Beating the Beetles in the Lab with Dr. Kelli Hoover
The following is a narrative by biochemistry and molecular biology undergraduate student Pratiti Roy as part of the “Students Telling the Stories of Science” series. The essay takes readers into the lab of Dr. Kelli Hoover, professor of entomology in the College of Agricultural Sciences. Hoover’s leads a team of researchers whose work revolves around the study of biological control methods for invasive insect species, particularly the Asian longhorned beetle.
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In the basement of the Agricultural Sciences and Industries Building is a dimly-lit room with floor-to-ceiling shelves of glass mason jars. They are unremarkable, filled with twigs and branches. As you move closer, however, your eyes register movement — mottled beetles as long as your thumb prowl among the wood, swinging their long, curving antennae.
These Asian longhorned beetles (ALBs) are an unsettling sight for most people, but Dr. Kelli Hoover, a professor of entomology in the College of Agricultural Sciences at Penn State University, is unfazed. That’s not surprising, considering that she has been studying these beetles for years.
She reaches for a jar and points out the small holes that dot the bolts of wood. “These are oviposition sites,” she explains. “Notches in the bark where the female beetles lay their eggs. Later, when the larvae hatch, they will bore into the wood. Were these larvae in the wild, they would have drilled their way into the tree, creating tunnels that would disrupt vessels in the tree that transport water and nutrients.”
A single larva can consume up to 1,000 cubic centimeters of wood in its lifetime — think of a one-liter Pepsi bottle filled with wood shavings. The damage doesn’t end in the larval stages, however; once the larvae grow into adult beetles, they will chew their way out of the tree and feed on its leaves and bark. Infested trees do not recover. They die a slow, inevitable death.
In 2001, a group of scientists in New York estimated that ALBs have the potential to wipe out up to 30.3 percent of the trees in the United States, which comes out to about 1.2 billion trees. ALBs are an invasive species originating from China, typically infesting hardwood trees like ash, birch, elm and maple.
Currently, it is found in three states: Ohio, Massachusetts, and New York. The U.S. Department of Agriculture has declared it eradicated from Illinois and New Jersey. However, ALBs have proved to be hard to control, and there is a constant fear they will spread to other states. There is no “cure,” no surefire way to eradicate the insect and help trees recover. Instead, ALB infestations must be monitored and quarantined.
These infestations are difficult to diagnose until it’s too late, as symptoms may not manifest until three to four years after infestation. The only current method of containment is simply cutting down any infested trees.
“The United States Department of Agriculture (USDA) has tried using systemic insecticide treatment of trees on the perimeter of the quarantine regions to prevent spread of the beetle, but it is unclear whether this is effective. Moreover, these insecticides are highly toxic to bees and other non-target insects,” Hoover stated.
While the situation seems bleak, Hoover and her lab members aim to find other viable control methods, including treatment of wood prior to shipping and development of pheromone-baited traps, which lure beetles with a chemical they use naturally to attract mates. In a paper published in 2016, members of Hoover’s lab, in conjunction with scientists from the USDA, investigated ALB digestion to see if it could provide any clues for possible control.
Charles Mason, a postdoctoral researcher working under Hoover and the primary author on the paper, explains, “The thrust of the research we do with this beetle is trying to identify what attributes allow it to consume a relatively recalcitrant diet source.”
ALBs are remarkable in that it feeds on wood. Wood is difficult to digest; it is nutrient-poor and filled with chemicals that the plant uses to defend itself from attacking insects. As of right now, not much is known about how wood-feeding beetles like ALB deal with these problems. Scientists hypothesize that these insects express key genes used to fight plant toxins and break down the complex carbohydrates present in the tissue they consume.
Every organism has its own set of instructions in its cells: DNA. If DNA is one long instruction manual, then genes are chapters in the manual; they are sections of the DNA that provide directions on making certain specific protein products. When a particular product is needed, the cell will “express” that gene more profusely — in other words, it will read the chapter for that product over and over again in order to make more of it.
In addition to studying the gene expression of the insects themselves, they looked at the microbes living in ALB guts. These bacteria and fungi are thought to metabolize cellulose into more manageable sugars and provide the host insect with nutrients not found in wood.
Knowing this, Hoover’s lab investigated what was going on in the guts of ALB larvae to find a more comprehensive explanation. The experiments were done on larvae as opposed to adult beetles because, as Erin Scully, a USDA researcher, explained in an email, “that is the life stage that feeds in the wood. The adults feed externally on leaves and twigs, which are still cool, but not as difficult to digest and nutrient deficient as the wood.”
Scully is a Penn State graduate who received her PhD while working in Hoover’s lab. She currently works as a researcher for the USDA in Kansas, studying pests that damage stored products, like grains and pastas. Despite being a thousand miles away, her collaboration with the Hoover lab has not ended. The larvae experiment actually used data she had collected while a student at Penn State. Years after the initial experiment, she worked from her office in Kansas with Mason to write the paper.
In the experiment, they compared levels of gene expression in maple-reared larvae to those of larvae raised on a nutritionally rich artificial diet to get a better understanding of how ALBs overcome the challenges of wood consumption.
The artificial diet provided the insects with all of the nutrients they needed. It had fiber in the form of cellulose, sugar, milk protein, and vitamins, among other things. While it may not sound appetizing, it was an improvement from the usual wood diet of these beetles, as it didn’t have any of the hard-to-digest carbs or harmful toxins found in wood. By comparing gene expression of the larvae on the two different diets, they were able to identify the genes that were highly expressed in the wood-feeding larvae, which were likely involved in making the wood-based diet nutritious.
As they suspected, in the wood-consuming larvae, there were higher levels of expression of genes involved in digesting plant cell walls, obtaining essential nutrients and detoxifying plant defenses. However, there were also higher levels of expression of genes involved in both making and breaking down proteins.
While this result was initially surprising, the researchers think it might be a mechanism to protect against plant toxins. Often, these toxins will affect proteins and cause damage; the insect degrades those proteins and makes new proteins to replace them.
The larvae on the artificial diet didn’t express these genes. They didn’t need to, as their diets were already nutritionally complete, with no toxins present. They would not be wasting their energy on expressing genes that were not necessary. The ability of ALBs to modulate its gene expression based on its diet is essential for wood-feeding insects because of the nutritional challenges they face.
These findings provide a foundation for further research in ALB metabolism — for example, investigating how different tree species affect gene expression or how gene expression changes through the life cycle of a beetle. In addition, since modern pest management tries to take advantage of plants’ built-in defenses, understanding how beetles fend them off can be helpful.
Scully hopes this research will help to identify gene targets to disrupt digestion, detoxification or nutrient recycling. By targeting these genes, scientists could potentially make it more difficult for ALB larvae to survive.
Although this research touches only on one particular aspect of the beetle, Hoover’s lab doesn’t stop there. They have their fingers in several different pies, taking a multi-pronged approach to characterize the beetle’s secrets to success.
Mason lists different tactics the lab is taking: studying a certain fungus present in the beetles’ guts, analysis of the microbes living in the gut, observation of the boring habits of the larvae, to name a few. All of these topics are meant to drive researchers closer to understanding how Asian longhorned beetles work. Mason sums up the aim of the lab succinctly: “We want to know what makes invasive species invasive.”
Mason suggests going down to the quarantine room again. The second trip is less intimidating. It helps that all of the lights are on this time, and that Mason talks so animatedly. Without breaking the conversation, he unscrews a jar, lifting a beetle off of its branch and onto his finger with the nonchalance of someone who does this a lot.
“They’re pretty charismatic,” Mason declares. Perhaps realizing that “charismatic” is usually not a word most people would use to describe beetles, he amends, “They’re cute.” The beetle wiggles its antennae in agreement.
Strangely enough, he isn’t wrong. He places the beetle back on its branch, then twirls the branch in his hands as he continues to explain his research. He shuffles the branch from one hand to another, and you have to wonder if the beetle is getting disoriented from this treatment.
But don’t mistake Mason’s energy for flippancy — he is passionate about what he studies and strives to connect it all to the bigger picture. He explains of his approach, “Let’s say this beetle can feed on this tree, but not on that tree. One question is why. Alright, we can find out the reason why. My interest then is to say, but what happens next? I want to take what we can learn from the individual and apply it to the population.”
Ultimately, Hoover and her lab members are looking to demystify the workings of this beetle. Their research may seem trivial at first glance. However, as scientists like the Hoover lab researchers put the puzzle of the Asian longhorned beetle together bit by bit, piece by piece, they will get a better understanding of how it functions. Then we will find viable solutions to stop this invasive species from devastating our forests.