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Pests, Chemicals & Drilled Wood

Protect Your Bees

All creatures have their own pests, predators, and diseases to deal with and solitary hole-nesting bees are no different. Dealing with the pests and diseases of hole-nesting bees is usually a very simple process, just harvest cocoons and remove infestations. Familiarize yourself with the top three pests: parasitic wasps (mono & pteromalus), chalkbrood (a fungal infection), and pollen mites. Read on to find out what they look like, what they do, and how to deal with them.

Pests, Disease, and Small Predators

Parasitic wasp: Monodontomerus

(more likely to attack mason bees)

  • What it is: Also known as "mono", monodontomerus is a small black parasitic wasp. It looks about the size of a gnat (3/16" or 4mm) and hovers at nesting holes. It is attracted to the smell of mason bee cocoons.
  • What it does: Mono lays its eggs within developing bee larva. Mono females typically deposit their eggs through the side of a thin-walled nesting material, but has been known to get through back ends of wood trays as well. They are able to insert their ovipositor through mud or other thinner material. The bee larva spins its cocoon, and then is consumed from the inside out. Unfortunately, the mono larva are now nicely encased in a mason bee cocoon, which is tough to discern apart from normal cocoons. The wasp’s development time is very short and they chew out through the muddy mason bee chamber and attack other uncontaminated larva elsewhere in your nesting material.
  • What to do: Check your BeeGuardian bag periodically. If you find that mono emerged, you can easily kill them through the bag to prevent further damage from this pest. An alternative is to "candle" your cocoons one by one.

Parasitic wasp: Pteromalus

(More likely to attack leafcutter bees)

  • What it is: Also known as the Canadian chalcid, the pteromalus is one of the most common pests you'll see with developing leafcutter bees. Pteromalus are small, black, parasitic wasps that are the size of small gnats (3/16" or 4mm).
  • What it does: This pest waits until the nesting female is finished laying an egg, then sneaks into the hole and lays her eggs within the freshly laid leafcutter egg. These pteromalus larvae will overwinter with the leafcutter and emerge before the leafcutter bees do. This is the dangerous part. While the leafcutters are mid-development, the pteromalus can deposit her eggs through the sidewall of the leafy cocoons and produce another batch of pteromalus.
  • What to do: Protect your leafcutter bees in a fine mesh bag similar to the BeeGuardian. (Our leafcutter cocoons ship in the smaller LeafGuardian bag.) The pteromalus comes out about 9-12 days after incubation starts. When you see these tiny black chalcids in the bag, squish them. If found outside flying around your nesting bees, spray a fine mist of water to knock them out of the air and then squish them.
Various colors of chalkbrood infected mason bee larvae

Chalkbrood

  • What it is: A fungal spore that can be devastating to your entire colony. You can’t really see the fungal spore but what you see instead is an infected bee larva that has died from the infection. Chalkbrood cadavers are chalky and the skin breaks apart easily. The surface feels like the wing of a butterfly. A healthy cocoon somewhat resembles a fuzzy, plump raisin. Chalkbrood, resembles the letter "C" and can range in color from salmon/brown to gray/charcoal in color.
  • What it does: When ingested by a bee larva, the chalkbrood spore kills the larva and alters it into a mass of spores. Healthy mason bees emerging brush past this chalky larva-shaped spore and get the spores on themselves. They then spread the spore to your mason bee house, flowers, and other nesting holes. Other mason bees will bring the chalkbrood into new holes, allowing the rearing larvae to consume the spore and perpetuate the cycle. Two or three years of this cycle can devastate your entire mason bee colony.
  • What to do: If you find one situation of chalkbrood when harvesting your mason bees in the fall, open all nesting material. Dispose of all chalkbrood cadavers, being careful that other cocoons don't touch the chalkbrood. After you've harvested all cocoons, we recommend that you wash your cocoons with bleach/water mixture of 1 Tbl bleach to 1 cup water to kill the chalkbrood spores. Spot clean your reusable nesting tray with these recommendations.
pollen mites in a nesting tubepollen mites on a mason bee

Pollen Mites

  • What it is: Pollen mites, more specifically Krombein's hairy-footed mites, are clear and you need a magnifier to see them. What you can see easily is an orange mass that is mite feces. Pollen mites are found throughout North America; more in moist environments than arid. If you don't harvest your cocoons, the pollen mites are reintroduced into your yard which accelerates the decline of your nesting mason bees. The mites also stay within an opened hole waiting for more pollen delivered by an unsuspecting mason bee.
  • What it does: A pollen mite's role in life is to eat pollen. They hitchhike on the backs of insects from flower to flower to find more pollen. Unfortunately, they also hitchhike into nesting holes and eat the mason bee's pollen loaf. The pollen mite either eats the egg and then the pollen loaf, or just the pollen loaf and the larva then starves.
  • What to do: As you harvest your mason bee cocoons in the fall keep an eye out for signs of pollen mites. We recommend dry brushing reusable wooden trays with a stiff brush to remove debris and pollen mites. You might also bake your wood trays at 250 degrees for 20 minutes. Harvesting mason bee cocoons is the easiest and best way to reduce pollen mite infections.
bee house in a moat to keep ants away.

  Photo by Julianne Noonan

Ants

  • What it is: An ant colony may be attracted to your bee house because of the sweet smell of pollen and nectar.
  • What it does: Ants can get into your nesting holes through any contact with a surface such as a fence post or wall.
  • What to do: You could try double sided tape, but we've heard that ants will crawl across the tape. A sticky product like Tanglefoot or a layer of Vaseline can also work. Creating a moat of sorts similar to this picture might be the best method. Ensure that there are rocks or a stick or so in front of the house so that a bee that falls off doesn't drown.
adult earwig

Earwigs

  • What it is: Earwigs are opportunists and are more scavenger than predator.
  • What it does: Leftover pollen is a wonderful earwig treat. They may destroy a freshly laid egg if the nesting female has just laid it and has left to gather mud. However, this seems remote.
  • What to do: Loosely roll up a section of newspaper, bind it with a rubber band or string, and dampen it. Place this moisture "trap" inside the house, or physically attach it to the nesting house where you suspect the earwigs are gathering. Earwigs will be attracted to the moisture and crawl inside the coil. Dispose of the newspaper with the earwigs in your compost pile. Another solution is to build a moat by placing your mason bee house on bricks and set it in a tub of water (look at the ant section above for more details).
Adult stelis cuckoo beeStelis cuckoo bee cocoon

   Photo by Steve Peterson

Cuckoo Bees

  • What it is: You’ve heard of cuckoo birds and the bee world also has cuckoos that mimic their host bees and take advantage of their hard work. The best means of determining this pest's cocoon is by their spaghetti-shaped feces rather than mason bees' brown straight feces. The cocoon is also very firm in comparison to the mason bee.
  • What it does: One example of a cuckoo bee is the stelis bee. They lack the ability to transport pollen, and so they lay their eggs in the pollen loaf built by the mason bee. When the stelis egg hatches, it kills the mason bee egg or young larva and then consumes the remaining pollen loaf.
  • What to do: During cocoon harvest, if you find cocoons within your mason bee cocoons that are different in texture, color, and has different feces, remove it.

Sapygid Wasp

(Attacks mason bees)

  • What it is: A parasitic wasp, the sapygid female is about 1/3” long and black with yellow/white spots or a maroon band around the middle of her abdomen. The body is smooth and shiny with few hairs.
  • What it does: The female sapygid finds an unplugged leafcutter bee nest, she searches for the cell cap and inserts her ovipositor through the thin wall and lays an egg or two near the leafcutter bee egg. Sapygid eggs hatch within 1-2 days and consume any other sapygid larva and the leafcutter egg as well. It then consumes the pollen loaf, spins a cocoon, and overwinters along with the neighboring leafcutter bees. It hatches out about the same time as the leafcutter bees do.
  • What to do: If you find this parasitic wasp hovering near your leafcutter holes, spray it with a fine mist of water to stun it and then kill it.
Adult Ptinus beetlePtinus larva on a mason bee cocoon

  Photo by Steve Peterson

Ptinus Beetle

(Attacks mason bees)

  • What it is: A small beetle known around the world to attack food and even museum items.
  • What it does: The ptinus beetle eats the larvae and cocoons. If left unchecked, this scavenger has the potential to destroy many of your mason bees.
  • What to do: During cocoon harvest, remove all eggs of this pest.

Carpet Beetle Larva

(Attacks mason bees)

  • What it is: Carpet beetle larvae are hairy and look like caterpillars.
  • What it does: The carpet beetle larva eat mason bee cocoons. Mud plugs or divisions don't concern them. They are known to be scavengers and may just follow after another insect has ravaged your tube or reed.
  • What to do: Take the nesting material down after the mason bees are finished for the season. Store the holes upright in a BeeGuardian bag in a shed or garage. During harvest, as you sort through the nesting material and find this pest, eliminate them.

Large Predators

Hungry Birds

Give your bees some safe flying space by enveloping the bee house with wire mesh that has hole openings of at least ½-¾” and try to give the bees a space of 3” from their nesting holes to the mesh to approach and land. Fill the empty space above the nesting holes or in the attic of your bee house with twigs or crumbled paper to deter birds from nesting. Once holes are filled, remove temptation by placing the nesting material in a protected area.

Bears

Bears are attracted to the sweet smell of pollen and nectar and to the meal of bee larvae. Try to move nesting houses out of bear’s reach.

Rats & mice

After nesting activity is complete, remove nesting tubes from house and place capped ends facing up in a chew-proof container.

Racoons

Move the nesting house to 6’ up off the ground and make sure it’s hard for a racoon to climb and reach the nesting house.

Chemicals

We do not recommend using chemicals in a yard because they can place our local ecology out of balance. We believe when both prey and predator are in balance, our ecosystem is healthy. When you remove the pest, predators have no food source, and thus either starve or leave. When the pest shows up again, there seemingly is no recourse other than to reach for the chemical shelf for a solution. This creates a costly chemical cycle in your yard and garden. Avoiding chemicals may appear to tip the balance to the side of too many pests, but predators will find the pests and your yard will return to balance within a few years at most.

We have seen customer's lawn treatments, that did not include pesticides, cause solitary bees to leave a yard where they were previously happy to nest. We believe that bees do not like the smell of manmade chemicals. Your neighbor's chemical treatments may also cause your bees to nest elsewhere. Consider gently explaining your desire to raise native bees and a compromise is to ask them to avoid spraying during the months that your bees are actively pollinating.

Plants produce chemicals that are natural repellents for pests and insects like mosquitos and ticks: lavender, rosemary, lemongrass, and thyme are some good examples. Here is a great guide that explains how these plants can repel unwanted insects, their beneficial uses for us, and the plants listed are also great for feeding bees.

A wonderful alternative to chemical fertilizer is to top-dress your garden and make your own compost with food and plant scraps. The microbial relationship with plant roots is an ancient symbiotic relationship between beneficial bacteria, mycorrhizal fungi, and your plant’s roots. Supporting this natural interaction is a better and longer-lasting solution than chemical fertilizer.

Drilled Blocks of Wood

Drilled blocks of wood are harmful to hole-nesting bees because they can’t be opened for cocoon harvest and they are often too shallow to support a healthy ratio of male and female bees. Unlike carpenter bees, mason and leafcutter bees are too small and unable to clean out their nesting holes before reuse.

If you must use drilled blocks of wood:

  • Inspect and replace them every year to reduce the risk of disease and pest buildup.
  • Make sure the nesting hole is at least 6”/15cm deep to ensure a healthy ratio of male and female bees.
  • You can harvest bee cocoons from drilled blocks by inserting paper or cardboard nesting tubes into the drilled holes before the female bees begin to nest.

shift bees out of drilled blocks of wood before next season

Solitary bees like to see what they're nesting in. Place your old block of wood with the holes facing up inside a cardboard box or paper sack. Put a few pencil-sized holes in the sides of the box or sack and close it up so that healthy bees must emerge through the pencil-sized holes. Place the box or sack underneath the new nesting house. Healthy viable bees will crawl up out of their hole, crawl out the sack or box and look to nest elsewhere. Males may go back in looking for females, but this trick should work well. Throw the old block away after bees have emerged.

Bamboo

Bamboo is a natural material but it is also exotic and our native bees have not evolved to nest within it (even though they will use bamboo if nothing else is available). Bamboo openings can vary greatly in size and many will be either too large or too small. Bamboo tubes are such a strong material that they can't be opened for harvesting cocoons without harming the bees. Bamboo, although cheap, is not a good medium for raising mason bees. Bamboo also doesn’t allow the pollen loaf to breathe and mold can grow too easily. Please consider replacing bamboo tubes with natural reeds or EasyTear tubes.

Shift Bees out of Bamboo before next Season

Solitary bees like to see what they're nesting in. Place your bundle of bamboo with the holes facing up inside a cardboard box or paper sack. Put a few pencil-sized holes in the sides of the box or sack and close it up so that healthy bees must emerge through the pencil-sized holes. Place the box or sack underneath the new nesting house. Healthy viable bees will crawl up out of their hole, crawl out the sack or box and look to nest elsewhere. Males may go back in looking for females, but this trick should work well. Throw the bamboo away after bees have emerged.

Plastic Straws

Plastic straws are not a natural material. Plastic straws don’t allow excess moisture to be absorbed which can cause moldy cocoons. You also can't harvest cocoons from them easily. Consider replacing plastic straws with natural lake reeds or reusable wooden nesting trays.