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

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?

Rusty
Honey Bee Suite