While certainly too small to see from space, the gypsy moth caterpillar (Lymantria dispar) can cause enough change in a forest to be seen with Landsat’s 30-m resolution. A mere 1/16 inch long when they hatch in May, the caterpillar can reach 3 inches long by late June when they pupate. In that short time span, the pest feeds on the leaves of deciduous trees. Beginning in 2016, they caused noticeable change in the Northeastern United States.
While Landsat has not typically been used to monitor insect outbreaks in near-real time, new analytical tools combined with the open access to Landsat data make it possible to quantify insect damage over a large area at a level of detail not possible with aerial surveys alone.
The 2016 and 2017 images show the widespread defoliation of the trees in orange-brown, with a large area of heavy defoliation to the west of Providence, Rhode Island, and additional patches in eastern Connecticut. In 2018, defoliation was much less widespread and severe; however, continued defoliation is now mixed with mortality as individual trees succumb to multiple years of defoliation and other stressors including drought.
Gypsy moth caterpillars devour the leaves of hardwood trees, causing the greatest damage in late June as the larvae reach maturity. The caterpillars are hungry little buggers, and they’re not picky eaters. Even though they prefer oak and aspen, they feed on other deciduous trees such as maple, birch, poplar, willow, apple, and hawthorn, and can go after coniferous trees during a large outbreak.
During a dense population boom, they eat all the leaves on a tree and can even defoliate entire stands of trees. Beginning in 2016, the combination of visible and infrared imaging from Landsat revealed the extent of the damage the outbreak has caused.
Healthy forest is bright green, but as the time series progresses, much of the green turns to faded brown as the infestation spreads.
Though gypsy moth outbreaks are a fairly regular occurrence in the more southern portion of their invaded range, this outbreak was the first time gypsy moths caused significant defoliation in New England since the 1980s.
How did they get here?
An amateur entomologist Etienne Léopold Trouvelot brought gypsy moth eggs from France to Massachusetts in 1869. He was conducting experiments with the eggs possibly to see if the gypsy moth could be an alternative to the native silkworm. The insects somehow escaped from his home and have affected the forests of the Northeastern United States ever since.
The current gypsy moth outbreak is thought to be the result of a series of unusually dry springs in 2014, 2015, and 2016, which suppressed a fungus that keeps the moth population in check. The fungus (known as Entomophaga maimaiga in the scientific world) infects the caterpillars and causes high rates of mortality. However, researchers believe that during this outbreak, low precipitation during key periods in the gypsy moth life cycle resulted in lower amounts of this fungus, so the fungus was not as effective and the moth population was not kept in check.
A healthy tree can survive a defoliation by the caterpillars. It can usually produce new leaves in the same growing season. However, consecutive years of caterpillar attacks can cause tree mortality.
Landsat data can track the spread of the outbreak and monitor defoliation. Comparing newly acquired Landsat observations with long-term average conditions modeled from Landsat time series makes it possible to detect changes in vegetation “greenness” in near-real time.
These forest condition assessment maps represent changes in expected forest reflectance signatures compared to average patterns over an 11-year baseline. Blue pixels indicate forest conditions that are within the normal range of variability, while yellow, orange, and red show sustained decreases in vegetation greenness that coincide with caterpillar stages of the gypsy moth life cycle and are indicative of varying degrees of defoliation.
Black is non-forest, according to National Land Cover Database 2011 (NLCD 2011) classifications.
Data from Landsat not only detects initial outbreak and magnitude of the defoliation but also recovery later in the season.
The overall severity of gypsy moth defoliation was less in 2018, but the impacts on forest condition are still evident as both new defoliation and tree mortality. Between Springfield, Massachusetts, and the Quabbin Reservoir, changes in forest condition resulting from three years of above-average gypsy moth activity is revealed by Landsat imagery.
In fact, Landsat can detect both short-term and long-term forest disturbance of various causes. The gypsy moth outbreak is caused in part by an extreme weather event—drought. The pale horizontal line extending to the east from Springfield is the path of a tornado that touched down on June 1, 2011. Tornadoes are also an extreme weather event and unusual for this area.
The gypsy moth caterpillar has a larger effect on dominant species like oak, which can cause large-scale shifts in the very composition of the forest. With Landsat data and the resulting maps of infested areas, researchers can better understand the long-term impacts of outbreaks on the health of forests.
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