Schistocerca lineata
In our last blog on the History of UT Entomology, we looked at the damage screwworm flies were causing to livestock until the Sterile Insect Technique was introduced. In this blog, we will be looking at another infamous insect responsible for massive damage to resources such as crops. We begin with some of the work Dr. Greg Sword did at Brackenridge Field Lab (BFL) from 1993 to 1998 with grasshopper ecology, and how it led to insights into the biology of the desert locust, a notorious pest of Asia, Africa, and the Middle East.
What is the connection between locusts and grasshoppers? Locusts are the swarming phase of certain species of short-horned grasshoppers in the family Acrididae. Under certain conditions involving overcrowding, grasshoppers can change color and behavior and form swarms. It’s at this stage they are locusts. Nothing new to human history, locust swarms and the devastation they cause have been documented in ancient texts such as the Bible and Koran.
When Sword was just starting his graduate career at UT, he began to study a grasshopper named Schistocerca lineata. As he puts it: “I was primarily interested in the plants it ate and how its diet varied over time as well as among different populations.” At BFL, this grasshopper fed mainly on a plant called Ptelea trifoliata, also known as skunkbush. This plant is related to oranges, and is toxic.
Desert locusts.
When reared in isolation, the grasshoppers were green and cryptic, meaning they blended in with their environment. “But there was something unique about this grasshopper’s coloration,” Sword states. “I often found it to be yellow-and-black in the field, but by conducting experiments I was able to show that its color could vary from being green to yellow-and-black depending on its population density. So when reared in crowds, they would become bright yellow-and-black and conspicuous.”
Sword was able to demonstrate that the reason these grasshoppers turned yellow-and-black when in crowds was to signal to predators they were toxic and distasteful. However, the insects themselves are not toxic. It’s what is sitting in their guts that made them distasteful: skunkbush. This was the first ever discovery of density-dependent warning coloration in any animal and was published in the scientific journal, Nature.
So how does this work conducted at BFL relate to the infamous desert locust? “Schistocerca lineata is a New World relative of the desert locust,” Sword explains, “and it is well known that the desert locust can also change from green to yellow-and-black depending on its local population density.” This change in desert locust coloration commonly occurs prior to outbreaks and swarm formation.
“However, despite the fact that the desert locust had been the subject of human attention and study for centuries, no one knew at the time of my graduate research why they changed color in this manner,” Sword says. Based on his work at BFL, he received funding to conduct fieldwork on desert locusts in naturally occurring populations in the West African country of Mauritania. There he showed that locusts in pre-outbreak populations also feed on plants that make them toxic to their predators. At this stage, they are green and cryptic, making them difficult to spot by predators such as birds. But as locust populations grow and they change colors, predators learn to avoid the new yellow-and-black color patterns that the locusts express with they become crowded. Without predators, locust populations continue to expand, ultimately forming the devastating swarms for which they are so well known.
Greg Sword and a member of the Anti-Locust Center in Mauritania asking a man if he has seen any locusts during his travels in the vast Sahara desert.
Sword’s work forms the basis for the ongoing development of desert locust population monitoring schemes and population dynamics models that take into account the effects of locust host plant use and predator learning. The research goal is to identify areas where desert locust outbreaks are most likely to occur in order to prevent them, rather than react to massive locust outbreaks when the insect is inflicting massive damage to agriculture. Speaking on the value of BFL for his early research: “Access to a world class field laboratory played a huge part in my success as a student at UT. The ability to work daily, and sometimes both night and day, so close to campus and where I lived enabled me to conduct research in ways that simply wouldn’t have been possible if I had to try to travel to other sites across the state on a regular basis.”
Sword’s PhD supervisor, Dr. Lawrence Gilbert, agrees and adds that close proximity of the field site to more controlled settings, such as the laboratories at the Patterson Buidling on campus, not only allowed testing of hypotheses suggested by field observations, but led to fundamental discovery as well. According to Gilbert, Sword’s work at BFL is one of the more remarkable examples of work locally and that applies globally.
Sword currently is Professor and Charles R. Parencia Chair in Cotton Entomology at Texas A&M University.
For our other blogs in this series: read "Part 1: It Begins With Ants" here. Read "Part 2: The Fly Years" here. Read "Part 3: 'Oz's' Mosquitoes and Beetles" here. Read "Part 4: Screwworms" here.
