When Marianne Azevedo Silva arrived in the U.S. from Brazil a year and a half ago, she didn’t expect her postdoctoral research to lead her into a world invisible to the naked eye – and deep into the collections of U-M’s Herbarium.

After finishing her Ph.D., and her research on ant–plant interactions, Marianne was introduced to Professor Marjorie Weber, and soon made the move to U-M for a post-doc position in her lab. Now, Marianne studies another form of fascinating mutualism. Not between ants and plants, but between plants and mites.

Many plants have tiny structures called domatia—small pockets or tufts of hair on their leaves—that serve as homes for mites. These mites, in turn, help protect plants by preying on harmful herbivores such as fungi.

“If you look closely under a magnifier, you can see them running around,” Marianne says, pointing to the orange specks on a leaf. “They can be quite abundant—sometimes dozens or more in a single domatium.”

Marianne explains that mites use leaf veins like miniature highways, traveling from one part of the plant to another. While some are herbivorous, many are predatory, feeding on other tiny plant-eating insects. It’s a mutually beneficial arrangement: the plant provides shelter, and the mites provide protection.

In the warmer months, Marianne collects fresh leaves in the field, washes them in a special solution, filters out the mites, and uses DNA metabarcoding to identify them. After a conversation with Professor Weber, she wondered if Herbarium specimens might also hold traces of mites. That question led her to examine preserved plants in the collection—some dating back to the late 1800s—where she discovered evidence of mites (such as exoskeletons) and even their intact (but dried) bodies still clinging to the dried leaves.

“This is one of the things that really surprised me,” she says. “I had no idea that herbarium specimens could be such a rich source for studying these interactions.”

Most existing research on plant–mite mutualisms focuses on the plant side of the relationship. Marianne wants to better understand the mite side—what species they are, how abundant they can be, and how these interactions might influence the distribution of plant species over time. A combination of field work, herbarium data sampling, molecular tools, and phylogenetic analyses are helping her discover answers.

She’s already finding patterns. Plants with domatia are more likely to expand their occurrences up north towards temperate zones, suggesting the relationship with mites may support their distribution in certain climates. And because historical specimens allow her to “look back in time,” she can explore how these relationships have persisted—or changed—over more than a century.

Marianne’s research is a powerful example of how museum collections can be used in unexpected ways. “In 1870, no one collecting these plants knew that one day we’d be able to study their mites under a microscope,” she says. “That’s why preserving and sharing specimens is so important—you never know what new questions will be asked in the future.”

“Now, every time I see a leaf,” Marianne says, “I wonder what’s living there that we can’t see with the naked eye.”