feeding bees

The truth about zinc: is it toxic to honey bees?

When the zinc coating begins to corrode, it sheds as a grayish-white powder which may be toxic to bees.

We don’t actually know if zinc is toxic to honey bees or not, but it could be. Perhaps we should pass on galvinized bee feeders for now.

Just the thought of honey bees and zinc toxicity makes you all squirmy and anxious, right? Such a nail-biter of a topic is hard to resist, but here’s the spoiler: I don’t have an answer. Still, it’s a good question and I will explain why.

How is zinc toxic?

But first, a few words about zinc. Zinc is a metal, one of the elements necessary to living things in small quantities, but deleterious in high quantities. For humans, the Mayo Clinic recommends a daily allowance of 8 mg for women and 11 mg for men. But they warn us that “large amounts of zinc are toxic and can cause copper deficiency, anemia, and damage to the nervous system.”

According to the National Pesticide Information Center (NPIC), zinc sulfate—which is a combination of zinc and sulfur—is a registered herbicide that is commonly used to control moss. The toxicity of the product depends on the amount of zinc it contains. Other products, such as zinc strips, are used on roofs. When rain washes over the roof, the strip releases zinc carbonate, which kills moss, fungi, lichens, and algae.

NPIC goes on to make an odd statement:

 “Zinc salts are slightly toxic or practically non-toxic to birds and highly toxic to freshwater fish and invertebrates. No data were available on toxicity to bees.”

Maybe they were absent on the day the teacher mentioned that bees are invertebrates. Or maybe the phrase “freshwater fish and invertebrates” actually means “freshwater fish and freshwater invertebrates?” I don’t know.

In their defense, the website goes on to explain that, “Bees and other pollinators are not likely to be harmed because they do not eat much plant material.” That’s better. This statement is more helpful and comports with the information I found with the help of Peter Borst.

Zinc in the environment

An article by Jerry Bromenshenk titled, “Site-Specific and Regional Monitoring with Honey Bees: Case Study Comparisons” discusses the levels of heavy metals, including zinc, found in foraging honey bees. The research revealed that bees near the Anaconda Superfund site in Montana had high levels of arsenic, cadmium, and lead and low levels of copper, magnesium, manganese, and zinc, even though the soils were high in copper and zinc.

Taken together, you could conclude that since bees don’t eat the leaves, stems, and roots of plants—but only nectar and pollen—they are not getting toxic doses of these metals from the environment, even a highly polluted environment. Okay. Good news.

How much zinc is in a bee hive?

But the question that was posed to me by a reader was about candy boards. He asked if there was any harm in using hardware cloth in place of a plastic queen excluder for the bottom of a no-cook candy board. I thought about it for a while and responded that I didn’t see a problem.

For some reason, I mentioned this question to my husband who right away asked about zinc toxicity from corroding hardware cloth. It turns out that most standard hardware cloth is galvanized, which means the steel is coated with a protective layer of zinc that helps to prevent rust.

When zinc corrodes

The zinc layer can last a long time, but corrosion is hastened by certain factors, including continuously wet or damp conditions, high humidity, and the presence of acid (think honey and mite treatments). Does this sound bee-hive-ish? In a hive environment, the metal coating may quickly oxidize and shed as a light-colored powdery substance very high in zinc.

We know from experience that galvanized hardware cloth is frequently used in bee hives. It is used for bottom ventilation, top ventilation, mouse guards, robbing screens, moisture quilts, pollen traps, and queen excluders. But the bees are not generally eating from these surfaces. Oh sure, they’re going to lick them from time to time, but they are not actually consuming large quantities of food from them. But feeders are different. Is it possible that food served in galvanized mesh may be too high in zinc? Although I don’t know for sure, I think it’s possible.

Toxicity is hard to assess

Oftentimes, toxicity in a colony is hard to see. For example, some researchers have found that overheated fructose can produce enough hydroxymethylfurfural (HMF) to be toxic to bees. On the other hand, those same researchers think the HMF may kill a percentage of bees that is hard to detect by casual observation. If it killed 5% or even 10% of a colony over winter, would a beekeeper even notice? Probably not. The biggest loss may be a reduction of fitness that, in combination with other factors, could potentially bring a colony to its knees.

I believe it’s possible that high levels of zinc in the feed could behave in a similar way, perhaps taking out some, but not all, of the bees or perhaps weakening them in subtle ways. Remember, I’m speculating here. But after reading extensively about zinc in the diet of animals, I think we should be wary of feeding honey bees directly from galvanized wire. Or, if a substitute method is not readily available, perhaps we should replace the wire frequently before the zinc begins to corrode.

Is zinc toxicity an issue or not?

So what do you think? Could galvanized metal feeders be a problem in bee hives? If so, is it worth further study?

Honey Bee Suite

Zinc toxicity: When the zinc coating begins to corrode, it sheds as a grayish-white powder which may be toxic to bees.
When the zinc coating begins to corrode, it sheds as a grayish-white powder. Image by Manfred Richter from Pixabay

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  • For winter feeding I put the sugar bricks directly on the frames. Fondant is put in the milk jug bottoms I cut from the jug. They too go directly on the frames. I put a spacer over that and a quilt box on that.

  • Well OF COURSE it’s worth further study. I want us to know EVERYTHING. Plus I’m the FIRST to recommend work for someone not me.

    Also nowadays everything is bad for us. But is the zinc worse than the plastic, since I’ve read about beeks who won’t have any plastic in their hives.

    Also also, I see my bees collecting (water?) from all kinds of trash piles and under the burn barrel. Now I need to add heavy metal poisoning to my things to fret about without actually doing anything useful.

  • Rusty,

    Until proven by controlled studies that zinc is harmful to bees, the suppositions are exactly what you call them, speculation.


    • Carl,

      I think it would make a great master beekeeper project, especially for someone who wanted to schedule a project over the winter months. I hope someone takes it on.

  • > Until proven by controlled studies that zinc is harmful to bees,

    I would take the exact opposite approach: Until proven by controlled studies that zinc is safe for bees. There is a reason why galvanized tanks are no longer used for honey processing and storage.

    Pete B

  • You suggest replacing the zinc coated wire before it can rust. The zinc is all removed before the rust starts. I think the rusted wire would be safer than the fresh zinc coated wire. just my “speculation” on this subject

    • Clifford,

      In the next to last paragraph, I suggested replacing the wire before the zinc begins to corrode. At that point, the zinc would not have been released into the hive.

  • Back in the ’60s, stamped zinc queen excluders were common as well as the wire types that we still use. I don’t know that they ever caused toxicity problems for the bees. But, their use was discontinued — maybe for that very reason.

    I do know that zinc melts at a much lower temperature than does the steel in wire excluders. How do I know? I once tried using a torch to sterilize a zinc excluder. That works fine on steel but rapidly melts a giant hole in zinc. 🙁

      • Re Clifford’s comment and Rusty’s reply. Any galvanized material placed in the hive will begin to corrode the minute it is placed. Rate will depend on environmental conditions at its particular location.

  • Zinc works as a corrosion prevention on steel because it is more anodic than the steel. OK what I mean is that the zinc corrodes faster than the steel and it is doing so sort of as a battery. So as it corrodes the steel doesn’t until the day when all the zinc is used up. Then bingo the whole steel item rusts. Think about a galvanized trash can – you remember those right? Yeah I know – plastic…same for that watering can in your photo.

    Bottom line I like your frequent replacement idea. I have no cook candy boards on and use half inch rabbit wire mesh. I need to go check out a few that have seen a few years of use to check out if they have seen much corrosion or not…

    Not sure I am ready for plastic replacements though.

  • Hi Rusty,

    I know that welding galvanized is not something healthy. Even though we need a small dose of zinc. Too much of a good thing makes it not such a good thing. As for the small dose of zinc, the bees would get, from galvanized wires, I don’t think I’d get too upset. Beekeepers have been using galvanized equipment for at least a century. I had picked up a galvanized honey extractor. My wife wasn’t having anything to do with it. It wasn’t from the zinc coating, it was more of the dirt patina, that came with it. A friend of mine was thrilled to get it.

    And I make maple syrup. You can still get the old style galvanized sap buckets. So, it is still in use, for a lot of food grade containers.

    So I see you snuck in a reference to my memory. “The biggest loss may be a reduction of fitness that, in combination with other factors, could potentially bring a colony to its knees.” I can only assume, that these must be bee’s knees, correct? My memory is as long as the hair on those bee’s knees. Jus’ sayin’. Al

  • If in doubt, use a different material. So many safe materials that are useable to be messin’ with questionables. IMO

  • > Just spent most of an hour hunting around looking for zinc studies in terrestrial invertebrates.

    Yikes! That sounds familiar …


  • Peter B.

    If Zn is proved to be not harmful to bees, isn’t that the same as Zn is safe for bees? What’s the difference? I would bet that galvanized metal tanks are no longer used to process and store honey because they are heavy and may impart an unpleasant flavor to the honey. Do you have any data to support that honey from galvanized tanks has harmed people?


  • I made fondant frames from galvanized wire for a poor hive this past winter. The hive was overpopulated and only had 1 1/3 frames of honey from November to March. They made It! Strong hive how… found several full brood frames and two frames of honey. Queen is laying and doing well. My fondant had no metal corrosion present.

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