What's This? A Pictorial Guide of Things Found in Nesting Materials

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. 

Mud walls inside mason bee nesting chamber


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

Mason bee frass (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. 

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 chew hole in mason bee cocoon

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

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. 

Carpet beetle larvae inside nesting chamber

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.