What does a large number of scale insects form on plants?

Scale insects, which are small, sap-sucking pests, often form a protective covering that can appear like a crust on the surface of plants. This covering is composed of waxy secretions and body remnants of the insects themselves, leading to a hard outer layer that can be mistaken for bark or other plant structures. This crust can serve a dual purpose: it helps to protect the scales from environmental factors and natural predators while also making it more challenging for the plant to combat the infestation.

In contrast, while some pests might create powdery substances (often from mold or mildew), scale insects do not produce these. Additionally, they do not typically lead to fungus growth in a manner that would be characterized as a crust. The liquid droplets associated with scale insects, known as honeydew, are a byproduct of their feeding habits rather than a physical structure they create on the plant's surface. Thus, the appearance of a crust is specific to the damage and protective covering formed by scale insects, making it the best choice in this scenario.