Houdini Fly Update + How to Clean Your Cocoons

Fall Cocoon Cleaning Season Is Here & the Houdini Fly Hunt Continues

If You Spot Them, Please Report!

Your fall cocoon cleaning plays a critical role in protecting next spring’s mason bees. With hundreds of Houdini Fly reports coming in from Washington, Oregon, and British Columbia, researchers and bee advocates are sounding the alarm about the growing threat to mason bee populations. Whether you’ve already finished your mason bee harvest or are getting ready to start, this blog is packed with everything you need to protect next spring’s bees.

Rent Mason Bees has partnered with Abigail Lehner, lead researcher at the UC Davis Bee Lab, who recently received a grant to study Houdini Fly. As part of this collaborative effort, we need your help tracking this invasive species. Her reporting form is still open, so while you’re cleaning your nests this fall, please report any sightings.

CLICK HERE TO REPORT HOUDINI

We’re also back In the Lab with Abigail from UC Davis to share the latest Houdini Fly update, including where they’re being found and what signs to look for in your nests.

Plus, we’ll walk you through how to clean your own mason bee cocoons and how to identify everything inside your nesting material—from healthy cocoons to pollen mites, chalkbrood, Mono Wasp, frass and more.

Why Houdini Fly Is a Serious Threat

Houdini Fly is a kleptoparasite, meaning it sneaks into mason bee nesting chambers and lays its eggs inside. Its larvae eat the pollen provision meant for mason bee babies—starving them before they ever develop.

Here’s why their impact is so severe:

• A female mason bee lays around 15 eggs in her lifetime.

• A single Houdini Fly can lay up to 200 eggs.

• Their larvae develop inside the same chambers and emerge in spring alongside your bees.

• This 15-to-200 ratio can wipe out entire mason bee populations in a single season.

The good news?
You can remove 100% of Houdini Fly larvae simply by cleaning your nesting material in the fall.

What to Look For in Your Nesting Material

Houdini Fly larvae are easy to spot once you open your trays or tubes. Watch this video to learn how to identify Houdini fly larvae, pollen mites, chalkbrood, mono wasps and frass.

This is why fall is the most important time of year to care for your bees.
Please open your nesting material and inspect every tube or tray. Every larva removed is one less threat to your bees.

What Houdini Fly Looks Like in Spring

In spring, Houdini Fly emerges at the same time as your mason bees. They appear as:

• Tiny fruit-fly–sized insects

• Bright red eyes

• Fast-moving

Watch our Houdini Fly Video

How to Clean Your Mason Bee Cocoons

Cleaning your cocoons is the single most effective way to protect next year’s bees. Our step-by-step video walks you through cleaning:

• Stacking trays

• Cardboard tubes

• Bamboo (if it can be safely opened)

Protect the Next Generation of Mason Bees

Fall cleaning is how we break the cycle of parasites and predators.
By opening your nesting material and reporting Houdini Fly, you’re helping researchers track this invasive species and protect bees nationwide.

Long-Term Steps to Stop Houdini Flies:

1. Only Buy Mason Bee Cocoons that Have Been Cleaned:
   • Purchase mason bee cocoons that have been properly cleaned to ensure they are free from houdini fly larvae, chalkbrood and pollen mites.
2. Never buy bamboo or reed tubes filled with cocoons: 
   • They may be full or predators and are difficult to open and clean, making them a breeding ground for parasites.
   • Instead, choose clean, loose cocoons that you know have been cleaned.
3. Use Clean Nesting Materials Every Spring:
   • Set out fresh, clean nesting blocks or tubes at the start of each spring season.
   • Replace old, worn materials to prevent parasite buildup.
4. Harvest and Clean Cocoons in the Fall:
   • Open your nesting materials at the end of the season to clean cocoons and remove any pests.
   • Look for Houdini fly larvae—small, white maggots found inside the nesting cells. If you spot any… remove and dispose of them. Hot water or burn pile works. (don’t just dump them in yard waste bin or garbage… they will survive).

Thank you for being part of this effort to safeguard solitary bees.