What insect infests azaleas and other shrubs by feeding on the underside of leaves and leaves black grainy specks of excrement?

The correct answer is scale insects because they are known to infest azaleas and various shrubs by feeding on the plant materials, particularly the sap found on the underside of leaves. As scale insects extract sap, they excrete waste that appears as black, grainy specks, often referred to as honeydew or sooty mold. This waste can coat the leaves, leading to a blackened appearance that is a hallmark of scale infestation.

In contrast, spider mites are also pests that can affect plants, but they typically cause a fine webbing and stippling damage rather than the distinct black excrement associated with scale. Billbugs mainly target turfgrass and are not known to infest azaleas or shrubs in the same manner. Leafcutter ants are primarily defoliators that cut and carry leave fragments instead of feeding on the undersides of leaves like scale insects do.

Recognizing the characteristics of the pest and its typical feeding behavior is important for proper identification and management in ornamental and turf pest situations.