Estimating defoliation by insects in an agricultural field, specifically soybean, is performed by manually removing multiple leaf samples, visually inspecting the leaves for feeding, and assigning a value representing a “best guess” at the level of leaf material missing. These estimates can require considerable time and are subjective. The goal of this study was to design a low-cost system containing light sensors and a microcontroller that could remotely record and report long-term and real-time defoliation from the field. In experiments to evaluate the effectiveness of light sensors in measuring defoliation at different light intensities, sensors were subjected to two sets of simulated defoliation covers made out of card board (0, 25, 50, 75, or 100%) and paper (0, 15, 30, 50, 60, or 100%) while exposed to 7 different light intensities. The results indicated that, as light intensity increased, accuracy of the sensing system improved. This paper also exhibits the potential of the developed sensor for real-time defoliation estimation in the field.