Swarm Shift: How Locusts Switch Phases When Numbers Swell

By Ed Ricciuti

Until 1954, periodic plagues of the South American locust Schistocerca cancellata, sometimes covering almost 4 million square kilometers, ravaged the continent’s southern half, from southern Brazil to central Argentina and northern Chile. The invasions were halted by deployment of chemical pesticides in agricultural fields, preventing buildup of locust numbers and holding them at bay for 60 years. But, in 2015, like a recurring bad dream, the locust hordes returned and swarmed over much of Argentina.

Locust populations apparently had been building for years, Texas A&M University entomology professor Hojun Song, Ph.D., said in news reports at the time. Scientists suggested that warmer, wetter conditions may have caused environmental factors that triggered the swarms. Recently, a group of scientists including Song and colleagues from Texas A&M and several Argentine institutions, conducted a study exploring the process that leads to such locust “plagues.” Their findings are reported in a new paper published in July in the Annals of the Entomological Society of America.

Plague locusts have a split personality, with two phases. Young locusts, called nymphs, are inconspicuous loners. That is, until certain environmental conditions trigger significant changes in their behavior. They then display a gregarious phase, forming huge bands, or swarms. Physical changes, including morphology and development, accompany the behavioral shift.

Scientists have long pondered how the density of individuals sparks the transformation. Research on locusts has focused on the infamous desert locust S. gregaria of North Africa and the Middle East. The research reported by Song and colleagues fills a void in knowledge because S. cancellata has been little studied due to the long reprieve from plagues. However, the research suggests many similarities in the biology of the two locust cousins.

The swarming process involves a phenomenon known as locust phase polyphenism, in which environmental changes such as food shortages and crowding modify the insect’s observable characteristics such as behavior, morphology, coloration, life stages, and physiology. Organisms that exhibit such phases—Labrador retrievers of different colors are often cited as an example—are said to have “phenotypic plasticity.” It means that an organism can exhibit different phenotypes, different combinations of physical characteristics arising from the same genetic makeup.

“Because of the 60-year absence of major plagues in South America, there is virtually no modern study available that has quantified the effect of rearing density in S. cancellata,” the researchers write. “But now, the availability of a lab colony of S. cancellata enabled us to conduct rearing experiments to explicitly quantify the effect of rearing density in many traits that are associated with locust phase polyphenism.”

The researchers studied both nymphal and adult stages of the locust. They found that crowded nymphs in the last stage of development were much more active and more attracted to others of their kind than those experimentally kept in isolation. This finding corresponds with the fact that one of the defining characteristics of locust phase polyphenism is the ability to change behavior in response to changes in population density.

Normally, the locusts are quiet like your typical grasshopper, resting and feeding under cover, bursting into flight only when disturbed. When crowded, nymphs are jumpy and active, eating more than normal. Adults, who normally lay solitary egg pods, for reasons yet unknown sometimes lay them in groups so that nymphs appear in large groups that start moving en masse. The flightless young locusts travel in coordinated, marching bands. Adults both walk and fly, in the same organized fashion.

An increase in local population density, say the researchers, ultimately leads to the gregarious phase and swarming. “We demonstrated that the stimulus inducing plastic responses in behavior is clearly local population density,” says Martina E. Pocco, Ph.D., of Argentina’s Centro de Estudios Parasitológicos y de Vectores and lead author on the study.

The density of nymphs that were studied even impacted color. Crowded nymphs had heads colored pale orange to orange-red and bore large black areas on various parts of their bodies, which were reduced to black dots on isolated nymphs, with green or light brown heads.

Color changes, says Pocco, may provide warning coloration. Changes in head and body size may relate to increased competition for food under swarming conditions.

The researchers say that detailed experiments on the biology of the S. cancellata locust under both isolated and crowded conditions “are critically needed to fill in the gap in the knowledge of the South American locust.”

Among other findings was that the pronotum, the hard covering over the thorax, was smaller on crowded female nymphs than on isolated individuals, and the femur was also shorter. The reverse was true for males. Meanwhile, isolated nymphs took longer to develop and lived longer as adults. As for adults, crowded and isolated females were similar in overall size, although crowded females had larger heads. Crowded males were significantly larger than their isolated counterparts.

A key finding was that isolated nymphs of both sexes had about 50 percent more sensory hairs on the outer face of the hind legs as those raised under crowded conditions. The hairs, which detect mechanical stimuli such as pressure and vibration, trigger behavior leading to swarming when brushed in laboratory experiments. The hairs, explain the author, are the main structure for detecting density. The larger number of hairs on solitary nymphs may facilitate the location of others. Once gregarious, the extra hairs are not needed.

Taken together, the myriad findings of the research will add to information fundamental to evaluating the risk that a local locust population will suddenly turn gregarious and initiate a plague. Better understanding of the phases in locusts, say the authors, “is crucial for an earlier prevention” of destructive outbreaks.

“We think that further detailed evaluation in the field of the traits registered in our study would help detect the early signs of gregarization,” says Pocco. In other words, research in the laboratory may lead to a way to stop future plagues before they start.

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.