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.

Harebell ~ Campanula rotundifolia Pollinators & Floral Visitors

Harebell ~ Campanula rotundifolia

Typically, small to medium sized bees visit harebell flowers. The five-parted, nodding flowers have a prominent, large, central style that may restrict access to large bees such as bumble bees. Bumble bees could reach the nectaries with their long tongues, but I have not seen one attempt this. Every visiting insect has a different approach and objective while visiting the flowers, some affect the rate of the flower development process and others get away with feeding on nectar without aiding in cross-pollination.

Filament bases surround the nectar-producing disc at the bottom of the style. When harebell anthers release pollen, the tube formed by the filaments around the style traps pollen. As the style elongates, the pollen collecting hairs on the style push the pollen upwards, exposing the pollen to pollinators.

The pollen collecting hairs are stimulated by visiting insects, more stimulation with pollen feeding, and less with nectar feeding. The more the hairs are stimulated, the shorter the duration of the male phase. Hairs retract causing the remaining pollen to fall out of the nodding flower. The female phase then begins with the separation of the stigma. This process, accelerated by pollinators, ensures cross-pollination of the flowers.

Leafcutter Bees ~ Megachile spp.
Leafcutter bees are frequent visitors of harebell feeding on nectar. Their pollen-collecting hairs (scopa) on the underside on their abdomens come into contact with the pollen held on the style.

Digger Bees ~ Anthophora spp.
Long tongues allow digger bees to reach the nectar at the base of the style and avoid contact with pollen on the style. The flower in this image has released all the pollen, the filaments are drying up and the stigma is preparing to separate to receive pollen.

Green Sweat Bees ~ Agapostemon spp.
Green sweat bees visit harebell flowers for nectar. This bee is holding itself in the nodding flower by grasping onto the base of the style. The stigmas have separated and are ready to receive pollen transferred from visiting insects.

Small Carpenter Bees ~ Ceratina spp.
Small carpenter bees are some of the early visitors of harebell flower development. After the style elongates and the pollen collecting hairs are covered with pollen, small carpenter bees visit and feed on and collect pollen stuck stigma.











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

The Pollen-Collecting Hairs of Campanula (Campanulaceae). II. Function and Adaptive Significance in Relation to Pollination. Yvonne Nyman. American Journal of Botany , Vol. 80, No. 12 (Dec., 1993), pp. 1437-1443

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

Native Plant of the Week: Nodding Onion ~ Allium cernuum

Nodding Onion ~ Allium cernuum


With our ongoing drought in many parts of the continent, it's a good idea to investigate what native plants are thriving in this heat. Nodding Onion is one of these natives, loving hot, sunny locations in dry sandy soils. It is also apparently tolerant of loam or clay soils.

Very similar to Prairie Onion (Allium stellatum), Nodding Onion has drooping umbel flowerheads comprised of 6 parted light pink flowers. Flower stalk heights range from 12 -24 inches.

The basal leaves are linear and grass-like in appearance, and often go unnoticed because the foliage of neighboring plants will hide the onion leaves.

The leaves have an onion odor, not as strong as chives but quite apparent. Nodding onion works really well intermixed with shorter prairie natives at the edge of a planting. It looks fantastic planted with Wild Petunia, Side Oats Grama and Spotted Beebalm all of which flower at the same time.

The primary visitor to the tiny flowers are small Halictid bees. Look for them clinging to the underside of the nodding flowers.

The occasional Leafcutter Bee will also visit the flowers of the Nodding Onion in our yard.

And where the bees visit, you will often find Tumbling Flower Beetles feeding on pollen.

I also spotted the Weevil species this morning. I'm going to investigate later if the weevil is feeding on any part of the Nodding Onion plant.

Nodding Onion is native to many parts of North America, see map below for range.

Kartesz, J.T., The Biota of North America Program (BONAP).
2011. North American Plant Atlas. Chapel Hill, N.C.

Great St. John's Wort: A Favorite of Pollinators

Great St. John's Wort ~ Hypericum pyramidatum (Hypericum ascyron)


I was going to feature this St. John's Wort for the Native Plant of the Week, but realized I had already featured it in an earlier post.

Since that post, I have photographed many more pollinators on this native perennial.

Great St. John's Wort's native habitat includes mesic to wet areas, along the edges of wetlands and lakes. It tolerates full sun locations but prefers partially shaded locations.

Great St. John's Wort is a tall, upright bushy native. Visitors to our yard often ask if it's a shrub because of its large stature. It can reach heights of five feet in moist, rich soils, in our mesic yard, it grows to about three feet in height.

Pollinators love the large, open flowers with the numerous stamens.

Leafcutter Bees in the Megachile genus are common visitors. Their abdomens turn bright yellow as the pollen collects on their abdominal hairs.

Leafcutter Bees also like the large leaves of Great St. John's Wort and have been cutting and collecting pieces of them for their nesting materials.

Bumble Bees also like the flowers. Bombus griseocollis and Bombus bimaculatus are common Bumble Bee species that are visiting the flowers in our yard.

These Long Horned Beetles (Typocerus velutinus) have been abundant in the yard this year. Several of these beetles can be found on one Great St. John's Wort plant feeding on pollen.

Flies are attracted to the flowers too. I don't have any information on whether this Hypericum produces nectar or not, my best guess is that it does not.

The Syrphid flies like the one pictured here are probably consuming bits of pollen that have dropped onto the petals.

Greater Bee Flies (Bombylius major) seek nectar on the Great St. John's Wort, hovering above the flowers like hummingbirds and probe flowers with their long tubular mouthpart.

These are just a few examples of the many types of pollinators that visit Great St. John's Wort flowers. If you have a medium to moist location, this native perennial will brighten any partially shaded location with its extremely showy flowers.

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.