Which insect closely related to aphids feeds on conifers and causes galls?

The adelgid is a small sap-sucking insect closely related to aphids and is known for feeding on conifer trees. It specifically targets species such as hemlocks and spruce, where it can cause significant damage by extracting nutrients from the plant. One of the notable effects of adelgid feeding is the formation of galls, which are abnormal growths on plants that result from the insect’s feeding behavior. This relationship illustrates how certain insects can manipulate plant tissue, leading to galls and other harmful effects on the host plant.

In contrast, scale insects, mealybugs, and whiteflies, while also damaging to various plants, do not specifically induce the galls that adelgids do on conifers. Each of these insects has its own distinct interactions with plants, but they differ significantly in their feeding habits and the specific kinds of damage they cause. This makes the adelgid the correct choice in the context of the question regarding insects that feed on conifers and cause galls.