Brian D. Farrell is a man with many props. He bounds around his sunny corner office at the Museum of Comparative Zoology showing off his finds: a pile of 60-year-old lantern slides of Cuba, an ancient projector, the dog-eared 1938 field journal of P.J. Darlington Jr., a well-known zoogeographer and one of Farrell’s predecessors at the museum.
But Farrell’s most beloved props are the dozens of boxes of mounted beetles that litter his tables. The insects range in size from the 4-inch-long palm beetle to the minuscule maize weevil, and come from all corners of the Earth – Africa, the Himalayas, Massachusetts, the Caribbean: They have muscular horns or wispy antennae; they are long and thin, short and round, oval and nearly octagonal; and, most spectacularly, they are striped, spotted, and washed in brilliant reds, greens, yellows, silvers, and blues both iridescent and absolute.
“Sure,” says Farrell, a professor of biology and the MCZ’s entomology curator, “they look good in cases, but when they’re squirming around in your silo destroying millions of dollars’ worth of grain, they’re a little less attractive.”
Of the approximately 750,000 insect species described by scientists (versus 4,000 for mammals), 350,000 are beetles. They are the hardiest and by far the largest group on Earth: One out of every four animals is a beetle. Hence, they are our main competitors for food resources.
Grain weevils alone cost the global economy about $35 billion, or a third of the world’s grain crop, every year. The problem starts when a few adult grain weevils happen upon a nice cache of rice, wheat, or corn and start chomping away. The female lays each of her eggs in a single piece of the grain; the larvae hatch and grow inside, and the cycle begins again.
The USDA has spent millions of dollars to develop listening devices that are placed in silos to pick up the chewing sounds – not that they really help much, since there is no way to kill the insects once they’re on the grain. All the high tech tells us is whether the infestation is sufficiently developed to quarantine the silo, or whether there are few enough weevils that they can just be ground up along with the grain and kneaded into the bread sold in your local supermarket.
Various other beetle species damage dozens of crops including bamboo, palm trees, bananas, grasses, sugarcane, pines, and irises.
“My research is about the evolution of interactions of various sorts,” Farrell says, “including those with plants, those with fungi that help insects attack plants, and those with bacteria that help insects digest plants.” His main focus of late has been on bark beetles, which cause about $7 billion of timber damage in the United States alone.
“We spend tens of millions of dollars every year studying all aspects of their biology,” he says. “Why not spend another half million to understand their evolutionary relationships?”
Studying beetles’ evolutionary biology, for instance, gave Farrell insight to a question that had puzzled scientists for decades – possibly centuries: Why are there so many beetles? In a paper published in the July 1998 issue of Science, Farrell concluded that it’s because “every time they colonized plants, particularly flowering plants, their diversity leapt up by several orders of magnitude” as the plants developed defenses to combat them. Today, more than half of beetles are plant feeders – and they have a huge advantage over their predatory and scavenger brethren simply because plants are so plentiful. “So the reason why there are so many beetles collectively,” Farrell says, “is because of repeated origins of herbivory.”
Farrell and his colleagues in evolutionary biology will never run out of things to study about beetles: There’s evidence that the critters have been around for about a quarter of a billion years. “Today you’ll find some of the same beetles that were around 200 million years ago,” he says. “They’re living fossils, basically, still doing what they were doing back then.”
Among those are the bark beetles, which bore through the bark of a tree or log, dig a long tunnel just beneath the surface, and lay their eggs – which in time dig their own tunnels, creating an intricately patterned gallery that somewhat resembles an ant farm. Infestations can cost Canada and the United States up to tens of thousands of hectares of lumber a year.
“They stay where the action is,” Farrell says of the beetles, “where all the nutrients are and the tree’s vascular system is strongest. But trees aren’t willing victims; they have defenses, such as the resins that conifers pump out.”
The beetles, however, come prepared with their own weapons. “They carry fungi with them,” he says, “pathogens that are able to grow several millimeters a day. And those fungi, which they introduce in the galleries and inoculate at lots of different points, grow and help overwhelm the defenses of the plant. Subsequent to that the fungus ends up really feeding on one part of the plant, while the beetle builds its galleries in a slightly different part – they’re slightly antagonistic, because after all they’re eating the same thing.”
Half a dozen times during their evolution, however, the beetles have “subverted this relationship with the fungi entirely to their own end,” Farrell says. Because these species live in the heartwood, they “are not the forest killers. They’re basically going into logs that have just been stripped and utilizing what the bark beetles don’t use.”
They may therefore not be of great concern to loggers, but these beetles certainly have a fascinating story.
“What they do is they capture several cultivars” – that is, asexual species of fungi – “and basically make them their slaves. They carry them deep inside the middle of the tree and build their little galleries there, and they cultivate the fungus. The fungus feeds on the xylem, which is a vascular tissue of the plant, and the beetles eat the fungus and feed it to their larvae. They’re actually doing fungal husbandry, maintaining these gardens in perfect condition.”
The economic damage is less than that of the bark beetles, but is not insignificant in certain parts of the world. “Especially in tropical countries where these agriculturalists occur,” Farrell says, “they basically riddle lumber as soon as it’s at the mill. You take the bark off and these guys go ‘zip’ and they just make it look like a colander afterwards.”
Farrell’s upcoming research, during a sabbatical year in the Dominican Republic, will focus, in part, on these agriculturalists, or ambrosia beetles. He has already sketched out the evolutionary patterns of where each species came from, how many origins there were, and what the consequences of each origin were, lending a dynamic historical dimension to the current body of knowledge. But why is this important?
“I often give the analogy,” he says, “that it’s sort of like going to a former colonial country and trying to understand the politics and the current demographics without knowing the long-term history of the place. You sort of have to know both in order to understand the whole thing. And that’s what we’re contributing, is a perspective of evolutionary history to these kinds of interactions.”