Varroa Reduces Bees’ Surface Antimicrobial Defences

A recently published article in Scientific Reports suggests yet another way in which varroa mites are bad for honey bees. Varroa reduces bees’ surface antimicrobial defences. Apparently, many social insects do something called “venom bathing”, which basically involves coating their bodies with venom from sting glands. The main component of bee venom is melittin, which is antiseptic, antiviral, anti-fungal, and antimicrobial. So, bees use more than propolis to defend against pathogens.

Here’s a breakdown of the main points coming out of this research:

Pusceddu, M., Tragust, S., Theodorou, P. et al. Varroa destructor weakens the external immunity of western honey bees by impairing melittin production. Sci Rep 15, 30623 (2025). https://doi.org/10.1038/s41598-025-13440-2

More Than “Blood-Sucking Mites”

Beekeepers are familiar with Varroa destructor as the parasite that weakens colonies by feeding on fat-body tissue and transmitting viruses like deformed wing virus (DWV). But a new study in Scientific Reports reveals a subtler impact: varroa suppresses bees’ external immunity by lowering levels of melittin, a key antimicrobial peptide.

Think of melittin as part of the bee’s natural “surface disinfectant.” Without it, pathogens can colonise the cuticle more easily, leaving even apparently healthy bees vulnerable.

What Is Melittin?

• A major component of bee venom: About 40–50% of the dry weight of venom is melittin.

• Mode of action: Melittin disrupts microbial cell membranes, punching holes that kill bacteria and fungi.

• In bees: Present not only in venom but also on the cuticle, where it helps maintain a “clean” surface barrier.

• In humans: Extensively studied for potential therapeutic uses (antimicrobial, anti-cancer, anti-inflammatory).

So melittin is not just about stings—it’s part of the bee’s everyday immune toolkit.

The Study in Brief

• Journal: Scientific Reports (Nature Publishing Group).

• Date: 20 August 2025.

• Method: Laboratory study measuring melittin levels in bees with and without Varroa infestation.

• Species: Apis mellifera (workers).

• Finding: Varroa-parasitised bees had significantly lower melittin on their cuticle.

Why This Matters for Beekeepers

Traditionally, varroa is a threat because of:

1. Feeding damage — mites drain fat-body resources.

2. Virus transmission — especially DWV and related pathogens.

This research points to a third mechanism: varroa undermines bees’ antimicrobial armour. That means colonies are vulnerable in ways that aren’t immediately visible.

Practical Takeaways

Stick to integrated pest management (IPM):

• Monitor mites with sugar shake, alcohol wash, or sticky boards.

• Treat promptly using approved methods (oxalic acid, formic acid, thymol, amitraz as permitted).

• Rotate treatments to prevent resistance.

• Support colony hygiene. Many strong colonies rely on propolis envelopes, grooming, and hygienic behaviour. These are more effective if bees aren’t already immunocompromised.

Pitfalls and Uncertainties

• Lab vs field: Results were lab-based. The degree of melittin suppression in field colonies isn’t yet quantified.

• Other peptides: The study looked at melittin specifically; other antimicrobial peptides (defensins, apidaecins) may also be affected.

• Mechanism: We don’t yet know how varroa feeding reduces melittin production or deposition.

• Practical impact: Beekeepers can’t measure melittin levels.

Wider Scientific Context

• Previous work has shown that varroa suppresses bee immunity more broadly, e.g., reducing hemocyte counts.

• This is the first study directly linking mite feeding to reduced antimicrobial peptides on the cuticle.

• Together, these studies reinforce the picture of varroa as an immunosuppressive parasite, not just a virus vector.

Conclusion

Yes, I went and did another, albeit brief, article about varroa mites! Readers are probably sick of it, but if it means your bees aren’t sick, it’s worth it.

The Mole (my son, Alex) and I, are currently in the middle of harvesting honey and treating colonies with Formic Pro once the supers are off. Most colonies have a medium langstroth box on the floor (containing some uncapped stores) with a full deep box above that. The Formic Pro goes on top of the frames in the brood box, with the foil wrapper covering the tops of the pads, as recommended by Randy Oliver. If we had time, we’d be doing alcohol washes to check mite loads, but we don’t – so everything gets treated. Nucs get a different treatment (Thymovar) as they are more likely to be badly damaged by formic acid.