Studies suggest that insect pests may increase in northern forests due to increased metabolic activity in active periods and increased winter survival. Many insect species are limited by extremely cold temperatures, and evidence suggests that several insect species may increase in a warmer climate, including the hemlock woolly adelgid. Trees stressed by heat, drought, or disturbance are also typically more vulnerable to insect pests and diseases. Emerald ash borer and Dutch elm disease are expected to continue to limit ash and elm species.