Houdini Fly Alert for Mason Bee Raisers

Houdini Fly Alert for Mason Bee Raisers

We love mason bees for many reasons! They're early spring pollinators, a lot of fun to watch, and require very little maintenance compared to honey bees, making them perfect bees for busy gardeners and farmers.

Traditionally, spring mason bee care included three simple steps. Step one, Install the mason bee house—step two, release mason bee cocoons. Step three, sit back and relax while these super pollinators do their thing. However, the Houdini Fly, an invasive parasite of the mason bee, has added an important new step to our care routine!

What is the Houdini fly?

Cacoxenus indagator, more commonly called the Houdini fly, is a cleptoparasite. Cleptoparasites are thieves, meaning they steal food or prey from other animals. For bees, most cleptoparasites lay their eggs inside bee nests. Once the Houdini Fly eggs hatch, the larval parasites eat the pollen loaves intended for immature bees and sometimes will even eat the developing bees in the process.

The Houdini fly gets its name from its unique method of escaping from bee nests. Female Houdini flies lay their eggs in nest cells before the female mason bee can seal the nest with mud. The fly larvae quickly hatch and consume the pollen loaf before the mason bee larvae, causing the mason bee to starve. The flies then continue to grow in the nesting cavity undetected.

Following pupation, newly emerged adult flies have a brief window of time to escape from the nest before their bodies harden. Houdini flies use an anatomical adaptation called a "head blister" to perform their escape acts. The flies find a small crack in the nest's mud wall, insert their deflated head blisters, and then pump the blisters full of blood, which creates hydraulic pressure, causing the mud wall to fall apart. The flies repeat this process until they have created a small exit hole to squeeze their soft bodies through—it's not a quick process, but it's effective!

Click here to watch a video of the Houdini fly inflating its head.

What do Houdini flies look like?

Adult Houdini Fly Characteristics

  • Large, red eyes on the side of the head;
  • Dull brown with horizontal stripes on abdomen;
  • Wings fold over each other when resting;
  • Smaller than a house fly, but larger than a fruit fly;
  • Often found waiting on the outside of mason bee houses; and
  • Slow, hovering flight pattern—not very skittish making them easy to squish.

Houdini Fly Larvae Characteristics

  • Houdini fly larvae look like sticky white clusters inside the brood cell;
  • Multiple larvae per brood cell;
  • Often surrounded by curly orange/brown frass (poop).

Where did the Houdini fly come from?

The Houdini fly is thought to be an example of sending mason bees from Europe without carefully inspecting the nests before the move. The first documented sighting of the Houdini fly in the United States was in New York in 2011. Since then, they have spread across the country, with the first documented sighting in Washington state in 2019. However, mason beekeepers have reported they found Houdini fly maggots in their mason bee nests many years before 2019. Unfortunately, the Houdini fly is now believed to be permanently established in Washington.

But, there is good news!

Through good management practices, mason bee farmers and backyard bee raisers can reduce the impact of the Houdini fly on managed mason bees, which will help wild mason bee populations.

We cannot stress enough the importance of good management practices for managed pollinators!

How do I protect my mason bees from Houdini flies?

We're so glad you asked! But, before we go into specific spring and fall protection tips. There is still a lot we don't know about these parasites in North America. We request that if you spot Houdini flies, as adults around your bee house in the spring or larvae in your nesting materials in the fall, that you help the bee community track their spread!

You can do this in two ways:

1. Fill out our 2022 Spring Houdini Fly Survey to help Crown Bees and the researchers we work with identify newly infected areas and potential management practices. We know mason bees fly at different times across North America, so the survey will be available until July 2022.

2. You can upload a photograph of the Houdini fly adults or maggots to iNaturalist so others can track their spread.

Spring Protection Tips:

1. When you set up your mason bee house in the spring, make sure you use nesting materials that allow you to open, inspect, and harvest cocoons.

2. If you are purchasing mason bee cocoons to get your populations started, be sure to ask the producer how they harvested their cocoons and whether they inspected the cocoons for Houdini flies. Read this before purchasing mason bee cocoons to learn how to spot reputable producers!

3. You should consider controlling adult mason bee emergence if you purchased mason bees that arrived in wood blocks or unopened reeds. You can do this by placing the nesting materials in a BeeGuard Bag or another fine mesh bag and closing tightly. Check the bag daily, release any emerged mason bees, and squish any adult Houdini flies.

4. While you mason bees are flying, regularly check your bee house (every day if possible) and instantly kill any adult Houdini flies you see hovering around your nesting materials!

They are slow flyers and easy to squish. Or, if you don't love squishing things, some of our bee raisers have noticed that using a low-powered vacuum (like a dustbuster) works quite well at sucking up Houdini flies.

Fall Protection Tips:

Harvest mason bee cocoons - Open nesting materials in the fall and kill any Houdini fly larvae. The larvae look like sticky white clusters inside the brood cell, often surrounded by curly orange/brown frass (poop)—see image below.

When they are in the larval stage, it is easy to kill them in the fall. You can squish them at this stage, although it'll be a bit messy. Or, for those of you who don't love squishing things, you can submerge them in alcohol or soapy water before tossing them in the garbage.

Thank you for being diligent and keeping an eye out for the Houdini fly this spring and fall! Remember, nest monitoring and maintenance are essential to maintaining bee health! It's our responsibility as bee raisers to do all we can to reduce the incidences of pests, parasites, and diseases and allow managed bees to thrive.

Good Luck and Happy Pollinating!

~The Crown Bees Team

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