Native Bee Spotlight: Cuckoo Bees ~ Coelioxys spp.

Cuckoo Bees ~ Coelioxys spp.

A female cuckoo bee, Coelioxys sp. nectars on
hairy false goldenaster, Heterotheca villosa in late fall

There are many types of cuckoo bees in North America. In the Coelioxys genus, there are approximately 46 speces. The common name "cuckoo bee" is typically used for any bee species that lays its eggs in the nests of other bees. These bees are known as cleptoparasites, where the cuckoo bee larvae kill the host larvae and feed on the provisions (pollen and nectar) provided by the host bee.

Coelioxys cuckoo bees are common in the summer months; in central Minnesota I typically see them from June until October. Both males and females can be observed visiting flowers for nectar and females looking for, or waiting to enter a host's nest. These cuckoo bees lay their eggs in the nests of leafcutter bees, Megachile spp. Less frequently, they have been documented laying eggs in the nests of Anthophora, Centris and Euglossa spp.

A female cuckoo bee watches for the
female leafcutter bee to exit the nest
in the rock cavity.

The cuckoo bee flies closer to the entrance
anticipating the exit of the host bee.

Females Coelioxys bees actively look for a host's nest. Once a nest is found, the female cuckoo bee waits until the host bee leaves the nest to collect provisions. With a short window of opportunity, the cuckoo bee slips in the empty nest and looks for a fully provisioned brood cell to lay its eggs in.

The host leafcutter bee, Megachile sp. enters the nest in the
rock cavity carrying a piece of leaf to line or cap the brood cell.

Females have a sharply tapered or triangular-shaped abdomen. This acute point on the end of the abdomen is used to pierce through the layers of leaf pieces that line the brood cells of their host, leafcutter bees. The egg(s) is laid within the layers of leaves, or underneath the pollen mass hidden from sight in case the host leafcutter bee is still in the process of provisioning the nest. The cuckoo bee eggs are often different in size or appearance which may be another reason why the cuckoo bee hides the eggs.

The large sickle-like mandibles that the cuckoo bee larvae use to kill its siblings and host larva.
Illustration from: Michener, C. D. (2000). The bees of the world (Vol. 1). JHU Press.

The menacing part of this cleptoparasitic life cycle occurs after the cuckoo bee larva has hatched and begins to develop. The larva develops large sickle-shaped mandibles that are directed forward (instead of downward) to prepare to kill the host egg or young host larva. By the third or fourth instar, the Coelioxys larva has killed any sibling larvae and the host. It now has an empty brood cell stocked with pollen and nectar provisions to feed on and develop.

A female cuckoo bee nectars on
purple prairie clover, Dalea purpurea

The timing of adult emergence of cuckoo bees is very critical; there is a short window of opportunity to overlap the timing of the host's adult emergence. Coelioxys spp. have been documented emerging slightly earlier or around the same time as their host. A larvae will often develop into an adult the same year as the nest is constructed and more female cuckoo bees are produced earlier in the season.

A female cuckoo bee perches on foliage low to the ground
watching for a leafcutter bee to emerge from a nest in the ground.

Appearance:
Female: Tapered abdomen ending in an acute point
Male: Pronged or multi-spined abdomen
Both Male & Female: Hairs on the bottom of the eyes
Relatively hairless, dark gray - black, often pocked appearance
Many species have red legs

Native plants I have observed Coelioxys spp. foraging on:
Butterfly milkweed, Asclepias tuberosa
Wild Bergamot, Monarda fistulosa
Prairie Coreopsis, Coreopsis palmata
Hoary Vervain, Verbena stricta
Joe Pye Weed, Eutrochium maculatum Smooth Oxeye, Heliopsis helianthoides Black-Eyed Susan, Rudbeckia hirta
New England Aster, Symphyotrichum novae-angliae
Hairy False Goldenaster, Heterotheca villosa
Purple Prairie Clover, Dalea purpurea 

References:
Baker, J. R. (1971). Development and sexual dimorphism of larvae of the bee genus Coelioxys. Journal of the Kansas Entomological Society, 225-235.

Michener, C. D. (2000). The bees of the world (Vol. 1). JHU Press.

Rozen Jr, J. G., & Kamel, S. M. (2006). Interspecific variation in immature larvae of the cleptoparasitic bee genus Coelioxys (Hymenoptera: Megachilidae). Journal of the Kansas Entomological Society, 79(4), 348-358.

Scott, V. L., Kelley, S. T., & Strickler, K. (2000). Reproductive biology of two Coelioxys cleptoparasites in relation to their Megachile hosts (Hymenoptera: Megachilidae). Annals of the Entomological Society of America, 93(4), 941-948.

Milkweed Pollination - A Sticky Situation

Milkweed plants, Asclepias spp. have a unique method of transferring pollen from one plant to the other for cross-pollination. Pollen is aggregated in sac-like bundles called pollinia, located on either side of the stigmatic chamber. The two bundles are strung together with a gland (filament) at the top of the stigmatic chamber.

Bees, wasps, flies, beetles and butterflies visit the flowers for nectar. Milkweed plants typically produce a lot of nectar, it is replenished overnight, to the delight of nocturnal moths, and the remaining nectar is ready for the first diurnal visitors in the morning.

For accessing nectar, floral visitors prop themselves on one of the five flower hoods, sliding their tongues down the side of the hood where the nectar is held. They must be careful not to slip their leg down into the flower between the anthers.

When pulling their leg out, they could snag it on the filament holding together the sticky pollinia sacs.

Pollina stuck to the legs of a great
black wasp, Sphex pensylvanicus

The sticky pollinium sacs are carried to other milkweed plants on their legs and if the insect again ‘slips’ the pollinia can be inserted into the stigmatic chamber ensuring cross-pollination. Unable to pull their leg out, smaller bees can become trapped in the flowers and perish.

In a study by Fishbein and Venable (1996), small- and medium-sized bees, and medium-sized butterflies had the lowest removal rates of pollinia. Their study found that it was the larger bees, like bumble bees that were most effective at transferring pollinia from one plant to the other.

Leafcutter bee, Megachile sp. alighting
on top of the flower hoods to nectar.

Ivey et al. (2003), also found large carpenter bees, Xylocopa spp. and bumble bees, Bombus spp. effective pollinators, partly due to their foraging efficiency; they visit flowers methodically, probing all the hoods of a flower, and visiting more flowers per flowerhead.


Leafcutter bees are common visitors of milkweed feeding on nectar. They rarely snag pollinium sacs so are considered ineffective pollinators.

Small carpenter bees, Ceratina spp. prop themselves on the top of the hood and slide their tongue down the side of the hood to reach nectar.

Cuckoo bees, Coelioxys spp. are true nectar thieves. The have no pollen-collecting structures on their legs. They only visit flowers for nectar because they are cleptoparasites, laying their eggs in the nests of other bees.

This particular cuckoo bee is a cleptoparasite of leafcutter bees,  Megachile spp. Females have a tapered abdomen ending in a sharp point that is used to break through leafcutter brood cells.

Another example of a small-sized bee, yellow faced bees, Hylaeus spp. are frequent visitors to swamp milkweed in late summer for nectar.

Soldier beetles, Chauliognathus spp. also love to feed on nectar on milkweed plants but are rarely found carrying pollinia.

What seems to be a not-so-mutualistic relationship between floral visitor and plant, where floral visitors are exploiting nectar resources, cross-pollination is still occurring due to the effective visitation by large bees, who transfer the polllinium sacs from one plant to another.


References
Fishbein, M., & Venable, D. L. (1996). Diversity and temporal change in the effective pollinators of Asclepias tuberosa. Ecology, 1061-1073.

Ivey, C. T., Martinez, P., & Wyatt, R. (2003). Variation in pollinator effectiveness in swamp milkweed, Asclepias incarnata (Apocynaceae). American Journal of Botany, 90(2), 214-225.

Kephart, S. R. (1983). The partitioning of pollinators among three species of Asclepias. Ecology, 120-133.

Hoary Vervain Insect Visitors

Hoary Vervain ~ Verbena stricta


I planted more Hoary Vervain a few years ago in our landscape because it likes dry, sandy locations in full sun. Now, I'm enjoying a significant increase in the diversity of pollinators and insect visitors coming to Hoary Vervain.

Small Skipper Butterflies like the tubular five-parted flowers. Pictured here, Peck's Skipper (Polites peckius), one of the easier small skippers to identify with its distinct markings.

A second Skipper, my best guess is Fiery Skipper (Hylephila phyleus), although we are a little north of its range. Perhaps due to our early spring warm-up this Skipper got a head start on its migration north and traveled a little further.

Cabbage White butterflies (Pieris rapae) are also frequent visitors seeking out nectar.

Hoary Vervain flowers from the bottom upwards, on the tall narrow flower spike. The flowering time can therefore last from July into September providing a nice source of nectar for insects.

Some of the first Soldier Beetles in our yard this season are on Hoary Vervain. They like to feed on both nectar and pollen on many types of flowers.

Soldier Beetles are predators of other insects and are therefore considered natural enemies. Falling in the family Cantharidae, some soldier beetle adults will feed on aphids, and insects eggs and larvae providing biological control.
(Michigan State University, Natural Enemies)

This attractive, large Syrphid fly was using its long mouthpart to sponge up some nectar.

Syrphid flies are also predators, some larvae feed on aphids.

Bumble Bees also like the Hoary Vervain flowers. Less frequent visitors, probably because some preferred native species such as Wild Bergamot flower at the same time.

These medium-sized fast moving Long Horned Bees (Melissodes species) would stop on the Hoary Vervain just long enough so I could snap a photo.

This particular bee likes to fly around other bees nectaring and take small jabs at them. They seem to spend their time doing this more than landing on flowers for pollen or nectar.

Two of the new bee species discoveries this season have occurred on Hoary Vervain.

This Cuckoo Leafcutting Bee (Coelioxys species) is a cleptoparasite of Leafcutter Bees. With no means to collect pollen, its only visiting the Hoary Vervain for nectar. Read more about the Cuckoo Leafcutting Bee here: Another New Native Bee Discovery.

The second discovery, just last week was another Cuckoo (cleptoparasitic) Bee in the Genus Triepeolus. With bright orange legs and black and white striped body, it is an extremely attractive bee.

These bees lay their eggs in the nests of Long-Horned Bees (Melissodes species).

If you have an open, sunny location with medium to dry soils, I highly recommend Hoary Vervain. Its upright stature, coarse leaves and long blooming purple flowers make it a great addition to any landscape or prairie.

Another New Native Bee Discovery

Leafcutter Bee

I'm amazed at the number of new native bees we've seen this year in our yard. Many of the new ones are cleptoparasites on other bee species, otherwise known as Cuckoo Bees.

Read earlier posts here about Nomada Cuckoo Bees, and another post about a Sphecodes Sweat Bee that's also a Cuckoo Bee - both new sightings this year.

The latest new bee is a cleptoparasite of Leafcutter Bees (Megachile species). Leafcutter Bees are different from other native bee species - instead of collecting pollen on their leg combs, they collect pollen on hair patches on their abdomens.

Leafcutter bees nest in cavities or hollow stems. Females cut circular pieces of leaves to wrap brood cells. The cell's sides are made with oval pieces, and it is closed with a number of circular pieces. (Attracting Native Pollinators)

This week I photographed this new Cuckoo Bee (Coelioxys species) nectaring on Prairie Coreopsis and Butterfly Milkweed. I thought it could be a type of wasp at first, with its pointed abdomen.

These Coelioxys Cuckoo Bees are in the same family as Leafcutter Bees (Megachilidae). Like the other Cuckoo Bees I wrote about this spring, they don't have any pollen collecting capabilities. They visit flowers for nectar only.

"Protruding from the last segment are short spines used to break through the brood cell walls of their host species during egg laying".

"The female enters a nest while the host is foraging and hides her egg by laying it in the wall or behid the food masses of almost finished brood cells." (Attracting Native Pollinators)

Like other cleptoparasites, the larvae of the Cuckoo Bee hatch and kill the host bee larvae. They then eat the pollen supplied by the host bee.

It has been fascinating to observe the order of arrival of native bees in our landscape. Starting with common Bumble and Mining Bees, the next season Leafcutter, Digger, Carpenter and Sweat Bees, then this year Carder Bees and many types of cleptoparasitic Cuckoo Bees.