More Drama at the Bee Block

I was going to write at some point about how one of the other benefits of putting up a bee block ("bee condo") for solitary bees and wasps is that their parasites have a harder time finding them there than in a more natural situation. Well, last week I was proven completely wrong about that. On the positive side, I identified two new tenants in the block in our backyard.

Female mason bee, Heriades sp., on milkweed, before I had identified it

Early in the week I happened to see an insect fly from one of our blooming milkweed plants and literally make a bee line into one of the smaller holes in our bee block. I was not able to make an identification, so swiftly did it disappear.

Stakeout: female Sapygid wasp patiently eyeing potential hosts

Friday, June 24, I happened to glance at the bee block and noticed a very slender insect in one of the corners. I broke out the camera, took a couple pictures, and identified it as a sapygid wasp, family Sapygidae. Sapygids are parasitic on bees in the families Megachilidae and Apidae, and mason wasps in the family Vespidae, subfamily Eumeninae. The larvae of the sapygids either feed on the food stored for the host's offspring, or live as ectoparasites on the host larvae. Great.

Territorial dispute? Another sapygid joins the first one

Oh, here comes *another* sapygid. There were two on the nest block for only a brief time, and though it is not easy to separate the genders, I had no way of telling if it was a pair, or I believe they were two competing females. Males have even longer antennae and a blunter tip to the abdomen.

While photographing one of the sapygids, I got lucky. One of my pictures captured the mystery lodger in the bee block from earlier in the week.

Target detected: Sapygid wasp spies Heriades mason bee giving away its nest location

The insect coming to one of the smallest diameter holes is a type of mason bee in the genus Heriades. There are only eleven species of Heriades in North America north of Mexico, and five species in Colorado. Only Heriades cressoni has been officially recorded in El Paso County, but our bee block resident could still be one of the other species.

The female bee uses a pre-existing tunnel, such as an old beetle boring in a dead tree or log, or a hollow twig, in which to make her nest. She creates a series of cells with a partition of plant resin dividing one from another. She usually leaves one empty cell at the entrance before sealing the completed nest with a resin plug. Should a parasite break in, it might be duped into thinking the nest was empty; or at least dissuaded from having to break through a second "door."

Female Heriades sp. mason bee collecting pollen beneath her abdomen

The mother bee gathers pollen with a brush of hairs on the underside of her abdomen. Back at the nest she mixes the pollen with nectar she regurgitates from her crop (an internal food storage organ). This is "bee bread" that she fashions into a ball or "loaf" for each cell. She then lays an egg in the cell, and finally seals it with the resin partition. She repeats this scenario until the nest cavity is filled.

The sapygid wasp that had been waiting patiently for just the right moment to lay her own eggs in the bee nest finally made her move. She quickly backed halfway into the hole on the bee block and presumably deposited one or more eggs before fleeing the scene. This is the strategy that many, if not most, bee parasites use. They stake out a nest in the process of being provisioned, and enter when the host bee leaves to gather more nectar and pollen, or material used in making a partition between cells.

Game on! Female sapygid wasp laying eggs in Heriades mason bee nest

Another surprise happened when I saw a mason wasp approach and then enter another hole on the block. This one is different from the one I wrote about a couple weeks ago. It is smaller and more ornately patterned. It is either Parancistrocerus sp. or Stenodynerus sp. One cannot tell the two apart from images of live specimens in most cases. Even with actual specimens under a microscope it is difficult to make an identification. Like most cavity-nesting mason wasps, paralyzed caterpillars will serve as food for the larval offspring.

Female mason wasp, Parancistrocerus sp. or Stenodynerus sp.

I can hardly wait to see what comes next to our bee apartment building. Please comment if you yourself have a bee block (or more than one), and share what is coming and going from it, what parasites or predators you have seen lurking, and any other pertinent observations.

Sources: Krombein, Karl V. 1967. Trap-nesting Wasps and Bees: Life Histories, Nests, and Associates. Washington, DC: Smithsonian Press. 570 pp.
Scott, Virginia L., John S. Ascher, Terry Griswold, and César R. Nufio. 2011. "The Bees of Colorado (Hymenoptera: Apoidea: Anthophila)," Natural History Inventory of Colorado No. 23, University of Colorado Museum of Natural History, Boulder, Colorado. 112 pp.
Scott, Virginia. 2015. "Resin Plugs (part 1) and Their Bugs," The Bees' Needs Blog.

A "Boring" Place to Nest

Last week I talked about the benefits of putting out bee blocks ("bee condos") for solitary bees and wasps to nest in. I thought it might help to understand why this is important in light of where these insects normally nest in nature.

Passaloecus aphid wasp near her nest hole

Dead, dying, and weakened trees are exploited by a number of different wood-boring beetles. The adult female beetles lay their eggs in bark crevices and the larvae that hatch then bore into the wood, feeding as they go. Eventually they enter the pupa stage near the surface of the wood. The adult beetle that emerges from the pupa then chews its way out. The result is a round or oval exit hole.

Solitary bees and wasps make use of these abandoned beetle borings. In fact, competition for the right size holes is keen. A wasp can modify the entrance to some degree by chewing-off wood fibers to expand the size, or plastering mud or resin to shrink the opening. That is a lot of effort, though, so wasps will look high and low for a hole that fits them perfectly.

A typical deathwatch beetle, family Ptinidae, that makes the holes that many wasps nest in

Among the most coveted of holes are those made by emerging deathwatch beetles in the family Ptinidae (formerly Anobiidae). These are tiny insects, and you might be hard-pressed to fit an entire pin or tack (head and all) into one of their perfectly round exit holes. Deathwatch beetles are also prolific, and so there will be many holes in a given dead tree. It may look like someone fired a load of buckshot into the bole.

A super-slender keyhole wasp, Trypoxylon sp., about to enter her hole

Last week I came across just such a tree, a cottonwood, that was teeming with wasps coming and going from various holes. I recognized at least three genera: Symmorphus mason wasps, Trypoxylon keyhole wasps, and Passaloecus aphid wasps. None measured more than about six millimeters. Mason wasps harvest caterpillars as food for their offspring. Keyhole wasps hunt spiders, and aphid wasps go after aphids. Some yellow-faced bees, also known as masked bees, genus Hylaeus, are also known to use such holes.

A cuckoo wasp, family Chrysididae, bides her time

Meanwhile, metallic green cuckoo wasps, family Chrysididae, and wild carrot wasps, family Gasteruptiidae, were looking for opportunities to lay their own eggs in the nests of the other wasps, at the expense of the host's offspring. Mostly, though, there was competition for nest holes, and male wasps harassing the females for mating opportunities. A male or two might jump on a female, and they would tumble off, leaving the door open for another female wasp to claim the nest hole.

Want to witness this action yourself? Look for dead, standing or half-fallen trees with bark stripped off, and usually in a shaded location or at least on the shady side of the tree itself. You might be surprised to see a wasp enter a hole head first, then exit the hole the same way. Somewhere inside there is enough room for the wasp to turn around.

A mason wasp, Symmorphus sp., checks out her potential new digs

You may have trouble finding dead trees, and this is why solitary wasps and bees need our help. Property owners, and even managers of natural parks, will cut down trees deemed a potential hazard, such that they can avoid potential liability issues including litigation should a tree fall on a person, vehicle, home, or other valuable object. The consequences of this include a housing shortage for everything from flying squirrels, woodpeckers, swallows, wrens, owls, and other birds, to solitary bees and wasps.

Please consider building or buying a simple, but quality, bee block and installing it somewhere on your property. You can find resources to do that in last week's blog post. Let me know what tenants you get as a result. We know precious little about most insects that are not considered economically important in a good or bad way, so all observations are important. Thank you.

The Mason Wasp Euodynerus foraminatus Endorses Bee Blocks

Those "bee blocks," chunks of wood with holes of varying diameters drilled into them, are all the rage now for attracting solitary native bees like leafcutter bees and mason bees. They do work, no matter where you live, and I have proof. Ok, so the first occupant in our newly-hung bee block is actually a wasp, but that is also a good thing, and I'll explain why.

A bee block need not be as fancy as the one shown here. There are some standards you should adhere to, though, and a thorough explanation is found here. Solitary bees come in a variety of sizes, so having a good selection of various diameter holes is helpful.

Why am I excited that a solitary wasp is using our block? Wasps are "flower visitors" that go to flowers strictly for nectar to fuel their activities (think gas station, not grocery store), but they do a fair bit of pollinating in the process. The biggest benefit of wasps, though, is that they are parasitoids of insects we consider pests.

The scenario for the mason wasp using our nest box, Euodynerus foraminatus, goes like this: The female wasp catches and paralyzes a caterpillar, and flies it back to the nest hole where she stashes the victim, and usually several more, as a larder of food for a single larva offspring. She lays an egg on the first or last victim, and then erects a mud partition. She repeats the steps, such that one of those holes you drilled might end up filled with several cells, each holding numerous caterpillars that won't be eating your garden plants. The preferred prey of E. foraminatus are larvae of moths in the families Oecophoridae, Tortricidae, Gelechiidae, Pyralidae, Crambidae (subfamily Pyraustinae). All of these include some notorious pests, as well as caterpillars that conceal themselves in rolled or tied leaves that other wasps won't go to the trouble of extracting.

We know at least one nest tunnel is complete because it has been plugged with mud. You can identify the different types of bees and wasps using a "bee condo" by the kinds of closures they make. Some megachilids (mason and leafcutter bees) will make the final closure with a plug of chewed-up leaves. Other bees will plug a nest hole with plant resins.

Besides the comings and goings of the bee block inhabitants, you can get some other kinds of drama. While watching the bee block for signs of activity last Friday, I noticed a crab spider hanging out under the "eave" on our bee block. Sure enough, when the wasp approached the spider reacted, hanging of the roof and opening its "arms" in a potentially lethal embrace. The wasp ignored or never noticed the threat, but I ushered the spider to another part of our yard anyway.

You may think that there is no way one of these boxes would work in your yard. The above image is of our yard, behind our townhouse unit. Look at it! It is literally like ten-by-twelve feet. Surely, if we can make this work, so can you. It takes virtually nothing to put one up, and it offers both critical bee real estate for the insects, and hours of entertainment for you and your family. Since solitary bees and wasps do not have vast quantities of offspring and honey stores to defend, they are not going to sting you. Besides, they are too busy working.

Back to that mason wasp for a second. Euodynerus foraminatus is a very widespread species, found coast to coast in the United States, plus all Canadian provinces, and down into Mexico, too. There is a very good chance you could attract one to your own bee block.

Sources

: Buck, M., Marshall, S.A. and Cheung D.K.B. 2008. "Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic region," Canadian Journal of Arthropod Identification No. 5: 492 pp. (PDF version).
Stone, H.B. 2014. "Insects of the Week: Eumenids and Chrysidids," The Bees' Needs Blog.