouthparts, called stylets, into plants and drawing sap out of the tissue on both indoor and outdoor plants. Damage is not often significant at low pest levels. However, at higher numbers they can cause leaf yellowing and curling as the plant weakens, which is what you are currently seeing on your plants. Mealybugs also excrete honeydew, which makes the plant sticky and encourages the growth of sooty moulds. Mealybugs are a common greenhouse pest that affect ornamentals, houseplants, as well as outdoor shrubs.
These pests are difficult to control. The first thing you should do when you discover these pests is to isolate the infected plants, as you have notice they will spread readily between plants.
Your next line of defense is to physically remove as many mealybugs as possible. If the plant is small, you could remove the pests individually, knocking them into a container of soapy water. Or, you could spray the plant with a hard blast of water: a handheld shower-head on a high pressure setting will do the job. Collect and dispose of the bugs that fall off, and then pick off and dispose of any adults that remain on the plant.
Lastly, you can also apply an insecticidal soap made for indoor plants. Make sure to follow the manufacturer’s instructions.
The University of Maryland provides additional information on mealybugs and how to control them: Mealy Bugs on Indoor Plants
November 12, 2021