SummaryIn order to explain and predict yield losses caused by European corn borer larvae,Ostrinia nubilalisHbn. (Lepidoptera: Pyralidae), a mechanistic model was proposed to describe cavity appearance, their lengthwise extension, and their within‐plant distribution. It integrates main dynamical processes of larvae: mortality, development and within‐plant distribution. After its estimation and test under field conditions in 1988 and 1989, its validation, based upon X‐ray photographs, is presented. This method allows cavity appearance and lengthwise extension to be followed without damaging the plants. This study confirms the relevance of the damage model and the rate of cavity lengthwise extension estimated wi