How mark-recapture methods can validate wild bee sampling protocols

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In a recent study published in Apidology, researchers at North Carolina State University are using mark-recapture techniques to assess the effectiveness of common bee sampling methods. During the study, bees were briefly held for tagging using a “bee squeezer” device. (Photo by Molly Briggs)

By Emma Briggs

Emma Briggs

Emma Briggs

Wild bees are an ecologically important and incredibly diverse group of insects. With approximately 20,000 described species worldwide, they provide essential pollination services in a wide range of natural and agricultural environments. Unfortunately, wild bees are experiencing unprecedented population declines, likely due to anthropogenic stressors such as pesticide exposure, habitat loss, introduced species (including non-native honey bees), and climate change. climatic.

With these declines comes a critical need to monitor bee populations. If we want to conserve wild bees, we need to know which species are declining (and how fast).

Bee Sampling Blind Spots

Current wild bee monitoring protocols incorporate several sampling methods, including aerial nets, pot traps (also called bee bowls), and paddle traps. However, there has been much controversy surrounding the effectiveness of these methods – see previous entomology today articles “Building a better bee trap: researchers say bee bowls are overused” and “What’s the best way to monitor wild bees?” just for two examples exploring this question.

These sampling techniques only provide an index of the size of the bee population. In other words, they give a count of bees captured during a sampling period, not a count of all bees in a population. A challenge with index sampling methods is that you never know whether the bees you sampled represent, say, 1% of the population or 50%, and whether that percentage is constant across species and one habitat to another. Indeed, it appears that is not the case.

For example, tray traps typically capture large numbers of small sweat bees but few large bumblebees and carpenter bees, and it seems unlikely that these capture rates are a good indicator of the relative abundance of each species. This uncertainty brings us to the heart of the bee monitoring debate: countless researchers rely on aerial nets, tray traps and paddle traps to provide information on the abundance and composition of bee communities. bees, but how do we know that their capture rates accurately reflect the underlying bee populations?

Mark-recapture to measure “detectability”

To answer this question, our research team – from North Carolina State University, the North Carolina Wildlife Resources Commission (NCWRC), and the US Geological Survey’s North Carolina Cooperative Fish and Wildlife Research Unit – has conducted a mark-recapture experiment, the results of which we published this month in the journal Apidology. Mark-recapture studies are different from traditional methods of sampling bees because they incorporate information not only on capture rates, but also on detectability – or the likelihood that an individual in the population will actually be observed during sampling. They do this by first tagging a subset of bees in the population and then measuring detection rates of tagged bees over subsequent sampling periods. With this information on detectability, mark-recapture studies can estimate the total number of bees in the population.

marked bee

During the mark-recapture study, captured bees were marked on the thorax with paint pens. Bees captured over multiple days of surveying were given multiple paint dots. (Photo by Emma Briggs)

This ability to generate absolute population estimates is crucial, as we can use these estimates to verify ground capture rates from traditional sampling methods. For example, if we compare mark-recapture population estimates to catch rates from an index sampling method such as tray traps, we can determine whether the traps provide an accurate representation of abundance and of the species composition of that bee community. Although researchers have long suspected that traditional methods of bee sampling are biased, this type of comparison can take the guesswork out and begin to quantify those biases.

Our study took place in the summer of 2020 at three sites in the Butner-Falls of Neuse Game Lands near Raleigh, North Carolina. We selected these sites as our research partner, the NCWRC, was already conducting aerial net and tray trap sampling for an ongoing study. Since the NCWRC was already doing index sampling, this was the perfect opportunity for us to step in and compare its catch rates to mark-recapture estimates. For the mark-recapture study, we focused on four common bee taxa: eastern carpenter bees (Xylocopa virginica), sheet cutters (leaf beetle spp.), two-spotted beetles (Melissodes bimaculatus) and green bees (augochlorella sp., Augochlora puraand Augochloropsis spp.).

The study was divided into five two-week sampling periods in which three days were devoted to the mark-recapture study, followed by one day of sampling with flat traps and aerial nets by the NCWRC. During the mark-recapture survey days, we captured the bees at the sites using insect nets and transferred them to a “bee squeezer”. Once restrained, we marked the bees on the thorax with a colored dot using paint pens. We used a different paint color on each sampling day, and if a bee was captured on multiple sampling days, it could sport up to three different paint colors. Conducting field research at the start of the pandemic was not easy: we faced bureaucratic delays, transportation problems and supply shortages. But, with a bit of luck (and a lot of effort), we managed to finish our field work before accessing our closed sites for the hunting season.

bee net

The results of the mark-recapture study indicate that aerial nets more accurately reflect the abundance of the underlying bee community than tray traps. (Photo by Henry Ritterpusch)

Surprising findings, but questions remain

After collecting and processing our data, we used linear regression to compare absolute population estimates from the mark-recapture study with catch rates from aerial nets and tray traps. What we found surprised us. Tray traps, which are widely used in bee tracking studies, did not correlate well with mark-recapture estimates. In contrast, aerial net capture rates were well correlated, indicating that this sampling method likely provides a better representation of the abundance and species composition of the underlying bee community. Although tray traps are still useful in bee diversity surveys, further research is needed to identify the conditions under which they act as a reliable indicator of abundance.

While studies comparing catch rates from index sampling methods to mark-recapture population estimates are common in other taxa (e.g., mammals, birds), we are the first (to our knowledge) to attempt one for aerial nets and tray traps in the wild. the bees. Additional mark-recapture studies are needed to evaluate other sampling methods such as paddle traps or nest counts, especially at a multispecies community level. To be clear, we are not suggesting that mark-recapture methods replace all other bee sampling techniques – they are far too labor intensive and time consuming to do so. And they have their own set of assumptions that must be adhered to in any study. However, mark-recapture methods provide a convenient means of monitoring selected bee species and evaluating the performance of other potentially flawed sampling methods.

Emma Briggs is a graduate student at the Warnell School of Forestry and Natural Resources at the University of Georgia in Athens, Georgia. Twitter: @emma_b321. Email: emma.briggs@uga.edu.

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