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Providing shelter for native bees

Last month I had the opportunity to run a workshop on protecting native bees for 250+ kids at Camp Micah in Bridgton, ME. Like humans, bees need three things: food, shelter, and water. In my workshop, the campers focused on shelter—we built 200 bee “hotels” to donate to the Honeybee Conservancy for their Sponsor-A-Hive program.

We hear a lot about honey bees, which make their homes in hives, but most bees are solitary and make their homes in less conspicuous manner. Mining bees (Andrena species), as their name suggests, make their home by digging tunnels in bare soil. In addition to digging tunnels, cellophane bees (Colletes species) line their nests with a clear protective secretion that resembles…you guessed it..cellophane! To provide shelter for these types of bees, leave your garden un-mulched.

Hanging by a couple of our research hives, typical man-made honey bee hives, at Cummings School of Veterinary Medicine at Tufts University. Photo: Rachael E. Bonoan

Blue orchard mason bee (Osmia lignaria) female.
Photo: USGS Bee Inventory and Monitoring Lab

Mason bees and leaf-cutter bees also nest in tunnels, but they do so a bit differently. These bees use ready-made tunnels in wood, hollow sticks, or dried-out plant stems. Female mason and leaf-cutter bees collect pollen and nectar to make a “food ball,” which she shoves to the very bottom of the nest. She then lays an egg on top of this food ball and makes a divider out of either mud (mason bees) or leaves (leaf-cutter bees). The momma bee then collects materials to make another food ball, which she puts in front of her “divider,” lays another egg, and the cycle continues until the nest is full of food and baby bees.

Cross-section of a blue orchard mason bee (Osmia lignaria) nest with mud dividers, orange-yellow food balls, and bright white bee eggs.
Photo: USDA ARS

The baby bees hatch out of their eggs, eat their nutritious food ball and develop from larvae, to pupae, to adult. In mason bees, pupae spin a cozy cocoon in which they complete their development to adult. The adult mason bees stay inside their cocoon until the weather is just right. In early spring, they chew their way out and emerge into the bright new world. To provide shelter for these bees, leave some of the larger, dried out stems in your garden. Or, like the campers, you can make a bee hotel!

Bee hotels don’t have to be five-star. They can be as simple as taking some dried out stems or reeds, creating a bundle, and securing the bundle with twine. You can hang this bundle somewhere near your garden (where the bees have food!) or in a tree. A variety of tunnel sizes ensures a variety of bees can use your bee hotel—bees come in many shapes and sizes. To provide enough space for the momma bee and her babies, the tunnels should be about 4 – 10 mm in diameter and about 15 cm (6 inches) long. If you don’t have dried-out stems readily available, you can purchase small cardboard tubes or paper straws to make your bundle. Avoid using plastic straws or bamboo as they don’t let the nutritious food ball breathe and may harbor mold.

You can add some amenities to your bee hotel in the form of PVC. A piece of PVC pipe 2 – 4 inches in diameter and a few inches longer than your tubes allows for some protection from the elements. Simply place a cap at one end of the PVC and pack your tubes in until they fit snugly. Again, use twine or if needed, zip ties, to secure your bee hotel. To keep birds and other possible predators out, you can add a security system with 1-inch wire mesh loosely secured to the front of your bee hotel. If possible, face the entrance of your bee hotel to the south so the bees get lots of warm morning sun (and a nice view).

When constructing your bee hotel, think about making it as big as the food (flowers) in your general area will support—you don’t want to raise too many bees and not have enough food. A meadow of wildflowers can support more/larger bee hotels than a small urban garden. To avoid spreading disease, replace the hotel’s linens (the tunnels) every year or two. In March and April, watch the entrance to your bee hotel to see how many bees emerge!

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Solitary wasps are fierce, fascinating, and totally harmless

In mid to late summer in the northeastern US, several species of large solitary wasp (belonging to the families Sphecidae and Crabronidae) frequent gardens, parks, and other open spaces. Despite their threatening appearance, solitary wasps are totally harmless. They are more interested in hunting other invertebrates–like spiders, flies, and bees–than they are in you. Solitary wasps are carnivores that capture and paralyze insects or spiders to feed their young, with many species specializing on particular types of prey. Unlike hornets, yellowjackets, and other social wasps, solitary wasp females build and provision nests independently of one another. Nesting locations differ among species and may include a variety of cavities both above and below ground.

Great Black Wasp (*Sphex* *pensylvanicus*) nectaring on Hairy Mountain Mint (*Pycnanthemum* *verticillatum* var. *pilosum*)

Digger wasps in the genus Sphex nest in the ground. In the northeast, the Great Golden Digger Wasp (Sphex ichnumoneus) and Great Black Wasp (Sphex pensylvanicus) are two particularly common species that can be seen drinking nectar from milkweeds, mountain mints, and other flowers. Females hunt katydids, stinging and paralyzing their prey before dragging it back to the nest. Although they are solitary, digger wasps sometimes aggregate, with many females constructing nests in close proximity. Each nest consists of a main tunnel with a number of side tunnels, each of which ends in a brood cell in which an egg is laid after the cell is provisioned with several katydids. When bringing paralyzed prey back to the nest, female Sphex leave the prey item outside the nest entrance while investigating the nest interior before dragging the prey down. If the prey item is moved slightly, the wasp will retrieve it and inspect the nest yet again. Sphex’s automatic nest-checking routine has captured the attention of several philosophers interested in the contrasting ideas of instinct and free will, inspiring the coining of the word “sphexish” (used to describe actions that appear thought-out and deliberate but are instead actually quite mindless).

Golden Digger Wasp (*Sphex ichnumoneus*) nectaring on Hairy Mountain Mint (*Pycnanthemum* *verticillatum* var. *pilosum*)

Isodontia grass-carrying wasps are a common sight around houses, gathering dry blades of grass and stuffing them into a crevice to furnish a nest. Grass-carrying wasps are predators of katydids and tree crickets and, like the digger wasps, leave their prey alive, but paralyzed, for their larvae to feed on.

Grass-carrying Wasp (*Isodontia* sp.) nectaring on Hairy Mountain Mint (*Pycnanthemum* *verticillatum* var. *pilosum*)

The giant cicada killer wasps (Sphecius sp.) are hard not to notice. Reaching lengths of an inch or more, these are among the largest wasps in North America. Even so, these formidable-looking insects are typically harmless. Females are not aggressive and although males may behave aggressively, they are unable to sting. Cicada killers sometimes form nesting aggregations, with many females utilizing the same patch of bare soil while males hover about looking for opportunities to mate. As their common name suggests, cicada killers hunt cicadas, paralyzing them and then flying back to their nest while carrying a prey item heavier than themselves. The wasp larva consumes the cicada and emerges as an adult the following summer.

Other solitary wasps hunt soft-bodied prey. The thread-waisted wasps in the genus Ammophila are a group of impossibly-skinny caterpillar predators. They can often be seen flying with a caterpillar slung underneath their body, toting their paralyzed prey back to an underground nest. Interestingly, after completing their nests and filling the tunnel with sand, some thread-waisted wasps have been observed using a small stone held between their jaws to tamp down soil at the former nest entrance, a behavior sometimes considered to be an example of tool use!

Thread-waisted Wasp (*Ammophila* sp.) with caterpillar prey

Though they may lack the charisma of butterflies, bees, and other favorite garden insects, solitary wasps are a diverse group that play an essential part in regulating numbers of herbivorous insects. By leaving patches of bare soil for nesting and planting milkweeds (Asclepias sp.), mountain mints (Pycnanthemum sp.), joe-pye weeds (Eupatorium sp.) , and other favorite nectar plants, you can encourage the presence of these beneficial insects in your yard and enjoy their pest-control services and enthralling behaviors.

Photo Credits: Max McCarthy

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The butterflies who are raised by ants

Silvery blue caterpillar. Photo: Atticus Murphy

What are these ants doing, clustering around a caterpillar? If you guessed eating, you’d be right, but probably not in the way you imagined.

These ants are engaged in what’s called “tending,” and far from being harmed by the interaction, the soft and vulnerable caterpillar is likely a beneficiary. In fact, the caterpillar has a suite of complex adaptations that seem aimed at keeping ants nearby. Most striking among these is the dorsal nectary organ, a gland that secretes a nutritious liquid high in sugar. Foraging worker ants eagerly consume the food and bring it back to their colonies. The cost to the caterpillar is only the cost of producing these little nutrition packets.

A less attractive ant and a silvery blue caterpillar. Photo: Atticus Murphy

But why would a caterpillar want a murderous cadre of ants clustered around it? The answer is protection. For one thing, when you manage to get the bullies on your side, they won’t bully you anymore: that is, the pacified ants are no longer a threat to the caterpillar. And in general, being a caterpillar is very dangerous. They have soft bodies, often feed in the open, and are not known for their quick movement, making them easy prey. In addition to being eaten directly, there are a huge diversity of parasitoids in the insect world, who lay eggs inside caterpillars’ bodies and eat their way out. This kills the caterpillar. A standing guard of ants, who generally protect their food sources and each other, lowers the caterpillar’s risk of being parasitized. Thus, because this interaction is often mutually beneficial, we call it a mutualism, meaning that both the ants and the caterpillars do better because of it: ants get food and caterpillars get protection.

Ants tending a silvery blue caterpillar, who is releasing a droplet from the dorsal nectary organ (the tiny glimmer in the center of the photo). This is located at the rear end of the caterpillar. Photo: Atticus Murphy.

In order to keep their attendants friendly, the caterpillar can also release a potent cocktail of chemicals that mimic ant pheromones, encouraging the ants to stick around, and hopefully keeping them from trying a bite of caterpillar. This cocktail is so effective that sometimes the ants can’t distinguish the scent of the caterpillar from their own kind. If the ants are absent and a predator approaches, some caterpillars also make use of specialized organs that produce noises or fragrances, attracting ants from farther away.

*An adult female Silvery Blue lays an egg on lupine: within 3 days the egg will hatch, and within a week it will be old enough to attract ant attendants. Photo: Rachael Bonoan.*

The butterfly species in the pictures above is the one I worked with this summer, the silvery blue (Glaucopsyche lygdamus). It’s common across the U.S., but this interaction is a global phenomenon, occurring in hundreds of butterfly species that can be found on every continent except Antarctica. And with a diversity of species comes a diversity of interactions: many different ant-caterpillar pairings have emerged, and unique quirks abound. Perhaps the most captivating variations on the theme are the parasitic blue butterflies. These dastardly caterpillars have taken the usual mutually beneficial interaction and tilted things decidedly in their own favor by truly pretending to be baby ants. After spending some time feeding on a host plant like most caterpillars do, these species use their unusually effective chemical mimicry to induce ants to take them inside the actual nest, where the caterpillars are either fed alongside the real ant young, or more sinisterly, the caterpillar devours the ant young, growing fat by pillaging their hosts until they’re ready to emerge as adults.

*The Large Blue butterfly, a parasitic relative of the Silvery Blue. Photo: Ann Collier.*

The ant-tending of these butterflies is not just an interesting quirk of natural history, but for some species may be the key to their continued existence. The classic example of this possibility is the large blue butterfly (Phengaris arion) of Britain, which is a parasite of Myrmica ants. This butterfly was on the decline for decades in the British Isles and was an early beneficiary of an intensive conservation campaign. Unfortunately, this campaign failed, and by the 1970s, the species teetered on the edge of extinction in spite of years of efforts. The conservationists were perplexed. They had carefully cultivated healthy patches of the host plant, Thymus, and there looked to be plenty of ants in the area, so why were the butterflies still declining?

It took a careful reexamination of the already well-known dependence on Myrmica ants to understand what had occurred. The large blue was an unrecognized specialist, a butterfly who relied not just on Myrmica ants to survive, but on a particular species of Myrmica ant. This species was so crucial that even close relatives were totally unsuitable and could not successfully “raise” caterpillars to adulthood. While there were indeed plenty of Thymus plants and plenty of Myrmica ants, the ants were of the wrong species! The large blue tragically went extinct in Britain before this new knowledge could be put in practice, but it has since been successfully reintroduced.

So, the next time you see a blue butterfly, remember that it might well have relied on an unruly bunch of ant nannies to survive into its winged form. Remember also that these butterflies provide still another example of the myriad ways in which our pollinators are dependent on an entire healthy ecosystem and its component parts, not just on their host plants.

Further Reading:

Pierce, N. E., M. F. Braby, A. Heath, D. J. Lohman, J. Mathew, D. B. Rand, and M. A. Travassos. 2002. The ecology and evolution of ant association in the Lycaenidae. Annual Review of Entomology 47:733–771.

Thomas, J. A., D. J. Simcox, and R. T. Clarke. 2009. Successful Conservation of a Threatened Maculinea Butterfly. Science 325:80–83.