Toxic Nectar: How Pollinators are Exposed to Metals in Urban Landscapes

Bees and butterflies rely heavily on nectar for survival—but in today’s cities, native blooms are scarce, and a hidden danger may be lurking in the few that remain. Modern urban environments introduce a host of pollutants, and recent research reveals that nectar and pollen in these areas can contain trace metals like arsenic, lead, cadmium, and chromium.  

Dr. Sarah Scott joins us to share her findings on how these contaminants may be harming pollinators, affecting not only their health but also their foraging patterns and behavior. High concentrations of metals pose a serious risk—and as urban spaces continue to expand, we need to know how to maintain the delicate balance between urban nature and pollinator health.

Sarah is a postdoctoral researcher studying wild bee welfare at Newcastle University in England. You can read more of her work here and this is the paper we discussed.

Good to know

Sarah’s study focused on plant species thriving in vacant city lots. These often-unintentionally planted spaces were found to contain plant species such as chicory, clover, dandelion, and queen anne’s lace, which grow in the absence of human interventions. Her findings indicate that these plants absorb varying levels of metals depending on their species and the environmental conditions such as soil pH and organic matter. 

Transcript

Jacy: [00:00:00] Welcome to the Bees Knees. A podcast wild about native bees. Wild and native bees are under threat worldwide. In each episode, we look at actionable things we can do to support these adorable little guys whose pollination work is crucial for maintaining biodiversity. I’m Jacy Meyer and I thank you for being here.

We know bees and butterflies sip nectar as their primary food source. But what if that nectar contains something unexpected like arsenic and lead and other trace metals? Today we’re speaking with Dr. Sarah Scott, whose recent study found pollinators. Foraging in urban landscapes might be ingesting harmful metals through the very nectar they rely on for survival.

But how does this happen? What does it mean for pollinators and potentially even for us? So what initially inspired you to investigate Trace Metals in Nectar? [00:01:00]

Sarah: So I studied the effect of trace metals on Bumblebee Health and behavior for my PhD. And while I was digging through the literature, just kinda seeing what was out there, it was really surprising that even though people have been looking at honeybees as bio indicators of meta pollution since the eighties, there really wasn’t that much work kind of really pinning down where they were coming in contact with the metals and so.

From that, I really kind of wanted to take a look at kind of what these sources would be in the environment and it made sense to start with their food sources, so nectar and pollen. I wanted to look at plant species specifically that were blooming in vacant lot systems. So I did my PhD in an urban ecology lab that focused in the city of Cleveland and looking at the species that just voluntarily grew in those places and kind of what they offered for the bees.

Jacy: So you mentioned the A landscape that you studied. Can you tell us a little bit more about what was [00:02:00] growing there or not growing there, and then the metals that you found.

Sarah: So most of this research took place in Cleveland, Ohio, which is a post-industrial city in the United States of America, and it has a really large history of manufacturing and steel production, which .

Has led to, unfortunately, a lot of contamination within the city. But then in addition to that, the decline of those industries have also led to an abundance of vacant lots and vacant buildings. And so our lab focused on looking at the biodiversity of insects and plants that were in these vacant lot systems.

And a lot of this, the vacant lots that I sampled, we were just finding volunteer plant species that are not planted intentionally by people, but. Grow in these kinda unused spaces. So species include chicory, red and white clover. There’s lots of dandelion and a few other, you know, queen anne’s [00:03:00] lace pretty common things that you’d typically see.

And so I was looking at metals that are pretty common in urban and industrial areas. So arsenic, cadmium, chromium, and lead. A lot of the soils are contaminated with pretty high concentrations of lead, which is, you know, the, which you typically hear of when you think of metal contamination in cities.

Jacy: Now, you also found that different plants had different amounts of metals.

Do you have any ideas, about why do you think causes these differences?

Sarah: Yeah, so metal uptake is quite complicated. It varies based on the plant species, but also the metal species. And there’s a lot of like external abiotic factors that also influence bioavailability, um, including pH and the amount of organic matter that you find in the soil.

And so in general, there’s three different groups of plants with that vary in their ability to take up metal. So there’s um, metal excluders, which [00:04:00] are plant species that. Don’t take up metals very much into their biomass. So biomass being roots shoots and leaves. There’s plant indicators which are, are metal indicators, which are plants that uptake metals that are reflective of what you’d find in the soil.

And then there’s plant hyper accumulators that are metal, hyper accumulators that take up metals from the soil and concentrate them at levels that are a hundred to a thousand fold higher than plant species that don’t take up those metals. And so. There’s a lot of research on these different plants and it’s believed that they’ve evolved these really efficient mechanisms to taking up metals, to take up essential metals.

But then a lot of the times, these trace metals that are non-essential are also taken up at the same time, there’s also ideas that these plants will use these metals as a form of defense against our rivery animals that are trying to eat that.

Jacy: So. The metals that you [00:05:00] listed sound disturbing. What is the effect on bees who’ve been exposed to them?

Sarah: So the effects of metals ranges all the way from mortality down to we’re seeing changes in their foraging behavior, exposure to high concentrations of metals as you’d expect results in mortality with for bees. Um, so it’s metals generally cause havoc, uh, if you’re exposed at high levels, but then we’re also seeing.

That exposure to high concentrations of metals results in changes in their morphology, so differences in their eye, head and wing sizes. We’re also seeing elevated brood mortality and a delay in brood development time. Unfortunately, most of the research has been only focused on honeybees with just a few studies on bumblebees and even fewer on solitary.

So just as a, a caveat for most of the effects that I’m going to be talking about, have been studied in honeybees. Um, negative effects of lead on memory and learning in honey [00:06:00] bees, which given that they’re foraging behavior, they need to go out and find the resources and then remember how to get back could be really challenging for their overall success in, in pollination.

We’ve also, there’s been studies that have found that bees will forage for shorter durations on metal contaminated resources, either just nectar or plants that have been grown. In contaminated soil so that can have a negative effect, can downstream on pollination services. There’s an effect on the gut microbiota.

So exposure to selenium causes changes in that gut microbiota, which, um, that is responsible for anything from nutrition and nutrient availability to immune function and reproduction. So. There’s a lot that we still need to do to understand that, but um, that seems like a, a potentially stressful impact of metals.

[00:07:00] Um, yeah. And then overall, uh, metals also, uh, negatively impact the immuno competence of different bees. And so it makes them a little bit less able to deal with external stressors.

Jacy: So you mentioned that these areas in Cleveland were quite industrialized. Do you believe that in maybe less industrialized urban areas, the level of metals would be as high in these plants?

Sarah: So unfortunately there’s kind of metal contamination everywhere that both where people are concentrated and naturally metals are both naturally occurring and, uh, byproducts of human development and. I think there would definitely be higher concentrations around these industrial sites. There still are present in areas with less of that presence, and I’ve been working on another project looking [00:08:00] at kind of a continuation of, of this paper, but in areas that we.

Would consider safe or like low metal areas and finding not the same concentrations, but still finding quite a bit of metals in the nectar. So it’s kind of everywhere. And the nice thing is we still see bees everywhere. So I think that they’re still able to process and deal with some of that, but yeah.

Jacy: So besides nectar, what other ways can bees get exposed to metals in the environment?

Sarah: Bees are likely exposed through multiple different pathways. So either through consumption of contaminated resources, but also direct contact with contaminated air or soil or water. And if we’re thinking about most bees, a lot of them are solitary soil nesting, so that puts them in direct contact with contamination.

I also have found a few papers that, um, looked at bees, um, picking up particulate from the [00:09:00] air as they’re flying. So if, if you think about them, they carry a little bit of a charge and so I like to compare them to like little swiffers that fly through the air and, and attract these, these particular matter.

And then as they’re flying, can groom those particles into the collected pollen. So. It’s kind of coming at them from every direction. So

Jacy: where would you like to see the research go in this area?

Sarah: So there’s still a lot that we don’t know about the effect of metals on bees in general. And as I kind of highlighted earlier, most of the work right now is with honeybees.

And so I’d love to see. More work focusing on the effect of these metals on solitary species. How these soil nesting species are, if they’re more able to detoxify and process these metals. Yeah. And more focused on a lot on sub-lethal effects. So these behavioral observations that we’re seeing. ’cause I think there could be a lot of challenges for their [00:10:00] foraging efficiency and pollination.

So yeah, just more work on solitary.

Jacy: If you could encourage people to do just one thing to support native bees, what would it be?

Sarah: The main message that I wanna make sure is really clear with this is I. Don’t want to be discouraging people from planting flowers. So while we are seeing metals that are at concentrations that could start to have sub-lethal effects with the bees, it is more important to have resources for bees in general.

So they’re much able, more able to tolerate a little bit of metal exposure than no food resources at all. And so I really wanted to be clear with this study that this is just trying to raise awareness of. People being more aware of the health of the land and the health of the soil before necessarily attracting pollinators into that area.

And there’s little changes that people can do. So testing soil, looking into the history of the lands, I. And really identifying if [00:11:00] a potential area that is targeted for pollinator habitat is safe, first off. And then if not just making small changes, you know, planting species that aren’t necessarily as attractive to pollinators or I.

Mowing plant species before bloom if it is highly contaminated. But yeah, just keep planting flowers, being aware of the soil health and planting flowers. There are soil testing kits available through, there’s private. Companies. Um, and then depending on where people are located, some universities will offer these services for a fee.

And yeah, just being more in tune with the soil, kinda the whole system. And yeah, thinking critically about not only providing food for bees, but also the quality of food for those bees.

Jacy: It’s clear that what happens in our urban environments doesn’t just stay in the soil. It makes its way into flowers nectar, and ultimately the creatures that rely on them.

Thanks so much [00:12:00] to Sarah for joining us and to you for listening. Sarah’s actionable advice, think about the soil. Our bees absolutely need flowers, but flowers need healthy soil as much as our ground nesters. Until next time, stay grounded.