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

The adelgid is a type of sap-sucking insect closely related to aphids and is known for its significant impact on coniferous trees. Specifically, adelgids primarily feed on conifers, like spruce and hemlock, where they can cause galls—abnormal growths on the plants. These galls form as a reaction to the adelgid's feeding, leading to deformities and potentially weakening the tree's health.

Understanding the role of adelgids in the ecosystem and their detrimental effects on forestry is crucial for pest management and conservation efforts. While scale insects, mealybugs, and whiteflies are also pests that cause various issues on plants, they do not specifically target conifers in the same way adelgids do, nor are they known for gall formation on these trees. This distinctive behavior and feeding pattern of adelgids make them the correct answer to the question.