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Finicky or voracious? A multiyear study explores the dietary preferences of insects

Monarch butterfly feeding on milkweed in Shenandoah National Park.

Before Shannon Murphy became an insect and arthropod expert, she was just like the rest of us: not a fan of bugs. 

“I was interested in ecology because I enjoyed being outdoors and I liked hiking and I enjoyed being in the woods,” Murphy says, recalling her undergraduate studies. “I started working as an honors thesis student, and my research was on Abert’s squirrels.”

Murphy wanted to know why the “super cute” tufted-ear mammals were such picky eaters, snacking only on the phloem of ponderosa pine trees. But when it came time for graduate school at Cornell University, she decided to take a new route, focusing on invertebrates. 

After just a few insect courses, Murphy, now an associate professor of biology at DU’s College of Natural Sciences and Mathematics, began to understand how special insects really are. 

“I realized that they are just one of the most amazing groups of organisms on Earth,” says Murphy, who has built her career studying insect and spider intricacies. “They provide so many ecosystem services to us as humans, and basically nothing in the world would work right without them.”

Through a new grant awarded by the National Science Foundation (NSF), Murphy will spend the next four years alongside postdoctoral fellow Dhaval Vyas and collaborators from Georgetown University and University of Massachusetts Boston, working to understand the role diet plays in insect diversity.

“There are 10 times more butterflies and moths out there than all of the birds and mammals you can think of,” Murphy says.

But while science has diligently categorized diversity among beetles, moths and flies, it still hasn’t brought us closer to understanding just why and how insects have evolved into such a wide variety of species. 

Of the many herbivorous insects inhabiting our homes, gardens, farms and forests, about 90% are considered “specialists,” meaning they have tailored their diets to only a single or very few plant species. Take monarch butterflies, for example. Milkweed, their favorite meal as caterpillars, has defenses such as latex sap and toxins that prevent most insects from digging in. Monarchs, however, have evolved to withstand these defenses and have tailored their diets around this plant. They even co-opt its chemical defenses to protect themselves from predators. 

The remaining 10% of insects are considered generalists. They eat whatever is available and tasty, despite the many deterrents plants devise (so-called bottom-up effects) and the additional impact of natural enemies, such as predators and parasitic insects (top-down effects). For years, ecologists have disagreed over whether bottom-up effects have the greater influence on insect diets or whether top-down effects do. 

“This grant is to figure out how top-down and bottom-up interactions affect insects with a broad diet differently than those with a narrower diet,” Murphy says. Working with Vyas and a number of undergraduate students, Murphy will employ fall webworm caterpillars, a moth species she’s worked with for over a decade, to reveal new insights about insect diets. 

“What we have found is that fall webworms have this huge range in diet breadth. They can eat over 400 species of plants worldwide. But here in Colorado, there are some populations that eat only two to four species, some eat 15 species, and then out east there are other populations that eat a lot more,” she explains. “So, we are looking to see how the plants and natural enemies may be driving the evolution of diet breadth.”

This research doesn’t just seek to answer an
important question that has intrigued scientists for decades. It also aims to provide information useful to the agricultural industry. 

“Several of our worst pest insects on agricultural crops are generalists,” Murphy says. “How do they eat so many different host plant species? We don’t really know that right now.”

Beyond the lab, Murphy is putting insects and spiders to work in the classroom. As part of the grant, she and Vyas are bringing their research and passion for science to students all the way from kindergarten to graduate school through a “Caterpillar Road Show,” designed to encourage an early passion for science.

“A lot of students, especially when they are younger, feel that you only see a lot of really cool animals on nature shows if they are in Africa or the Amazon,” Murphy says. “Here we are showing them, ‘Look at all the really cool things that are in your own backyard.’”

The grant also creates scholarships for four girls to attend DU Sci-Tech, a summer camp for middle-school-aged girls, cofounded by Murphy and two other DU science professors, Robin Tinghitella and Jennifer Hoffman. 

Although Murphy is keen to share her love of insects with curious students of all ages, it’s really a passion for science she’s hoping to instill. This project, she says, is making that work possible. Just this January, three of her undergraduate research assistants published a paper on the fall webworms in the DU Undergraduate Research Journal. 

“It’s just so rewarding to see them blossom and grow as scientists,” Murphy says. “Whether they grow to love insects or not is still to be determined. But all of them grow as scientists, and it’s just so much fun to see them become interested in the scientific process and become excited about the research they are doing.”

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