3 Mason Bee Predators Every Host Should Know

Once your mason bees stop flying, your job as a host isn’t finished. Proper fall harvesting and maintenance give you the opportunity to open nesting materials, inspect what happened during the season, identify predators and other problems, remove compromised material, clean reusable nesting equipment and prepare healthy mason bee cocoons for winter.

Most importantly, it gives you an opportunity to address problems before the next generation of mason bees emerges in spring.

If you simply leave unmanaged nesting materials in your bee house year after year, you may also be leaving behind the very pests, parasites and diseases that can harm your bees.

Pollen mites, chalkbrood and Houdini flies can all cause problems for mason bees, and you may never know they’re there just by looking at the outside of your bee house. From the outside, your mason bee house may look perfectly healthy. You may watch your bees coming and going all spring, carrying pollen and mud into their nesting cavities and filling your nesting block.

But what’s happening behind those mud walls?

In our Mason Bee Mayhem video, we take you inside mason bee nesting chambers for an up-close look at three predators every mason bee host should learn to recognize.

1. Pollen Mites

Pollen mites are tiny, but when their numbers build up inside a mason bee nest, they can create big problems.

A female mason bee works incredibly hard collecting pollen and nectar to create a food provision for each of her offspring. She places an egg on that provision and seals the nesting chamber with mud.

Unfortunately, pollen mites can also end up inside those chambers. They feed on the pollen provisions that were intended for the developing mason bee. When large numbers of mites are present, they can consume the food the larva needs to survive and develop.

From the outside, you may have no idea they’re there.

During fall harvest, opening appropriate nesting materials allows you to see what’s actually happening inside. In our video, we show you an up-close example of pollen mites so you’ll know what to look for in your own nesting materials.

2. Chalkbrood

Chalkbrood is different from the other predators we’re showing you because chalkbrood is actually a fungal disease.

Developing mason bee larvae can become infected when they consume pollen contaminated with fungal spores. Instead of developing into healthy adult bees, infected larvae die and eventually become hardened, chalk-like remains inside the nesting chamber.

The concern doesn’t end with that individual bee. Chalkbrood spores can remain in contaminated nesting materials, potentially exposing future generations of mason bees.

That’s one reason proper nesting materials and annual maintenance are so important. Continuously using contaminated nesting materials without proper cleaning or replacement can allow problems to carry over from one season to the next.

We’ll show you what chalkbrood looks like inside a real mason bee nest so you’ll have a better idea of what to watch for when harvesting your own bees.

3. Houdini Flies

Houdini flies are an invasive parasite of mason bees, and they can be particularly troublesome because of what happens inside the nesting chamber.

Female Houdini flies enter mason bee nesting cavities and lay their eggs near the pollen and nectar provisions that mother mason bees have carefully collected for their offspring. When the fly eggs hatch, Houdini fly larvae consume the food intended for the developing mason bee, often leaving the bee without enough food to survive.

If the infested nesting material isn’t opened and properly managed, Houdini flies can remain hidden inside and emerge the following spring alongside your mason bees. The numbers can multiply quickly. A female Houdini fly can lay hundreds of eggs, while a female mason bee produces only a relatively small number of offspring during her lifetime. That means an unmanaged infestation can produce many more Houdini flies ready to target mason bee nests the following season.

This is why identifying and removing Houdini flies during fall harvest is so important. What starts as a few hidden larvae inside a nesting block can become a much bigger problem for future generations of mason bees.

When you open an affected nesting chamber, one of the first signs you may notice is curly brown frass. Frass is the waste left behind by the Houdini fly larvae.

Look beneath the frass and you may find the larvae themselves: small, yellowish, worm-like maggots hiding inside the nesting chamber.

If infested nesting materials aren’t opened and properly managed, Houdini flies can remain hidden inside and emerge the following spring alongside your mason bees. That allows the cycle to begin again as the adult flies search for new mason bee nests.

What starts as a problem hidden inside one nesting block can become a much larger problem for future generations of bees.

Why Fall Mason Bee Care Is So Important

This is the part we really want every mason bee host to understand. Your bee house can look perfectly healthy from the outside.

You can watch female mason bees flying back and forth all spring. You can see them bringing in pollen. You can watch them collect mud. You can see nesting holes fill and think everything went perfectly.

But you cannot see what’s happening inside those individual nesting chambers.

Pollen mites may be consuming pollen provisions. Chalkbrood may have infected a developing larva. Houdini fly larvae may be hiding beneath piles of frass.

It’s all happening behind the mud walls.

That’s why putting up a bee house is only the beginning.

See What’s Really Inside a Mason Bee Nest

At Rent Mason Bees, we harvest, clean and inspect millions of mason bee cocoons each year.

Opening this many nesting cavities gives us an incredible opportunity to see what’s actually happening inside mason bee nests. We find predators, parasites, pests and diseases that would be almost impossible to detect by simply looking at the outside of a bee house.

That’s why we’re sharing what we see.

Our Mason Bee Mayhem videos take you inside real nesting chambers so you can see these problems for yourself and learn what to look for when caring for your own mason bees.

Learn what to look for. Know what doesn’t belong. And give your mason bees the proper care they need to thrive year after year. 

Learn How to Harvest & Clean Your Own Mason Bees

Want to care for your own mason bees? Our step-by-step video walks you through harvesting your nesting materials and cleaning your mason bee cocoons:

See How We Clean 3 Million Mason Bee Cocoons

Go behind the scenes at Rent Mason Bees and see what happens during our annual fall harvest:

Learn More About Mason Bee Predators

Want to learn more about the things that can harm mason bees and what you can do about them?

Learn More About Mason Bee Predators

Don’t just put up a bee house and forget about it. Fall care matters. Healthy bees start with healthy nesting materials.