🐝 The Perils of Pollinia and More about Milkweed 🦋

By Mary Free, Extension Master Gardener

A leafcutter bee (Megachile) carrying pollinia.
Photo © 2020 Mary Free

Imagine being a pollinator drawn to a fragrant flower that offers abundant nectar. In your hurry to sip from the nectar pool, you falter on the flower’s slippery, uneven surface and your leg slips into a crevice (the stigmatic slit). The slit is lined with bristles that restrict your movements so that the only way out is up.



As you fight to free yourself, something (the corpusculum) clasps your leg. If you manage to escape without losing a claw or worse (OUCH!), then you will find yourself carrying pollen-filled, saddlebag-like sacs (paired pollinia attached to the corpusculum by translator arms). Unlike most flowers whose pollen grains are loose and powdery, the pollen of milkweeds and orchids is bound together and transferred in a single unit, a pollinium.

Diagram created by Mary Free

If you are hungry and hasty (and keep slipping into stigmatic slits), then the pollinia you carry can multiply quickly and weigh you down. You can rid yourself of these sacs during a flower visit if a pollinium slips into a stigmatic slit where the stigmatic chamber is empty. If that is in a different flower of the same species, then you have achieved cross-pollination. You still have to break the translator arm though in order to fly on your way. Better get busy because you have plenty of pollinia to deliver…

…that is unless you pictured yourself as a small pollinator. In that case you might still be trapped inside the flower and will probably perish. If you imagined yourself as a butterfly, those delicate legs would have had a low success rate in removing the pollinia. However, if you chose to be a honey bee (Apis mellifera), a brown-belted bumble bee (Bombus griseocollis), or a digger wasp (Sphex ichneumoneus, S. pensylvanicus), then congratulations! Studies showed that they were the most effective pollinators of various milkweed plants, although honey bees likely left more parts behind. In those studies, the prize for worst pollinator, but best nectar robber goes to the 865 American bumble bees (B. pensylvanicus) that visited Asclepias syriaca (common milkweed) and extracted only four pollinia among them. They were, however, more successful pollinators when they visited other milkweed species.

This video shows European honey bees carrying pollinia of Asclepias tuberosa on their legs and tarsi. In the first slow motion clip, the honey bee’s tarsus–with pollinia attached–seems to have slipped into a stigmatic slit. Did it make a delivery or a pick up?  Video © 2020 Mary Free

Milkweed (Asclepias spp.), of course, is the host plant for monarch butterflies. A. syriaca is said to be the most fragrant milkweed and produces nectar both day and night, which is extremely attractive to pollinators. It is not as attractive to those with small gardens, however, as it can be unruly and spread very aggressively by rhizomes. It also is not quite as attractive to monarchs as a host plant.

Eastern tiger swallowtail and a bumble bee feed on Asclepias syriaca (common milkweed) at the Monarch Waystation in historic Ball-Carlin Cemetery in Arlington, Virginia in June. Cirsium discolor (field thistle) appears in the foreground and Penstemon digitalis ‘Husker Red,’ (beardtongue) and Echinacea purpurea (purple coneflower) in the background.     Photo © 2020 Mary Free

In experiments with nine milkweed species, monarch egg counts were highest on Asclepias incarnata (swamp milkweed). In choice experiments with four milkweed species, monarch “females laid 1.7 times more eggs on A. incarnata than on Asclepias syriaca…, 14.9 times more eggs than on A. tuberosa…Females laid nine times more eggs on A. syriaca than on A. tuberosa.” (Pocius, 2018)

Monarch butterfly on Asclepias incarnata (swamp milkweed) in July.
Photo © Henry Staples and Claudia Gerwin

 Monarch egg counts were higher when more than one species was present. Consider Asclepias incarnata and Asclepias tuberosa (butterfly-weed), both of which are better behaved in smaller gardens. They require different habitats–swamp milkweed tolerates wetter soil, clay, and some shade while butterfly-weed tolerates dry/poor soil and drought but prefers full sun. The length and timing of flowering also differs somewhat. Having both plants in a landscape would be advantageous to monarchs, because if a season is wetter or drier or hotter than normal, then at least one of these milkweed species should still perform well and be a viable and healthy host.

Female monarchs deposit their eggs on milkweed during the breeding season (late spring through summer in Northern Virginia) and their caterpillars devour the leaves as they grow. During this time, adult monarchs may feed on milkweed flowers, but they usually have many other nectar choices. Monarchs migrate from the northeast through Northern Virginia in September and October on their way back to Mexico. They require the sustenance of nectar-rich, late-season bloomers like asters (e.g., Symphyotrichum novae-angliae, S. oblongifolium), goldenrod (Solidago spp.), and native thistle so they have the energy they need to make the journey.

Asters rank second, after goldenrod, in supporting Lepidoptera, according to Doug Tallamy’s Bringing Nature Home. This video shows migrating monarch butterflies foraging on asters, including Symphyotrichum novae-angliae, in early October. Video © Mary Free



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