Think Twice Before Mixing Miticides: What Every NZ Beekeeper Should Know About Nurse Bee Toxicity

Why Read This? Because protecting your nurse bees means protecting your colony’s future. This new study reveals how common varroa treatments can backfire, especially when mixed.

When varroa mites threaten your hives, your first instinct might be to throw everything you've got at them—synthetics, organics, combinations. But before you do, consider this: a recent 2025 study published in Insects found that mixing treatments may do more harm than good, especially to your nurse bees.

Researchers tested three synthetic miticides (fluvalinate, coumaphos, amitraz) and two organic acids (formic and oxalic) on nurse bees at field-relevant doses. They didn’t just look at the individual effects. They examined what happens when these are used in sequence, like many beekeepers do in real-world varroa control.

Key Findings for Beekeepers:

• Synthetic miticides alone = low acute toxicity. Nurse bees exposed to fluvalinate, coumaphos, or amitraz by themselves showed high survival (over 90% after 72 hours).

  
  

• Organic acids are not so kind. Both formic acid and oxalic acid significantly reduced nurse bee survival, especially formic acid, which caused death within hours in some cases.

  
  

• Mixing doesn’t always make it worse, but it can. When synthetic miticides were followed by organics (24 hours apart), the organics still did the most damage. No strong synergistic toxicity was found, but the damage was already done by the organic acids.

  
  

• One combo stood out: fluvalinate + formic acid. This pairing didn’t show added toxicity, suggesting some potential compatibility. But other mixes, like amitraz + formic acid, led to worse outcomes.

  
  

Why This Matters in New Zealand

In NZ, we are increasingly turning to “natural” methods like formic and oxalic acids to manage rising resistance to synthetics. But this research is a reminder that natural doesn’t always mean safe, especially when it comes to your in-hive workforce. Nurse bees care for your brood and keep colony functions running. Killing them off, even unintentionally, risks delayed collapses down the line.

If you’re switching between miticides or layering treatments, this study is a must-read. It emphasizes the need to plan your varroa strategy, not just react to mite loads.

Takeaway Tips:

• Don’t assume that rotating miticides is risk-free, especially if your timing is tight.

• Formic acid can be highly toxic to nurse bees - use only when absolutely necessary, and follow best-practice application methods.

• Monitor not just mites, but colony recovery and brood care after treatments.

• If you're uncertain, consider talking to an apicultural scientist or consultant before combining treatments.

  
  

Need help planning your varroa strategy this season?

Avoid guesswork, because the wrong mix could cost your colony more than just mites.

Based on: Kim et al. (2025), "Toxicity of Consecutive Treatments Combining Synthetic and Organic Miticides to Nurse Bees of Apis mellifera", Insects 16(657). Read the study here: https://www.mdpi.com/2075-4450/16/7/657