Parasites and Diseases of Leafcutter Bees
Note: If you have already read through Parasites and Diseases of Mason Bees [link], you'll notice many of the same parasites and diseases also affect leafcutter 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 leafcutter 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 leafcutter 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.
Summer - What To Look For When Leafcutter Bees Are Flying
Chalcid Wasps (Pteromalus venustus) - Body Snatchers
Characteristics: 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.
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 Pteromalus 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 [link] article for more information on nesting materials.
2. Pteromalus emerge 1-2 weeks before leafcutter bees and parasitize developing larva. Therefore, completed nests should 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. Checking for adult Chalcid wasps is especially important during the incubation period for leafcutter bees. Several generations of wasps can occur during the incubation period, and unless you remove adult wasps, high losses of bees will occur.
3. Nest materials and loose cocoons can also be protected by surrounding stored bees with yellow sticky cards, similar to those we put in house plants to trap gnats and other small insects. Only use sticky cards during leafcutter storage and incubation; once the adult bees emerge, they are no longer necessary.
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.
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.
Spring - What To Look For During Harvesting
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 and contaminate the flowers they visit.
What You Can Do: The most effective control method is harvesting your leafcutter bee cocoons in the spring!
1. Harvesting means removing the cocoons from the nesting materials, [link to harvesting leafcutter bees article]. Unlike mason bees where you can visually see infected larvae, you won't see Chalkbrood with leafcutter bees; it's hidden inside the leafy cocoons. When you harvest and remove the cocoons from their nesting cavities the adult bees do not have to crawl through infected cells as they emerge - reducing the spread.
2. Nesting materials should 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. Previously used nest materials, like reusable wood trays, should be thoroughly disinfected by submerging in a bleach-water solution (1:3 by volume) for five minutes. Clean wood trays several weeks before leafcutter start flying to give them ample time to dry and air out!
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 alfalfa leafcutter bee differ by their longer, more pointed abdomen with short distinct spines at the tip.
- Cuckoo bees tend to emerge before alfalfa leafcutter bees. Allow your bees to emerge in a BeeGuard Bag or another transparent, breathable bag, and check the early emergers to see if they are true leafcutters or cuckoo bees.
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.
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 leaf-capped ends and nesting walls and feed on pollen loaves and bee larvae.
What You Can Do: After female leafcutter 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.
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 leaf-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 leafcutter 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.
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.