Whiteflies are small gnat-like insects, covered in a powdery white wax that feed on the sap of plants. This causes leaf-drop and weakens the plant in general. Whiteflies secrete a sticky substance called honeydew while they feed. The flying adults lay eggs on the tops of leaves, but it is the immature nymph stage, feeding unnoticed on the underside of the leaves, that does the real damage. Yellow Sticky Cards are a good way to trap the flying adults and prevent new eggs. Spray an insecticide or the the Green Solution (recipe in the Glossary) to get rid of the feeding nymphs on the underside of the leaves.