When caterpillars descend on a forest, they can strip every leaf from every tree in just weeks. For American forests threatened by the European spongy moth, that destruction has been nearly impossible to track from the ground. But now, scientists at Cornell University have found a way to watch the devastation unfold from space—and warn land managers before it's too late.
The researchers built a computer program that reads satellite images of forests and spots the telltale signs of a spongy moth invasion. deciduous trees healthy one month can lose all their leaves the next, then sprout new ones just weeks later—exactly the pattern these caterpillars leave behind. The system uses images from the European Space Agency's Sentinel-2 satellite, which monitors land conditions from orbit.
"Our ability to actually track and project where we're going to see high risks means we can have land managers on alert, paying more attention and possibly spraying insecticides and handling management activities," said Cameron Scholl, a doctoral student at Cornell who led the study. Finding egg masses and larvae currently requires teams to trudge through large forests in spring—slow, costly work. "Any bit of information that tells people this is where you should be putting your efforts can help."
The moths, native to Europe, Asia, and North Africa, first arrived in Massachusetts in 1869, likely hitching a ride on ships. Without natural predators in America, they have spread from North Carolina all the way north to Ontario, and as far west as Wisconsin. The caterpillars feed on roughly 300 different tree species. "When they reach high populations, they will eat almost all the leaves off every tree," Scholl said. Hardwood trees can usually recover by growing fresh leaves later in the season. But evergreen trees like pines and spruces cannot replace their needles so easily, making them more likely to die after a severe outbreak.
The study, published in the journal Forest Ecology and Management, also uncovered a clue that could help predict outbreaks before they start: warm, dry summers seem to signal heavy caterpillar populations the following year. The reason lies with a natural fungus that normally keeps moth numbers in check—this fungus needs moisture to work. When summers are dry, more caterpillars survive to become adults and lay eggs, hatching into ravenous hordes the next spring.
For now, the satellite system gives forest managers something they never had before: a way to see exactly where the moths are striking, and to act before entire woodlands are stripped bare.