Whiteflies can seriously injure plants by sucking juices from them, causing leaves to yellow, shrivel, and drop prematurely. However, experts say there is little to worry about. Whiteflies suck plant juices and, in turn, produce a sticky substance known as honeydew. Honeydew left on its own can cause fungal diseases to form on leaves. How to Identify Whiteflies on Plants. Whiteflies can cause two types of damage to a plant. They are very similar to a mosquito. White flies harm your plants by sucking the sap, juice, and moisture from the leaves. Whiteflies are sucking insects and excrete honeydew which can cause a secondary problem…..sooty mold. “They damage plants, but besides irritating people they do little else in the way of harm to humans,” Ebrahim Azizkhani, an entomologist, told Borna News Agency. Whitefly Bites. White flies may not be harmful to humans, but they can damage and kill your favorite ornamental trees and bushes. The first is considered to be “direct” damage. Lifecycle of Whiteflies. Whitefly damage-College of Agricultural and Human Resources (left), Whitefly damage-Top Turf Insect Damage (right) Diet and Damage Host plants include strawberries, house plants, greenhouse plants, vegetables and some ornamentals. Because chemical insecticides can be toxic to humans, pets and beneficial insects, it's best to save them for the worst whitefly infestations. Whiteflies are not known to bite humans, but they do have piercing mouthparts they use to extract fluids from plants. Due to whitefly feeding, plants will quickly become extremely weak … The whitefly is a species that thrives in warm environments and cannot survive the cold.