During mason bee cocoon harvesting, you'll find all sorts of things inside the nesting materials. As expected, you'll find healthy mason bee cocoons, mud, and mason bee frass (poop). However, you may also find Chalkbrood, pollen mites (or other parasites), pollen loaves from bee eggs that never hatched, and possibly even dead mason bee larvae. The combination makes harvesting a fascinating but messy process.

Below is a pictorial guide to help you identify the things you may come across while harvesting. Click on any link to learn more! 

Healthy Mason Bee Cocoon 

A healthy mason bee cocoon will appear dark grey or brown, ovular in shape, and firm to the touch.

Healthy Mason Bee Cocoon


Female mason bees build the walls of their nesting chambers out of mud or other "masonry" products to give each egg a safe place to develop into an adult bee. You'll find a lot of dried mud inside a productive mason bee nest. 



The small, dark pellets you find alongside the mason bee cocoons are frass or larva poop. 


Pollen Loaf

Not all eggs will hatch, and not all larvae will reach maturity. In this case, you may find partial or complete pollen loaves inside the nesting materials. They are typically yellow or orange in color, and will have a sticky texture. 

One or two intact pollen balls per tray or handful of nesting tubes are natural. However, if you find a lot of untouched or partially intact pollen balls during harvesting, there is cause for concern. Learn more here.

Pollen loaf

Pollen Mites

Pollen mites are typically white, yellow, or red and are at first difficult to pick out from individual pollen grains when viewing an infested nest. But, if you shake out the contents of the nest, you'll be able to see the mites move around. They are found throughout North America and are more common in humid environments.

Pollen mites inside nesting chamber


The single most destructive disease of cavity-nesting bees is the fungal pathogen called chalkbrood. Dead bee larvae will have a C-shaped, chalky, discolored appearance ranging from brown to grey, black, or white—depending on the stage of fungal development.

Chalkbrood infected mason bee cocoon

Houdini Fly Larvae

Female Houdini flies lay their eggs in nest cells before the female mason bee can seal the nest. The fly larvae quickly hatch and consume the pollen loaf before the mason bee larvae, which causes them to starve. Houdini fly larvae look like sticky white clusters inside the brood cell, often surrounded by curly orange/brown frass (poop).

Houdini fly larvae infested mason bee nesting chamber
Close up of Houdini fly larvae

Monodontomerus (Mono)

Mono is a chalcid wasps, which are some of the most destructive parasites of mason and leafcutter bees. The female wasp use their needle-like ovipositor to paralyze the bee larvae by inserting it through the cocoon wall. 

After paralyzing the bee larva, the female lays her eggs inside the cocoon. Upon hatching, the wasp larva consumes the bee and completes its development inside the cocoon undetected. The wasps emerge as adults from the cocoons by chewing a small hole in the side.

Evidence of mono is small holes in cocoons and BeeTubes. 

Mono wasp exiting the chew hole
Mono hole in mason bee cocoon, Oregon State University Extension

European Wool Carder Bees

The European wool carder bee (Anthidium manicatum) is a solitary, cavity-nesting bee species in the Megachilidae family, whose members include mason and leafcutter bees. 

The wool carder bee is native to Europe, parts of Western Asia, and North Africa. Since its discovery in New York in the '60s, the wool carder is now widespread throughout the United States.

Wool carder bees get their name because of the unique way they build their nests (see image below). Female wool carder bees collect the soft downy hair of fuzzy plants to use in building nests for their offspring. Some people find male wool carder bees very charismatic and enjoy watching them chase after other bees that invade their territory. In contrast, others would call them bullies based on male bees' aggressive tendencies toward other bees when defending floral resources.

Adult Wool Carder Bee

Adult European Wool Carder Bee

European Wool Carder Bee Nest

Wool carder bee nest. Image courtesy of Jeannie Davidson.

Live Larvae, Beneficial Solitary Wasps 

Solitary wasps are common beneficial insects in landscapes. They hunt and capture small insects (not your bees) or spiders to feed their young. They are not aggressive toward people and rarely sting. Some species of beneficial wasps will take up residence in your bee house.

If you come across any developing wasp larvae, you should try to close the nesting materials back up and store them overwinter in a BeeGuard Bag in your unheated garage or shed. If you must remove wild bee larvae, store them overwinter in a smaller CocoonGuard Bag.

Unidentified wasp larvae, Image by G. Fletcher

Spider-Using Solitary Wasps 

Trypoxylon is a genus of solitary wasps in the family Crabronidae that are active hunters of spiders. Female wasps paralyze spiders with a venomous sting and place them inside their nests as food for their developing larvae.

Depending on the species, they will either construct their own nest from mud or find cavities that already exist. Their use of existing holes is why you'll often find Trypoxylon cocoons in your mason bee nesting materials during harvesting. Cocoons are easy to identify, as they look like little black bullets or torpedoes. Solitary wasps that use mud tend to sonicate or vibrate the mud, resulting in a smooth, cement-like capped end.

Don't worry; these beneficial wasps that will go on to help pollinate your yard and garden next season!

Trypoxylon, Spider Using Solitary Wasp
Trypoxylon lactitarse

Grass from Solitary Wasps 

Some species of solitary wasps build their nests using blades of grass. 

Grass from grass carrying wasp inside nesting chamber

Carpet Beetle Larvae 

Carpet beetle larvae are small, hairy, and look like short fat caterpillars. The larvae can burrow through the tough mud-capped ends and nesting walls and devour pollen loaves and bee larvae. 


Six-Spotted Spider Beetle

You'll find the Six-Spotted Spider Beetle (Ptinus sexpunctatus) near or inside solitary bee nests. The larvae of the spider beetle are commensals in the nests of mason and leafcutter bees. Commensal relationships are relationships where one organism gets food or other benefits from another organism without hurting or helping it.

Among other things, they feed on the remains and moltings of dead insects, the food provisions intended for the developing bees, and bee frass (poop).

While the six-spotted spider beetle isn't known to be a significant threat to your mason or leafcutter bee populations, it is an invasive species in North America. To prevent the beetle from spreading to new locations, remove and kill any six-spotted spider beetles you find in your bee nests. 

Ptinus sexpunctatus, 6 spotted spider beetle


Bee-sure to check out our Parasites and Diseases of Mason Bees and our All-Season Pests and Predators of Cavity-Nesting Bees to learn more details about what to watch out for both when mason bees are actively nesting and during harvesting.