The gypsy moth is one of the most devastating forest pests in North America. In late spring, gypsy moth larvae hatch from eggs laid the previous summer. During the next forty days, tens of thousands of these caterpillars eat up to one square foot of foliage each. The gypsy moth has established populations in several states, and dangerously fast-growing populations in several others.

The state of Ohio is a critical area in the suppression of the gypsy moth. At this time, the gypsy moth population is contained in the eastern part of the state. If the gypsy moth establishes a population in the western part of the state, however, the range of the gypsy moth could easily spread into states west and south of Ohio. Besides diminishing the aesthetic value of Ohio's forests, gypsy moths also cause real economic damage to the Ohio timber industry, which is estimated to be a $10 billion per year industry.

The Ohio Department of Agriculture currently uses aerial sketch mapping each year to assess the damage done by the gypsy moth. This procedure is unpleasant, and most people are unwilling to do it. The state departments of Ohio now have unprecedented access to Landsat data. The data are available in a timely manner, and at a relatively low cost. This presents a real opportunity for improvement of gypsy moth management in the state of Ohio.

Vegetation reflects infrared light and absorbs visible light. These unique properties allow the health of vegetation to be assessed using the ratio of Landsat Band 4 (near-infrared) to Band 3 (red). To determine the change that has occurred between two dates, the ratio values from two dates may be subtracted. To identify change that has been caused by the gypsy moth, an area should exhibit defoliation between early June and late June and subsequent refoliation between late June and late July. This type of change results in large ratio subtraction values between early June and late June and negative ratio subtraction values between late June and late July. These ratio subtraction values are analyzed using change vector analysis to further isolate areas where change has been caused by the gypsy moth.