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The diversity of modern herbivorous insects and their pressure on plant hosts generally increase with decreasing latitude. These observations imply that the diversity and intensity of herbivory should increase with rising temperatures at constant latitude. Insect damage on fossil leaves found in southwestern Wyoming, from the late Paleocene–early Eocene global warming interval, demonstrates this prediction. Early Eocene plants had more types of insect damage per host species and higher attack frequencies than late Paleocene plants. Herbivory was most elevated on the most abundant group, the birch family (Betulaceae). Change in the composition of the herbivore fauna during the Paleocene-Eocene interval is also indicated.