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 Fall/Winter
1997 |
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Getting the Poop on Beetle Defenses
Most of us would not think of piling our own weight in excrement on our backs in self-defense. But in the insect world, this sort of thing goes on all the time. Tim Morton, who has a B.S. in botany and an M.S. in plant chemistry from the University of South Florida, discovered this intriguing behavior by accident while working on a Ph.D. in chemical ecology at the State University of New York in Stony Brook. "One day I was looking at some plants along a path on campus and noticed insect larvae on the leaves. The larvae had feces all over their backs," he recalls. "I knew they were three-lined potato beetle larvae and thought they must be using the feces, called frass in entomological circles, for defense. But I didn't know why they were feeding on that particular plant." This question became the focus of Morton's doctoral research. Two years ago he came to Penn State to complete his Ph.D. with Jack Schultz, an entomology professor noted for his work in the chemical ecology of plants and insects. Morton's study is the first to document how insects use particular chemical compounds within a plant's structure to protect themselves from predators and how this sophisticated defense system evolved. "Insects don't just suddenly land on plants with the most effective chemical defenses. There must be a series of steps," says Shultz. "Tim is looking at which plant compounds repel predators of leaf-eating insects." Morton started out by studying the three-lined potato beetle, which is in the Chrysomelidae family of leaf-eating beetles. These insects consume food that is full of harmful chemicals, but while they are constantly exposed to predators, their survival rate is very high. The frass-coated larvae Morton had noticed in Stony Brook were feeding on nightshade (Solanum dulcamara), a plant high in alkaloids–nitrogen-based compounds that are toxic to animals. Alkaloids include morphine, a derivative of poppies, and nicotine, a derivative of the tobacco plant. When the beetle larvae eat nightshade leaves, they absorb the useful nutrients and excrete the toxins, which are concentrated in their frass. The larva forms a fecal shield by excreting waste through its anus, located in the middle of its back. Muscular contractions continually move the frass higher onto the larva's back until it is fully covered. "No one had analyzed the chemistry of a fecal defense to see which compounds repel predators. This study was perfect for me because I like to do things that haven't been done before," Morton says. "At first it was kind of disgusting, but it's one of those things you just get used to after a while." Morton established the effectiveness of the fecal shield by scraping frass off some of the larvae and placing them in with a predator–in this case, ants. The larvae with fecal shields were left alone while those whose shields had been removed were eaten. Morton also fed some larvae lettuce, which is nearly free of toxins, to see if predators were repulsed by nonchemical frass. "The ants liked the larvae covered with lettuce feces just fine, so we knew the potent frass defense had to be chemically based," Morton says. To chemically analyze the frass, Morton had to collect at least one gram of fecal material. He says scraping feces off about 400 larvae, a dirty job in anyone's book, nets about a gram of frass. "One day, it took me about four hours to scrape a half a gram," he says. "I thought, I must be the only person in America who is currently making a living scraping feces off beetles." Morton worked with Ralph Mumma, distinguished professor of environmental quality, and Robert Minard, senior lecturer in chemistry in the Eberly College of Science, to identify the alkaloids found in the plants and determine how the beetles were able to employ the alkaloids for defense against predators. To analyze the frass, Morton used a mass spectrometer, which breaks down compounds into a chemical component "fingerprint." He discovered the beetles feed on, or more accurately, metabolize the sugars that plants often attach to alkaloids when they store them. Thus, the insects use the sugars for energy but do not metabolize the alkaloid in their digestive system, keeping the potent compound for their dorsal defense. After analyzing the potent fecal shields formed by the potato beetle larvae, Morton turned his attention to another leaf eater, the six-spotted beetle (Neolema sexpunctata). This insect feeds on dayflower (Commelina communis). Chemical analysis of this larva's frass showed high levels of a substance that very closely resembles a compound called phytol, which is found in all plants. Phytol usually anchors chlorophyll to a plant membrane, but some plant species have the compound unattached to anything, called "free" phytol. Through chemical analysis, Morton discovered that all the plants that the six-spotted beetles feed upon have this free form of phytol. The beetles use the chemical compound by metabolizing just 2 of its 20 carbon compounds, allowing the remaining 18 carbons to pass through undigested for use as a chemical defense. Frass with the phytol derivative repulsed ants just as effectively as that with alkaloids. Because phytol is found in more species of plants than are alkaloids and because the alkaloid-containing plants are a relatively recent addition to the flora, Morton believes that the beetles using phytol in their fecal shields might represent an earlier stage in the evolution of insect chemical defenses. "By studying chemical ecology, we can see how insects adapt and evolve for survival in specific environments," Schultz says. "Tim's work eventually will let us see how using plant chemicals to repel predators got started." Morton, who loves to teach and hopes to become a university professor, finds that Penn State's willingness to let him explore new territory has further fueled his interest in science–even if that means getting down and dirty with beetle feces or being referred to as a fecologist. "I grew up on a farm near Jamestown, New York, and I knew I didn't want to be a farmer, so I sort of avoided the outdoors and spent a lot of time reading encyclopedias and other books," he says. "When we were kids sitting out under the stars, wondering what we'd do when we grew up, everybody wanted to be an astronaut. I thought, it's probably pretty hard to become an astronaut, but I always knew I could be a scientist." – John Wall |
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