What type of harm do whiteflies inflict on plants?

Whiteflies are pests that primarily feed on plant sap, which they extract from the leaves and stems of plants. This feeding behavior is classified as sap-sucking and can lead to significant stress on the plant. As the whiteflies remove sap, they not only deplete essential nutrients but also cause a reduction in the plant's overall vigor and health. Additionally, the removal of sap can result in other secondary effects, such as the proliferation of sooty mold due to the honeydew excreted by the whiteflies.

The effect of sap-sucking is crucial to understand, as it directly impacts the plant's ability to photosynthesize and grow. If left unchecked, a severe infestation can lead to yellowing leaves, reduced flowering and fruiting, and ultimately, plant decline or death. The other harm-related options involve effects that are not typical of whitefly behavior; for instance, while whiteflies can lead to leaf curling indirectly through their feeding and the presence of viral infections they transmit, their direct mode of harm lies in sap extraction.