It is well recognized that host-specialized folivores prefer to feed on young in comparison to old leaves. However, the capacity of young leaf feeders to track specific leaf developmental stages has not been clearly demonstrated. Using three insect folivores and two plant species, we show that nitrogen (N) fertilization changes leaf development and that herbivores track these changes in leaf development. Nicotiana tabacum L. and Populus deltoides Bartram were fertilized at two and three rates of N addition, respectively. Plants with high rates of N supply had faster growth, greater leaf area, and faster leaf initiation rates than plants receiving low rates of N supply. Most important, all N addition treatments changed the position on the stem where leaves reached 95% full expansion; with leaves on plants receiving high rates of N addition reaching 95% full expansion further from the stem apex (e.g., leaf position 5 versus leaf position 3). Feeding assays with Trichoplusia ni Hubner on N. tabacum, and Plagiodera versicolora Laicharting and Chrysomela scripta F. on P. deltoides showed that these insect species preferred to feed on leaves at a specific degree of leaf expansion. This preference was exhibited independent of leaf position and N addition rate.