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Predator Alert – Invasive Houdini Fly


The Washington State Department of Agriculture (WSDA) has confirmed reports of Houdini fly (Cacoxenus indagator) in Washington State. The Houdini fly is a non-native kleptoparasite. What that means is that it does not attack mason bees directly, but lays its eggs on the pollen meant for the mason bee young inside of the nesting blocks. When the Houdini maggots hatch, they consume the food before the mason bee larvae, which causes them to starve. When the flies are fully formed, they escape from the sealed chambers, earning them the name “Houdini” fly.

Each cell in this nesting block originally contained a singular mason bee larva on top of a pollen food source. This photo shows multiple Houdini fly larvae in each cell. The mason bees in each cell did not survive because the Houdini fly larvae devoured their food sources.


Here at Rent Mason Bees, we are taking this very seriously. The Houdini fly could cause a significant decline in the mason bee population, which will then impact farmers and growers of crops. Olivia Shangrow, bee biologist said, “Houdini flies pose a significant threat to mason bees. We need to teach people how to identify this fly while populations are small so that we can help control it adequately and protect our native mason bee populations.”

Our top priority is to provide clean, healthy cocoons to our renters and control and eliminate as best we can the predators that can harm them. When you return your nesting block back to us at the end of the season, one of the steps we take when harvesting the cocoons is to pick out Houdini fly larvae and dispose of them.

When we harvest the cocoons during the fall, it is very obvious which nesting cells have been infested with Houdini fly larvae. In addition to visually inspecting the mason bee nesting blocks, we also place every cocoon over a light board to check for viability as well as any other pests that may be contained inside of the fully formed cocoons. We can tell which cocoons have been compromised because light will shine all the way through the cocoon.



We are committed to conserving the health of mason bee populations so these are necessary steps in protecting our bees to make sure the next spring we release healthy mason bees to our ecosystem, farms and backyards.


The Houdini fly is similar in appearance to a fruit fly and has red eyes.


Unfortunately, the Washington State Department of Agriculture does not feel eradication of the invasive species is possible. Instead, they recommend limiting its spread through the work of mason bee producers and enthusiasts. Collectively as a group we can all work together to lower the spread of the predator Houdini fly. Most of the prevention occurs during the harvesting of your mason bee cocoons, and by participating in the Rent Mason Bees program, we will harvest them for you.

How our hosts can help is if you see one of these predators, please dispose of it as you choose. They are slow moving, so they should be easy to catch.

Thank you for supporting solitary bees!  -Your Rent Mason Bee Team

This video shows how the Houdini flies can eventually emerge from the mason bee nesting cells if the larvae are not removed in the fall.


Reader Interactions


  1. Barbara Anne Brower says

    Any possibility of trapping the adults, or any advantage to placing mason bee housing in sites that discourage the flies? My shed with multiple mason bee “hotels” has LOTS of these pests, and it seems as if the right sort of trap could get them but not the bees?

    • Thyra McKelvie says

      I’m not sure of a houdin fly trap because they are drawn to the mason bee houses. The best way to rid houdini flies is to harvest your cocoons and clean your nesting blocks every year. They lay thousands of sticky white larva that look like little maggots. Those larva stay in the nesting blocks and form into a grown fly and emerge the following spring with the mason bees. Are you blocks able to be pulled apart and cleaned?

  2. Al Harris says

    I tried spraying my mason bee house with vinegar which seems to keep the houdini flies at bay. I just hope the vinegar will not harm the mason bees when they emerge in the spring. Any other suggestions…?

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