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  • Satellite keeps an eye on beetles

May 2009
Rebecca C. Jernigan,

SALT LAKE CITY – In the early nineteenth century, a small Eurasian tree was imported into the US for ornamental use and erosion control. Known as a tamarisk or saltcedar, this tree is considered an invasive species by the National Park Service because it has overtaken large sections of riparian lands in the Southwest, choking out native species.

A saltcedar leaf beetle sits on a tamarisk plant.

A few years ago, to combat the tamarisk, scientists released saltcedar leaf beetles (Diorhabda elongata) imported from Kazakhstan into regions where the tamarisk was common. They believed that the insects, which feed on tamarisk leaves, would defoliate and eventually kill the trees. However, many saltcedars grow in locales that are difficult to reach – so how can researchers track the beetles’ progress while monitoring any adverse environmental outcomes?

Investigators at the University of Utah, led by assistant professor of geography Philip Dennison, used infrared satellite data from two instruments on NASA’s Terra Earth-observing satellite to get an overall view of the defoliation. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) provides high-resolution images, with each pixel covering an area of around 50 × 50 ft at a rate of one to three images each summer. The Moderate Resolution Imaging Spectroradiometer (MODIS), having pixels that cover about 820 × 820 ft, offers less detail but provides daily images. According to Dennison, the ASTER images enabled the researchers to map the areas of defoliation, while the MODIS images allowed them to detect changes in vegetation over time. They selected NASA satellite imagery for their study because the agency provides the data at low cost.

Satellite imagery from 2006 (left) and 2007 (right) exemplifies the defoliation of a large number of tamarisk trees near the Colorado and Dolores rivers. In these infrared images, tamarisk foliage, which absorbs red light and reflects near-infrared light, is bright red. The sections showing less red in 2007 represent regions defoliated by saltcedar leaf beetles. Images courtesy of Philip Dennison, University of Utah.

The areas studied included four sites along the Colorado River and one on the Dolores River, a tributary. They examined the entire 589 acres, identifying 56 polygon-shaped areas totaling 57 acres where beetles had defoliated the tamarisk trees. The team compared images from 2005, 2006 and 2007 and discovered a small decrease in foliage from 2005 to 2006 but a large decrease between 2006 and 2007. Because the tamarisk is thought to deplete the water supply, the team used the satellite data also to estimate “evapotranspiration,” or evaporation of water from the soil and the use of it by plants. It is not yet known whether repeated defoliation will ultimately kill the plants or simply reduce their overall number of leaves.

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