Slideshow: Low-Quality Plants Power Locust Outbreak

Before 2004, Arianne Cease cared little about insects. Then a locust swarm swept through the village she was working in as a Peace Corps volunteer in Africa, and in a few hours it stripped the bark and leaves from hundreds of seedlings she was growing for her forestry project and gobbled almost everything else green in the area. Now, as a graduate student at Arizona State University, Tempe, Cease is getting to the bottom of what makes at least one kind of locust such a menace. It's a junk-food junkie. By munching grasses most other plant-eaters consider not worth the effort, the insect thrives in overgrazed grasslands until its populations swell so much that they move onto crops and pastures, wreaking havoc on local farmers.

The work suggests that in the case of this species, reducing the amount of livestock grazing might stave off outbreaks. It's "evidence that human activities might influence population densities," says Daniel Berner, an entomologist at the University of Basel in Switzerland, who was not involved in the work.

Researchers have long assumed that locusts swarm when their numbers get so high that they run out of good plants to eat and must move on to greener pastures, so to speak. "Greener" typically means containing more nitrogen, which is a key building block for proteins. The more plants have of it, the more nutritious they are for insects, livestock, and other herbivores. "Ecologists generally assume that plants high in nitrogen will facilitate insect outbreaks," says Fiona Clissold, a nutritional physiological ecologist at the University of Sydney in Australia.

But when Cease and her colleagues started investigating the dominant locust in north Asian grasslands, they were surprised to learn that individual insects grow more poorly on fertilized grass, which is high in nitrogen. The researchers spent several summers in Inner Mongolia following up on this finding. When given a choice between low-nitrogen grass and high-nitrogen grass, the insects consistently opted for the low-quality grass, Cease and her colleagues report online today in Science. The locusts were also more common in degraded pastures, where sheep had eaten most of the high-nitrogen plants. The insects preferred the plants there to plants collected from pristine areas.

Next, the researchers fed the locusts artificial diets to find out what ratio of carbohydrates to protein (which contains nitrogen) best suited them. Locusts tend to have low protein requirements, but this species needed the smallest amount "of any locusts studied to date," Cease says, indicating a preference for low-protein foods.

The work adds to growing evidence that each species has an optimum ratio of carbohydrates to protein in its diet and that too much of either can be a bad thing. Although low-protein foods translate into poorer survival for many organisms, "this locust is king of what we typically consider to be junk food," Cease explains. "This locust has evolved to take advantage of areas other grasshoppers are not able to use."

"This is the first paper that really makes a link between lower plant quality and enhanced performance," says Spencer Behmer, a physiological ecologist at Texas A&M University in College Station, who was not part of the study. It also helps make the connection between increased overgrazing and land degradation in the Eurasian grasslands and a surge in locust outbreaks, adds Stephen Simpson, a comparative nutritional ecologist at the University of Sydney, who also was not involved in the research. "Understanding this relationship will help in the development of land-management practices that reduce the locust threat."

Cease's group found very few locusts in areas with no more than six sheep per hectare; but where there were nine or more sheep per hectare, outbreaks were common. "The only way to really control [this locust] is to limit overgrazing," Behmer says. Moreover, once an outbreak seems imminent, fertilizing a field might one day provide an alternative to pesticide treatments, Cease says.

Cease wants to test other locust species to determine their ideal nitrogen-to-carbohydrate ratios. In particular, she hopes to return to Africa, where a close relative of the locust she studies is a threat to crops.