Invasive insects, drought and disease are killing some of Wisconsin’s oak trees. Many of the state’s oak trees are declining or dying as they face spongy moth infestations and drought conditions. One main culprit is the spongy moth, an invasive insect that was rampant in the state from 2022 through 2024. Catching decline early can keep trees healthyOak wilt happens quickly and can kill red oak trees within a month. But most of the other issues oak trees face take longer to kill the tree.