Bowl traps are little plastic bowls, in blue, yellow, and white, that are filled with soapy water and set out to catch bees. In a new article published in May 2020 in Annals of the Entomological Society of America, a group of researchers points out that bee bowls often don’t catch a representative sample of the bees seen on nearby plants, and they urge the bee research community to work toward developing new methods for monitoring bees. (Photo courtesy of Zachary M. Portman, Ph.D.)

By Paige Embry

Bowl traps are little plastic bowls, in blue, yellow, and white, that are filled with soapy water and set out to catch bees. The simplicity and benefits of these bowls are so alluring that it’s easy to overlook their flaws. A new paper published in late May in Annals of the Entomological Society of America assesses the good, the bad, and the ugly of the various methods used for monitoring bees but focuses on bowl traps (also known as bee bowls, pan traps, or Moericke traps), as they are the most commonly used method.

Paige Embry

Zach Portman, Ph.D., research scientist and bee taxonomist at the University of Minnesota and the lead author of the paper, says, “I see the field doubling down on ineffective monitoring methods because they are viewed as easier and more standardized. But, if we continue to try and monitor bees with bowl traps, I’m afraid that we’ll end up missing actual bee declines, and we’ll only realize it years or decades down the line.”

The three commonly used data-collection methods in bee monitoring studies are observations, netting, and bowl traps (along with similar, soapy water-based traps). All have their problems. Conducting observations takes a fair chunk of field time and an observer capable of identifying bees in real time. Capturing bees with nets also requires time and some experience to limit bias—it’s easy for novices to overlook anything other than the big, showy bees. The rise in popularity of bowl traps seems like an excellent alternative—cheap, reproducible, no expertise required, and no operator bias—but they aren’t the panacea they seem. Portman says, “In ecology, and science in general, it’s important to step back and ask ‘Are my methods actually measuring what I think they are measuring?’ And in the case of bee bowls, they are often used to answer questions that they are poorly suited to answer.”

In a new article published in May 2020 in Annals of the Entomological Society of America, University of Minnesota bee researchers Zachary M. Portman, Ph.D., Bethanne Bruninga-Socolar, Ph.D., and Daniel P. Cariveau, Ph.D. (left to right), urge the bee research community to work toward developing new and improved methods for monitoring bees. (Photo courtesy of Zachary M. Portman, Ph.D.)

One major problem with bee bowls is that they often don’t catch a representative sample of the bees seen on nearby plants. Instead, they may be filled with dozens of difficult-to-identify Lasioglossum or other halictid species and not much else. After capture, someone has to process all those bees: rinse off the soap, possibly blow-dry them, pin them, and label them. Then, a taxonomist with real identification chops will be needed to identify many of those halictids, and precious storage room must be given up to house them. So, if you’re going to collect a slew of halictids, it needs to be for a good reason. Some try to overcome the weaknesses of bowl traps by putting out bowls and netting bees, but that doesn’t fix the identification and storage problems.

Since many research projects focus on bee declines or range changes, collection methods should give a sense of the actual bee abundance, says Portman.

“One of the big flaws in a lot of ecological studies today is that they don’t actually measure changes in abundance. Instead, they measure changes in ‘relative abundance,'” says Portman. “For example, if you catch 50 specimens of a given species one year and 100 specimens of that species the next year, does that mean the abundance of that species has gone up? This question is impossible to answer unless you have some idea of what proportion of specimens get caught and how that is influenced by things such as weather and floral abundance. None of the commonly-used methods—bowls, netting, and observation—are well-equipped to address changes in true abundance, but they are often treated as if they are.”

Lasioglossum is the largest of genus of bees, with more than 1,700 species, all belonging to the family Halictidae, often known as “sweat bees.” In bee monitoring efforts, commonly used bowl traps can attract large numbers of halictid species, possibly skewing estimates of bee diversity in the studied area. A group of researchers argues for new methods for monitoring bees in an article published in May 2020 in Annals of the Entomological Society of America. (Photo via USGS Bee Inventory and Monitoring Lab on Flickr, public domain)

Bowl traps are little plastic bowls, in blue, yellow, and white, that are filled with soapy water and set out to catch bees. In a new article published in May 2020 in Annals of the Entomological Society of America, a group of researchers points out that bee bowls often don’t catch a representative sample of the bees seen on nearby plants, and they urge the bee research community to work toward developing new methods for monitoring bees. (Photo courtesy of Zachary M. Portman, Ph.D.)

Portman and his co-authors, fellow University of Minnesota bee researchers Bethanne Bruninga-Socolar, Ph.D., and Dan Cariveau, Ph.D., write that almost no studies look at the relationship between actual bee abundance and sampling method, although such studies have been done for other insects, like moths with light traps.

The researchers offer a variety of alternatives to the use of bee bowls, from targeted sampling to nesting censuses. Alternatives are needed because, as Portman says, “The use of bowl traps, especially for monitoring studies, continues to steadily increase. In some ways, it appears the field has hit some kind of feedback loop, where people use bowl traps because it seems like everyone else is doing it.”

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.