Note: If you have already read through Parasites and Diseases of Leafcutter Bees, you'll notice many of the same parasites and diseases also affect mason bees. However, there are few differences in this article, so make sure to give it a read as well!
To learn about common all-season pests and predators attracted to spring mason bees, summer leafcutter bees, and wild native bees, check out our All-Season Pests and Predators of Cavity-Nesting Bees.
Installing a bee house or hotel is a great way to encourage nesting, protect pollinators, and educate communities about bees' vital role in food production and ecosystem services. Unfortunately, productive bee nests are loaded with pollen, nectar, and bee larvae acting as a buffet for pests and parasites. The proximity of the bees to each other allows diseases 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 article highlights the common enemies of mason bees by season and methods for their control.
Important: Maintaining bee health takes more than just keeping an eye out for pests and parasites. Make sure you learn proper cleaning, harvesting, and storing techniques to ensure your mason bees are healthy and not contributing to disease build-up and spread.
Types of Parasites
We wanted to explain the two main groups of parasites commonly associated with mason and leafcutter bees before we get into the specific parasites you might find in and around your solitary bee house.
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 immature bees. Some cleptoparasites will even eat the developing bees in the process.
Parasitism by Body Snatching
Predatory and parasitic wasps are among the most common enemies of cavity-nesting bees, including mason bees. Sneaky females invade 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. Upon hatching, the wasp larvae kill and eat the bee.
Spring & Summer - What To Look For When Mason Bees Are Flying
Houdini Fly (Non-native, Introduced) - Cleptoparasite
Characteristics: Large, red eyes (Nicknamed "Devil Fly"; About the size of a fruit fly; dull brown with horizontal stripes on abdomen; when in rest its wings cross over each other; slow, hovering flight style.
The Houdini fly, named after its unique method of escaping from bee nests, parasitizes mason bees. 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. Once development is complete, the adult Houdini fly inflates its head to break through the mud walls to escape.
What You Can Do (Invasive 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 larvae. Larvae look like sticky white clusters inside the brood cell, often surrounded by curly orange/brown frass (poop).
3. Control adult mason bee emergence - If you cannot open nesting materials, place your nesting materials in a BeeGuard Bag or another 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. Some bee raisers have noticed that regularly using a low to medium powered vacuum around nesting materials to suck up Houdini flies has significantly reduced their numbers.
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!
Chalcid Wasps (Monodontomerus & Pteromalus venustus) - Body Snatchers
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 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.
What You Can Do:
1. The most critical control method for Chalcid wasps is the use of solid nesting materials free of entry points, especially at the back of the nest. Nesting materials without backs or that are too thin are easily penetrated by Chalcid wasps. Read our Bee-Safe Nesting Materials article for more information on nesting materials.
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. Mono wasps are often most active towards the end of the mason bee season. Therefore, completed nests should also be removed from the field and stored in a BeeGuard Bag or another 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. You can store questionable cocoons separately in a breathable, transparent container. Check the container often and destroy any parasitic wasps that emerge.
Sapygid Wasp - Cleptoparasite
Characteristics: Several different sapygid species are associated with solitary bees, so their appearance will vary, but adults are typically black and yellow and measure up to about 1/2 inch long (12.7 millimeters).
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. These wasps then 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 Bee - Cleptoparasite
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.
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 the host bee.
What You Can Do: Since these bees tend to resemble their host bees, it can be tough to determine the pollinator and 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.
- When harvesting, look for Cuckoo bee cocoons. The cocoons have a prominent nipple on one end, and you'll see long curly frass (poop). Discard any cuckoo bee cocoons you find.
Chrysidid Wasp - Body Snatcher
Characteristics: About 1⁄2 inch in length (12.7 millimeters) and are typically metallic green in color.
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.
Fall - What To Look For During Harvesting
Pollen Mites - Cleptoparasite
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 and are more common in humid environments.
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 cannot 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, the adults transport the mites to new cells and flowers during the foraging and nesting process. Without proper cleaning and harvesting, these mites can quickly spread throughout your entire bee house - ultimately decimating your population of mason bees.
What You Can Do:
1. Do not reuse nesting tubes because the pollen mites can easily spread. Clean reusable wood trays 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 next), are two reasons you shouldn't use drilled blocks of wood as nesting materials.
Chalkbrood Fungus - Gut Killer
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! Adult bees pick up Chalkbrood from flowers and transfer the spores of the fungus to the bee larvae through nest building. As the eggs hatch, the bee larvae consume the infected pollen, and the spores germinate in the larva's gut. 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 following season, emerging adults will pick up spores released from these dead larvae as they crawl towards the cavity exit. These spores then go on to contaminate the flowers they visit and the new generation of bees. Contaminated eggs and pollen result in the new generation of bees dying before reaching their adult stage.
Above: Healthy Brood. White and glossy appearance.
Above: Chalkbrood. Grey/black and chalky appearance.
What You Can Do: The most effective control method is harvesting your mason bee cocoons in the fall!
1. Harvesting simply means removing the cocoons from the nesting materials, then cleaning and storing them for winter. Harvesting is 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 bee cocoons by opening nesting holes and throw away all chalkbrood cadavers. BE CAREFUL not to let other cocoons touch the chalkbrood.
2. There are two ways to clean your cocoons and nesting materials of Chalkbrood. We recommend washing mason bee cocoons with cool water, pat dry, and spritz with our CleanBee solution (a safe bleach alternative). Alternatively, you could wash mason bee cocoons with cool water and a mild bleach solution (Ratio: 1 tsp. Bleach to 1 Cup Water), rinse with water, and pat them dry - the more chemical-based approach.
3. Nesting materials must either be cleaned or replaced before the following season. For this reason, we don't recommend drilled blocks of wood or other materials that cannot be opened and properly cleaned.
Product Recommendation: You can spray CleanBee on harvested mason bee cocoons that have been washed and dried to prevent Chalkbrood. You can also apply CleanBee to the reusable wood trays after washing. To conserve CleanBee, you only need to spray the chambers of the wood trays that had Chalkbrood present.
Houdini Fly (Non-native, Introduced) - Cleptoparasite
See Houdini fly information in the spring section above.
Remember: During harvesting, look for a mass of about 10 -20 while or pale yellow maggots with sticky curly frass (poop). Remove and kill all Houdini fly maggots you find during harvesting.
If you have bamboo tubes or drilled blocks of wood that you are unable to open, place the unopened nesting materials in a BeeGuard Bag. As the bees emerge in the spring, release the mason bees daily and kill any adult Houdini flies.
Dried Fruit Moth & Indian Meal Moth - Predators
Characteristics: Dried fruit moths have a wingspan of about 3/4 inch (19mm) and are light grey with darker grey patterns on their wings. Indian Meal Moths have a wingspan of about 5/8 inch (16mm) and are reddish-brown with a metallic sheen on their wings. Larvae of the moths are similar in appearance - thin, under an inch long, and look like pink or white grubs.
Left: Adult Indian Meal Moth, Center: Meal Moth Larvae, Top Right: Meal Moth Pupae
Female moths will often lay their eggs in small crevices near bee nests, and once they hatch, they wriggle their way to the nesting cavities. The moth larvae burrow through mud-capped ends and nesting walls and feed on pollen loaves and bee larvae.
What You Can Do: After female mason bees are finished nesting, remove nesting materials and store them upright in a BeeGuard Bag (or other breathable, transparent container) in a shed or garage that mimics outdoor temperatures. Remove and discard any moth pupae during harvesting.
Carpet Beetles - Predator
Characteristics: Carpet beetle larvae are small, hairy, and look like short fat caterpillars.
Like the Dried fruit moth and Indian meal moth, the larvae can burrow through the tough mud-capped ends and nesting walls and devour pollen loaves and bee larvae.
What You Can Do: The best control method is removing and storing nesting materials after female mason bees have completed their nesting. Store upright in a BeeGuard Bag (or other breathable, transparent container) in a shed or garage that mimics outdoor temperatures.
Check your nesting holes in early summer. If you find mud-capped ends with holes in them, remove and store those nesting materials in a separate BeeGuard Bag, as it could be a sign of a predator moving through your nesting materials.
Blister Beetles - Cleptoparasite
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.
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 visiting bees' scopa (hairs) and is 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, the mud walls restrict the larval beetle movement. However, within leafcutter bee nests, larval beetles may move between cells, destroying several eggs in the process.
What You Can Do: When you harvest cocoons, separate and discard blister beetle cocoons. 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.
Checkered Flower Beetles - Cleptoparasite
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, and have two spines at the tip of the abdomen.
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: Like other parasites, bee raisers can avoid most checkered flower beetle damage by promptly removing nests from the field at the end of the nesting season.
Cuckoo Bee - Cleptoparasite
See Cuckoo Bee information in the spring section above.
Remember: During harvesting, look for Cuckoo bee cocoons. They differ from mason bee 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.