• John Mackay

Oxalic Acid’s Impact on Bee Health

By John Mackay


The perfect storm of increasing varroa pressures, concerns of resistance and falling or no returns to beekeepers, has many beekeepers seeking lower cost or self-made varroa treatment. Proving popular is the use of materials soaked in oxalic acid and glycerine. Yet, little is known about the action of oxalic acid in killing varroa and while many have claimed success in treating varroa, others have reported substantial losses. Now there’s a new report suggesting we should look closer at the effects of oxalic acid.

‘Healthy stomach, healthy body’ has come to the realisation of ‘healthy stomach bacteria, healthy body’ i.e the stomach microbiome (bacterial community) and the relative levels of community members present relate not only to human health but our bees as well.

Oxalic acid-glycerine impregnated treatments, such as these "staples", have become popular with Kiwi beekeepers, but new research shows bee gut health could be compromised.

The major group of bacteria in healthy bees are the lactic acid bacteria, a large order of bacteria that ferment carbohydrates. Indeed, the discovery of this in bees led to the suggestion that honey was a fermented product! Way ahead of you kombucha, kefir and sourdough! Other recent work has looked at whether any benefits could be had in combatting AFB by feeding these bee-specific lactic acid bacteria (spoiler: nope!).

A new report just out last month looked at the effects of two neonicotinoids and oxalic acid on the bacterial makeup in a selection of bees (Resistance and Vulnerability of Honeybee (Apis mellifera) Gut Bacteria to Commonly Used Pesticides open access). Initially the authors looked at the effects of these pesticides on lactic acid bacteria when cultured on plates, using several example species. After firstly using the highest concentrations (where only oxalic acid showed any inhibitory effects), the researchers then used lower concentrations that might be more commonly encountered. However, a number of major lactic acid bacteria species were still inhibited by these lower concentrations.

The attention then turned to the bacterial levels in the bees themselves when they could feed on sugar water with the chemicals added, at what were considered lower concentrations. For the bees feeding on the chemicals, two of the three (including oxalic) showed increased bee deaths compared to the control (no chemical). Using the now-familiar genome sequencing methods to sequence all the bacteria present in the bee gut, bees feeding on oxalic acid-laced syrup showed the most marked differences in the relative levels of lactic acid bacteria species.

…As an aside, you might be thinking that feeding on oxalic acid might seem more akin to the dribble method of administering, rather than the presumed physical contact against the impregnated oxalic/glycerine materials in hives. However, many people report that the materials are chewed and removed by bees, presumably meaning that oxalic acid is ingested by bees as well. Back to the research though…

As a laboratory experiment, it has a number of differences and unknowns to normal beekeeper field settings – namely in ingestion vs physical contact of oxalic, as well as the potential differences in oxalic acid levels. However, it provides more information and understanding on the tolerance of bee health to oxalic acid. Similar work on bee gut composition has been performed in New Zealand, using the same sequencing methods. Check out apicultural conference talks from Dr Michelle Taylor at Plant & Food Research.

John Mackay is a molecular biologist and the technical director of Gisborne-based lab dnature diagnostics and logistics, as well as a hobby beekeeper.