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.

Native Bee Spotlight: Leafcutter Bees ~ Megachile spp.

Leafcutter Bees ~ Megachile spp.

Leafcutter Bees are a medium sized, long-tongued native bee. They have a flattened or deflated looking abdomen, and often visit flowers with their abdomen curved upwards.

We start to see Leafcutter Bees in our landscape in early June. Some of the first native plants they visit are Harebell and Pale Purple Coneflowers.

Instead of collecting pollen on combs on their legs, females collect pollen on the underside of their abdomen where they have long hairs.

A female nectars on Nodding Onion (Allium cernuum) showing off her pollen-laden belly.

If you don't see Leafcutter Bees on your native plants, look for circular or oval shaped cuts in the foliage like this.

Females use their sharp mandibles to cut pieces of leaves from foliage. I captured this female cutting an oval-shaped piece of leaf from our Great St. John's Wort (Hypericum pyramidatum) this summer. Leafcutter Bees use these pieces of leaves to line their brood cells where they build nest in pre-existing cavities such as in wood or hollow stems.

Look for Leafcutter Bee females collecting pollen and nectar from a wide variety of prairie plants such as this Gray Headed Coneflower (Ratibida pinnata).

Also feeding on nectar on Wild Bergamot in July. This Leafcutter Bee is taking advantage of a short cut created by a wasp. The wasp chewed a hole near the base of the tubular flower to get easy access to the nectar (nectar thievery).

Leafcutter Bee on Fragrant Hyssop (Agastache foeniculum), a licorice (anise-scented) prairie native.

In moister locations, Leafcutter Bees will visit Culver's Root (Veronicastrum virginicum).

Also Cup Plant (Silphium perfoliatum) in late summer.

















Source: Attracting Native Pollinators: The Xerces Society Guide, Protecting North America's Bees and Butterflies

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.