In the disappearing forests of South America, University entomologists create a record of life on Earth

Caddisfly Catchers: Discovering Biodiversity with Black Lights and Bedsheets

by Jennifer Amie

Caddisflies are found the world over, in virtually all types of fresh water, from swift-flowing rivers to still ponds. An important part of the aquatic food web, these widespread insects are of special interest to scientists both for their extraordinary evolutionary history and as a source of information about water quality.

Different species of insects—including caddisflies, mayflies, and stoneflies—are sensitive to particular types and levels of pollutants in the water they inhabit. In any river or stream, a population drop in one species may indicate a problem with water quality and may point to which contaminant is the culprit. In the United States and Europe, biomonitoring programs that track changes in insect populations are commonly used to evaluate water quality. In some parts of the world, however, such programs are difficult to implement—no matter how ecologically important the region is, or what environmental threats it faces—because it is impossible to monitor populations of species that science has yet to discover.

One such place is Southeastern Brazil, which contains some of the most endangered ecosystems in South America. Bounded by three mountain ranges and home to Brazil’s largest cities—including Rio de Janeiro and São Paulo—the area contains remnants of the once vast Atlantic rainforest, as well as rare sections of interior dry land forests known as cerrado. While much is known about the health of these terrestrial ecosystems, little is known about the condition of the watershed that sustains them and their neighboring communities. That’s because to implement biological monitoring programs, resource managers must first know which insect species live in a river or stream, and in what numbers. In Brazil, that critical baseline knowledge hardly exists.

For the past nine years, University of Minnesota professor of entomology Ralph Holzenthal has been working to remedy this crucial knowledge gap as part of a massive effort to discover and identify caddisfly populations throughout the Central and South American neotropics. Holzenthal and his team have spent up to two months each year trekking through forests, wading in streams, and setting up shop on riverbanks in Brazil, as well as establishing research programs in Costa Rica, Nicaragua, Venezuela, Peru, Bolivia, and Chile.

“We’re at the forefront of discovering biodiversity—out in the trenches, out in the field,” says Holzenthal. He and his team begin work before dusk by stringing an ordinary white bedsheet between two trees near the bank of a river. They mount a black light tube, powered by a 12-volt car battery, above the sheet and turn it on when the sun goes down. The light attracts flying insects to the sheet, where the researchers pluck off the caddisflies, capturing them in jars.

“It’s extremely important to know what is there and where it is,” says Holzenthal. “Southeastern Brazil, for example, is important from a biological, ecological, and conservation perspective. There is a certain urgency to the work because the Atlantic forest is being lost to development. Only 5 percent of the original forest remains.”

Holzenthal’s documentation of insect populations in the intact sections of the forest can be used as a yardstick to measure species loss in developed areas. But loss of habitat is not the only threat to a region whose watershed is beset by pesticide, herbicide, and fertilizer runoff; contamination from untreated sewage; siltation from logging; chemical pollutants; and, in some places, acid rain.

“Aquatic habitats are particularly impacted by human use,” says graduate student Desiree Robertson, a member of Holzenthal’s research team. “They’re like the sewers of the landscape if they’re not treated properly.”

Conservation measures and environmental protection are critical throughout the neotropics, and both aims are aided by the type of research Holzenthal's team does. “Conservation without documentation is incomplete,” says Roger Blahnik, a member of Holzenthal’s research team. “With limited resources, how do you select and prioritize what to conserve? You need to know what’s out there.”

In some places in South America, up to 75 percent of the caddisflies collected by Holzenthal's team turn out to be species previously unknown to science. When the team started work in Brazil in 1996, only 125 species had been recorded in the region. To date, the researchers have recorded 525 additional species.

“Before we collected in Brazil, no one knew that there were 650 species of caddisflies still hanging on after 500 years of European habitation in the region,” says Holzenthal. “What excites us more than anything is the fact that there are so many different kinds of caddisflies, and we’re discovering them and letting people know about them.”

The flies and larvae collected on expeditions are carefully prepared as museum specimens. “At the University of Minnesota, we’ve built one of the world’s best collections of caddisflies,” says Holzenthal, who is director of the University’s insect museum. The collection features a fully accessible database, illustrations of the insects, and specimens that are properly collected, well-labeled, and carefully maintained. Such collections serve as a library of scientific information that is available to researchers throughout the world.

Holzenthal’s graduate students and colleagues at the University examine the caddisflies under a powerful microscope, scrutinizing their anatomy to categorize them by species and build a caddisfly family tree. “If this taxonomy is done well,” says Holzenthal, “it has a certain immutability. It becomes part of the timeless record of life on Earth.”