Common Pests, Parasites, and Diseases of Cavity-Nesting Bees

Common Pests, Parasites, and Diseases of Cavity-Nesting Bees

Installing bee houses and hotels are great ways to protect pollinators, and educate communities about the important role bees play in food production and ecosystem services. Unfortunately, productive bee nests are a smorgasbord of food resources for pests and parasites and the proximity of the bees to each other allows the disease to spread easily.

Nest monitoring and maintenance are essential to maintaining bee health. Fortunately, proper care can significantly reduce the incidences of pests, parasites, and diseases and allow managed bees to thrive. This post highlights common enemies of leafcutter and mason bees as well as methods for their control. It is not a comprehensive approach to maintaining bee health and bee raisers should learn proper cleaning, harvesting, and storing techniques of mason and leafcutter bees to ensure they are not contributing to parasite and disease build-up. 

Pests and Predators  - Nest Nuisance

Ants & Earwigs 

Ants and earwigs are an occasional nuisance to mason and leafcutter bees. Damage is usually restricted to newly laid, exposed bee eggs. Earwigs, more common in the eastern US, will also scavenge on pollen provisions and leafy nest materials. Once nests are sealed with mud or leaves, the damage is usually minimal.

What You Can Do: Since these pests usually gain access to nests by crawling up shelter legs, coat the base of the shelter with a sticky product like Tanglefoot or a layer of Vaseline spread along the back of the surface where the bee house is mounted. For a non-toxic solution that does not kill the ants, try AntCant, which creates a slippery barrier that ants can't walk across. Make sure to remove any vegetation that is touching the shelter - it acts as a bridge for these insects.


Image by Melani Marfeld from Pixabay


Birds & Rodents 

Many species of birds will prey upon adult bees and woodpeckers and rodents can cause significant damage to both bee larvae and nesting materials. Once these predators recognize the bee nest as a food source, they are relentless.

What You Can Do: There is little you can do to protect your adult bees from birds once they leave the nest to mate and forage. It's a natural predator-prey relationship that has been around since the existence of animals. However, there are a few things you can do to protect the bees, larvae, and nesting materials inside your bee house.

Create a bubble on the front of your bee house with wire mesh to protect the shelter from intrusion. The size of the wire openings and the distance between the wire guard and the nesting materials are important to get right! Ideally, the openings should be about 3/4" and the bubble should be no more than 3 to 4 inches from the nesting materials. Openings smaller the 3/4" can damage bee wings. If the bubble is larger than 3 to 4 inches it can deter bees from nesting. If it is shorter, birds with longer beaks and squirrel arms will be able to reach the nesting materials.

Pro Tip: Bees can have trouble navigating in and out of wire mesh, so only use this method when absolutely necessary, not as a preventative measure. 

Product Recommendation: Crown Bees sells a protective bird guard that fits securely on our cedar houses.

Bird Guard for Crown Bees' Houses

Hornets & Paper Wasps 

Hornets and paper wasps are social insects, like honey bees. Adults feed on nectar and prey upon small insects to feed their offspring. They are often not a problem for mason and leafcutter bees unless they build their nests near or inside your bee house.

What You Can Do: Pesticides and chemicals will kill or deter your nesting bees and should be avoided. Wear protective clothing and spray the social wasp nest with high-pressure water from a hose. Remove the nesting holes from the bee house or protect them from the spray with a piece of cardboard. The best time to spray the social nest is in the late evening. Once removed, add twigs, rocks, or wadded paper to the empty space in the bee house to deter future social wasps. Another option is to let the nest run its course for the season because social wasps build a new nest every year.

Pro Tip: You can also deter social wasps from building nests by installing a fake wasp nest. Inflate a paper bag, cinch the end closed, and hang the paper bag under the eave of your house. Social wasps are territorial and do not want to build nests near each other.


Wasp Nest-Image by Eveline de Bruin from Pixabay

Image by Eveline de Bruin from Pixabay


Cleptoparasites - Parasitism by Theft

A Cleptoparasite is an animal that steals food or prey from another animal. For bees, most cleptoparasites lay their eggs inside bee nests and the larval parasites eat the pollen loaves intended for the immature bees. Some cleptoparasites will even eat the developing bees in the process. The following are cleptoparasites commonly associated with mason and leafcutter bees.

Pollen Mites

Characteristics: The 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, more common in humid environments.


Pollen Mites


Hairy-fingered mites, more commonly called pollen mites, are cleptoparasites of mason bees. As their name suggests, pollen mites eat pollen causing the immature bees to starve, and in some cases will even feed on the bee larva! Pollen mites are not able to break through the mud walls created by mason bees, but when new bees emerge and crawl through the nesting cavity, the mites from infected cells attach themselves to the newly emerged bees.

While these mites do not directly attack adult bees, they are transported to new cells during the nesting process, ultimately repeating the cycle.

What You Can Do:

1) Do not reuse nesting tubes because the pollen mites can easily spread. Reusable wood trays should be cleaned each season.

2) When you harvest mason bee cocoons in the fall, keep an eye out for signs of pollen mites. Harvesting mason bee cocoons is the easiest and best way to reduce pollen mite infections.

3) Hairy-fingered mites, along with the fungal disease chalkbrood (discussed later), are two reasons why drilled blocks of wood should not be used as nesting materials. 


Houdini Fly (Non-native, Introduced)

Characteristics: Large, red eyes (Nicknamed "Devil Fly"; About the size of a fruit fly; dull brown in color with horizontal stripes on abdomen; when in rest its wings cross over each other; slow, hovering flight style.


Adult Houdini Fly_Photo by Collin Purrington
Image by Colin Purrington


The Houdini fly, named after its remarkable method of escaping from bee nests, parasitizes mason bees. Female Houdini flies lay their eggs in nest cells before they are sealed. The fly larvae quickly hatch and consume the pollen loaf before the mason bee larvae, which causes them to starve. Once development is complete, the adult Houdini fly inflates its head to break through the mason bee nest mud walls to escape.

What You Can Do (WSDA Pest Alert):

1) Only use nesting materials that allow you to open, inspect, and harvest cocoons.

2) Harvest mason bee cocoons - Open nesting materials in the fall and kill Houdini fly maggots. Maggots look like sticky white clusters inside the brood cell, often surrounded by curly orange/brown frass (poop).


Houdini Fly Larvae & Frass


3) Control adult mason bee emergence - If you cannot open nesting materials, place your nesting materials in a fine mesh bag and close tightly. As the bees emerge, release the mason bees daily and kill any adult Houdini flies.

4) Instantly kill any adult Houdini flies you see hovering around your nesting materials.

5) Before purchasing mason bees, ask the provider how they harvested and whether they inspected the cocoon for Houdini flies. YES, Crown Bees does inspect ALL cocoons!


Sapygid Wasps

Characteristics: A number of different sapygid species are associated with solitary bees, so the appearance will vary, but adults are typically black and yellow and measure up to about 1/2 inch long (12.7 millimeters).


Sapygid Wasp


Sapygid wasps are cleptoparasites of both mason and leafcutter bees. The female oviposits her eggs into the nests of solitary bees and the developing wasp larvae consume both the developing bee and the pollen loaf. After consuming the pollen loaf, these wasps spin cocoons and overwinter undetected as adults alongside neighboring bees. Watch for Sapygid wasps in the summer! Sapygid wasps lay their eggs in the nests of bees while the female bee is away foraging.

What You Can Do: Watch for Sapygid wasps in the spring and summer! Sapygid wasps lay their eggs in the nests of bees while the female bee is away foraging. If you find this parasitic wasp hovering near your bee house, spray it with a fine mist of water to stun and then kill it.


Cuckoo Bees 

Characteristics: In most cases, cuckoo bees closely resemble their host species, typically only lacking the pollen-collecting scopal hairs on the underside of the abdomen.


Blue Orchard Mason Bee and Cuckoo Bee


Female cuckoo bees enter the bee nest while the rightful owner is out foraging and lays a single egg in the uncapped cell. Once the cuckoo egg hatches, the parasite larva kills the host and consumes its pollen loaf. Cuckoo bees lay only one generation per year and the development timeline closely matches that of the host bee.

What You Can Do: Since these bees tend to resemble their host bees, it can be tough to determine who is the pollinator and who is the imposter. However, there are a couple of distinctions to look out for:

Cuckoo bees of the blue orchard mason bee have a slightly smaller body, a lack of white facial hairs, and the absence of a pollen-collecting scopa. Cuckoo bee cocoons can be identified when compared to blue orchard cocoons by their prominent nipple on one end, and the presence of long curly frass (poop). Discard any cuckoo bee cocoons you find during harvesting.

Cuckoo bees of the alfalfa leafcutter bee differ by their longer, more pointed abdomen with short distinct spines at the tip. Cuckoo bees also tend to emerge before alfalfa leafcutter bees. Allow your bees to emerge in a transparent bag and check the early emergers to see if they are true leafcutters or cuckoo bees.


Leafcutter Bee and Cuckoo Bee


Blister Beetles

Beetles represent the most diverse insect order on Earth. The vast majority of them are beneficial, preying on crop pests and recycling nutrients. Only a few of them present challenges to cavity-nesting bees.

Characteristics: A few species of blister beetles are common cleptoparasites of cavity-nesting bees. Commonly called red and brown blister beetles after their coloration, they measure up to about 1⁄2 inch (12.7 millimeters) long as an adult.


Red Blister Beetle


Female blister beetles lay their eggs on the buds and flowers of many common weedy plants. Upon hatching, the tiny larva crawls to the top of the flower and waits for visiting bees. Using claw-studded legs, the larva clings to the scopa (hairs) of visiting bees and are transported back to the nest where they detach themselves. Inside the nest, the beetle larva consumes the pollen loaf and the bee egg. Within mason bee nests, beetle larval movement is normally restricted by the mud walls. However, within leafcutter bee nests, beetle larvae may move between cells, destroying several eggs in the process.

What You Can Do: Overwintering beetles encase themselves within a semi-translucent brown cocoon-like skin. The cocoons will look very different from both leafcutter and mason bee cocoons. When you harvest cocoons, separate and discard blister beetle cocoons.


Blister Beetle Cocoon


Checkered Flower Beetles

Characteristics: Adult beetles measure just over 1⁄2 inch in length (12.7 millimeters) and are dark blue with yellow, orange, or red-spotted patterns on their backs. The larvae have a somewhat worm-like appearance, are reddish in color, and have two spines at the tip of the abdomen.


Checkered Flower Beetle


Females lay their eggs near the entrances of bee nests. Upon hatching, the larva move between nest cavities consuming pollen loaves and bee eggs.

What You Can Do: As with many parasites, bee raisers can avoid most checkered flower beetle damage by promptly removing nests from the field at the end of the nesting season.


Parasitic  - Body Snatchers

Chalcid Wasps - Monodontomerus & Pteromalus venustus

Characteristics: Mono species are widespread throughout North America, and all are metallic green, blue, or black, with red eyes, and are 5/64 to 5/32 inch in length (~2 to 4 millimeters). Males are slightly smaller than females.

P. venustus females average up to 7/64 inch (~2.5 millimeters) long. Females are black with dark brown legs. Males are similar in size but have metallic green heads.


Chalcid Wasps


Chalcid wasps are some of the most destructive parasites of mason and leafcutter bees. This group includes both native (Monodontomerus) and nonnative (Pteromalus venustus).

The sneaky female wasp invades nests through small openings or weak spots in the nesting materials, or through incomplete or uncapped nesting holes. They use their needle-like ovipositor to paralyze the bee larvae by inserting it through the cocoon wall. Females can attack multiple cocoons, so loose cocoons are particularly susceptible and must be protected!

After paralyzing the bee larva, the female lays 10 - 50 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. These wasps develop very fast, so multiple generations can develop each season.



Images by Province of Manitoba, Canada


What You Can Do:

1) The most important control method for Chalcid wasps is the use of solid nesting materials free of entry points, especially at the back of the nest.

2) Mason bee cocoons that did not emerge within a few weeks of release may be full of parasitic wasps and should be destroyed.

3) Completed nests should also be removed from the field and stored in a breathable, mesh bag at the end of the nesting season. Occasionally check the bag and kill any adult wasps that may have emerged.

4) Small batches of mason bee cocoons can be hand sorted to remove chalcid wasps. Parasitized cocoons will feel softer than healthy cocoons and appear almost empty when squeezed lightly. Questionable cocoons can be stored separately in a breathable, transparent container. Check the container often and destroy any parasitic wasps that emerge.


Chrysidid Wasps

Characteristics: About 1⁄2 inch in length (12.7 millimeters), and are typically metallic green in color.


Chrysidid Wasp

Image by USGS Bee Inventory, Flickr


Multiple species of Chrysidid wasps also prey upon mason and leafcutter bees. These native wasps are not as destructive as Chalid wasps. Female wasps lay their eggs in nesting cavities while female bees are out foraging. Once the eggs hatch, the wasp larva attaches itself to the bee larva which it begins to consume. After feeding, the wasp larva then spins a thin, semi-transparent cocoon and overwinters as adults. Unlike Chalid wasps, Chrysidid wasps only produce one generation per year.

What You Can Do: These are minor native predators and often don't require control. You can watch for Chrysidid wasps in the spring and summer to determine if you have a problem. Their metallic color makes them easy to spot. If you spot a lot of these wasps hovering near your bee house, you can spray the wasp with a fine mist of water to stun and then kill it.


Chalkbood Fugus - Gut Killer

Last but not least, the single most destructive disease of cavity-nesting bees is the fungal pathogen called chalkbrood. Chalkbrood also affects honey bees, but it is a different species of chalkbrood.




Adult bees are not affected by the disease, but they do help spread it! Chalkbrood is picked up by adult bees from flowers and the spores of the fungus are transferred to the bee larvae through nest building. As the eggs hatch, the bee larvae consume the infested pollen, and the spores germinate in the gut of the larva. It's here that chalkbrood competes with the larva for food, resulting in starvation. Once the larva dies, the fungus continues to grow on the bee cadaver. The next season, emerging adults will pick up spores released from these dead larvae as they crawl towards the cavity exit and contaminate the flowers they visit.

What You Can Do: The most effective control method in the US is nest cleaning and maintenance.

1) Loose-cell management can be an effective method of controlling chalkbrood since the adult bees do not have to crawl through potentially infected cells to exit the nesting cavity. Harvest your mason and leafcutter bee cocoons by opening nesting holes and throw away all chalkbrood cadavers. BE CAREFUL not to let other cocoons touch the chalkbrood.

2) We recommend that you wash mason bee cocoons with cool water and a mild bleach solution (1:3 by volume), rinse with water, and pat them dry.

3) Nesting materials should either cleaned or replaced before the next season. For this reason, we don't recommend drilled blocks of wood or other materials that cannot be opened and properly cleaned. 

Product Recommendation: Spray Clean Bee (a safe bleach alternative) on harvested mason bee cocoons that have been washed and dried. You may also apply Clean Bee to both sides of reusable wood trays after washing.


We hope this post is helpful in identifying common pests, parasites, and diseases of mason and leafcutter bees! Insects are notoriously difficult to identify, so if you see something flying around your bee house and you’re not sure if it’s beneficial or harmful, don’t hesitate to ask an expert! Entomology Today published a great list of resources to help people identify the insects in their yards!


Good Luck and Happy Pollinating! 


Resource Highlight: Available to download as a PDF from SARE

Managing Alternative Pollinators: A Handbook for Beekeepers, Growers, and Conservationists is a first-of-its-kind, step-by-step, full-color guide for rearing and managing bumble bees, mason bees, leafcutter bees, and other bee species that provide pollination alternatives to the rapidly declining honey bee. Written by Eric Mader of the Xerces Society Pollinator Conservation Program; Professor of Entomology Marla Spivak; and Elaine Evans, author of “Befriending Bumble Bees,” the book includes expert information on the business and biology of pollination and how-to guidance on raising the alternative bee species.

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