Soybean aphids on a soybean leaf. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org.

By Edward R. Ricciuti

Ed Ricciuti

Unmanned aerial vehicles, better known as drones, are constantly being used for new purposes. They help locate lost people, deliver packages, and now have proven their worth at detecting when agricultural crops are being stressed by insect pests, according a paper published in the Journal of Economic Entomology.

The research turns on the fact that plants under stress reflect light waves differently than normal plants. Red reflectance from vegetation, for example, indicates chlorophyll content of the plant canopy and active photosynthesis, and near-infrared reflectance provides information about the cellular structure and intracellular air spaces within leaves, overall canopy coverage, and above-ground biomass.

The study found that that stress to soybean crops caused by the soybean aphid (Aphis glycines) can be detected by drone-based multispectral imagery, which photographs reflected light of several electromagnetic wavelengths in the same image. It can paint a high-resolution picture in wavelengths visible as well as invisible to the human eye and has potential for scouting the presence of not only the soybean aphid but other field crop pests. Combined in an index, the wavelengths detected provide a measure of overall plant health, which often correlates to crop yield.

A New Remote-Sensing Option

Satellites and piloted aircraft have been used for remote sensing of crops but are expensive, low-resolution, and limited by atmospheric conditions and orbital periods. More recently, changes in reflectance indicating stress caused by the aphid have been detected by ground-based measurements, with hand-held sensors, for example.

“Ground-based sensing is a recent development and provides far less coverage than from a drone,” says lead author Zachary Marston, PhD, of the University of Minnesota’s Department of Entomology. “Detecting soybean aphid-induced stress from a drone is the big win.”

Introduced from Asia in 2000, the soybean aphid has hit hard at the soybean-growing heartland of the north-central United States, which produces 75% of the nation’s crop. The United States leads the world in production of soybean, and it is the nation’s most widely grown field crop.

Infestations of the aphid are hard to prepare for because outbreaks can be erratic. Because outbreaks are unpredictable, current management recommendations call for routine scouting of soybean fields to monitor soybean aphid populations. Scouting, normally done visually, can be so time consuming that some farmers have opted instead for prophylactic applications of insecticides, risking development of insecticide resistance and harming non-target species and water quality. Before introduction of the aphid, pesticides were rarely used on soybeans. Since its introduction, there has been a 130-fold increase in pesticide application.

“It just isn’t realistic for conventional scouting to cover every plant in a field,” said Marston. “For soybean aphid, a scout often looks at less than one plant per acre. For the time being, however, conventional scouting is still the best way to determine the cause of stress in a field. We hope these findings will help change the way soybean fields are scouted by using the drone to cover the entire field first and identify areas that are likely infested with aphids or another problem. After problem spots are identified, the scout can walk directly to the areas most likely to have a problem.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.