Partridge Pea Puzzles

Bright yellow Partridge Pea (Chamaecrista fasciculata) flowers peek out from between the stems of taller grasses and flowering forbs in meadows, prairies, stream banks and other open areas from July through early September.

Partridge Pea (Chamaecrista fasciculata)

Partridge Pea’s flowers are tucked in the leaf axils down the length of the stem.

Partridge Pea (Chamaecrista fasciculata)

Each flower has five yellow petals, with one much longer than the other four, and another partially curled toward the center of the flower, where its reproductive parts are located. A 1992 study showed that the curved petal directs floral visitors to the flower’s reproductive parts, first to the pistil (female reproductive part), and then the stamens (male reproductive parts).[1]  The red smudges on the petals are part of the visual allure to pollinators.

Partridge Pea (Chamaecrista fasciculata)

In the Partridge Pea flower in the photo above you can see the three evenly sized petals at the top, one petal in the lower left that curls toward the center of the flower, and an over-sized petal at the lower right. The stamens are mostly clustered at the middle of the flower.  The pistil resembles a hook projecting from beneath the right-most stamen. It is visible at the top of the over-sized petal.  Imagine a pollinator coming in for a landing using the over-sized petal as a runway, guided by the curved petal, with the red smudges on the petals as beacons. The pollinator brushes first against the receptive stigma at the tip of the pistil, depositing pollen from the last flower visited, then moves on to harvest pollen from the stamens.

Bumble Bee harvesting pollen from Partridge Pea (Chamaecrista fasciculata) flower. Bumble Bees are adept at buzz pollination.

Partridge Pea flowers offer pollen as a reward to their visitors, but they don’t produce nectar. As a result, bees that collect pollen are the most likely visitors of the flowers.  But the bees have to have skills in order to harvest Partridge Pea’s pollen, since it requires special handling in order to access it.  The pollen is dispersed through a slit at the tip of the stamen’s anther.  Pollen can be shaken out of the anther as a result of buzz pollination, a technique in which a bee clings to the flower while vibrating its wing muscles without actually moving its wings.  ‘Milking’ the anther with a series of strokes is another method of successfully harvesting Partridge Pea’s pollen.[2]

Honey Bee harvesting pollen from Partridge Pea (Chamaecrista fasciculata) flower. Honey Bees can’t perform buzz pollination, so may be using the ‘milking’ technique.

Eastern Carpenter Bee harvesting pollen from Partridge Pea (Chamaecrista fasciculata) flower.

Butterflies aren’t interested in Partridge Pea flowers, since they don’t offer nectar. But several butterfly species use Partridge Pea as a food plant for their caterpillars, including the Sleepy Orange and Cloudless Sulphur.

Sleepy Orange drinking nectar from Winged Loosestrife (Lythrum alatum)

Cloudless Sulphur

Gray Hairstreak and caterpillar on Partridge Pea (Chamaecrista fasciculata). Can you see the tiny caterpillar clinging to a leaf in the lower left of the photo?

When I saw a Gray Hairstreak butterfly spending time walking around a Partridge Pea plant, it seemed possible that this was a female laying eggs. Gray Hairstreaks use some Pea family members as caterpillar food, including clovers and tick-trefoils, although I haven’t seen any confirmation that they would use Partridge Pea.

Gray Hairstreak on Partridge Pea (Chamaecrista fasciculata).

A closer look showed that the butterfly was visiting Partridge Pea for nectar after all, but it was nectar that is made available through extrafloral nectaries on the base of the stem of each leaf.  This was a great benefit for the butterfly, but not much help for the plant, since the butterfly offered no services in return.

Gray Hairstreak drinking nectar from an extrafloral nectary on Partridge Pea (Chamaecrista fasciculata)

When present, extrafloral nectaries are generally a plant’s adaptation to entice insects that are predators of herbivores to visit and protect the plant. Ants are especially important in this role, since caterpillars are a very desirable food for them.  Some wasps and lady beetles are also potential protectors of plants.  While they are interested in nectar for themselves, they are also on the hunt for insects to feed to their larvae.  The wasps and lady beetles may rid the plant of the caterpillars or other insects who would eat it.  Nectar is provided in exchange for this protection.

Partridge Pea’s extrafloral nectaries look like tiny open pots, glistening with nectar, an open invitation to thirsty insects cruising through, not all of whom will offer services to the plant.

The two round pot-like appendages near the base of the Partridge Pea (Chamaecrista fasciculata) leaf stems are the extrafloral nectaries. Notice the glistening drops of nectar oozing from them.

While I have seen ants working Partridge Pea extrafloral nectaries, I was surprised at the variety of insects I saw drinking from them at one location I visited. It makes me wonder whether the cost of providing this nectar is worth the protection gained from them.  In addition to the Gray Hairstreak, I watched while a Bumble Bee spent more time visiting the extrafloral nectaries than the flowers.

Bumble Bee drinking nectar from a Partridge Pea (Chamaecrista fasciculata) extrafloral nectary.

After visiting several extrafloral nectaries, the Bumble Bee moved on to a flower.

I saw several Paper Wasps visit the nectaries. Since these wasps hunt caterpillars to feed their larvae, they do have the potential to provide a service in exchange for a tasty drink.

Paper Wasp (Polistes species) drinking from an extrafloral nectary on Partridge Pea (Chamaecrista fasciculata)

Paper Wasp (Polistes species) drinking from an extrafloral nectary on Partridge Pea (Chamaecrista fasciculata)

‘That was tasty!’ Paper Wasp (Polistes species) on Partridge Pea (Chamaecrista fasciculata)

Some accounts of Partridge Pea say that the leaves will sometimes fold up when they are touched. I’ve tried it several times, but I have never had Partridge Pea respond to my touch.  However, I have seen Partridge Pea plants with their leaves folded, so I’m guessing the plant folds its leaves in response to some stimuli, but I haven’t found an explanation for what it might be.  Maybe it’s a mechanism to prevent excessive water loss on hot, dry, or windy days.  Or maybe the plant responds to the touch of a butterfly laying eggs, and wants to minimize the leaf surface available to her.  I wish I knew!

Partridge Pea (Chamaecrista fasciculata) with leaflets folded. What prompted this?

If the name didn’t give away its family heritage, the fruits identify Partridge Pea as a member of the Pea or Bean (Fabaceae) family.  These fruits are an important winter source of food for birds, especially Bobwhites and Greater Prairie Chicken.

Partridge Pea (Chamaecrista fasciculata) with fruits typical of the Pea (Fabaceae) family.

Partridge Pea is an annual, but reseeds itself readily.  It likes sun, and can tolerate poor, dry soils.  It helps to fertilize soils through its release of nitrogen, and is sometimes used in stream bank stabilization.  Partridge Pea is native from Rhode Island to Minnesota in the north, south as far as southeastern New Mexico, and from Texas to Florida.

Partridge Pea (Chamaecrista fasciculate)

Related Posts

Will Work for Food – Extrafloral Nectaries

Cloudless Sulphurs Are on the Move

Sleepy Orange Butterflies are Back

Resources

Cech, Rick; Tudor, Guy. Butterflies of the East Coast.  2005.

Eaton, Eric R.; Kauffman, Ken. Kaufman Field Guide to Insects of North America.  2007.

Rhoads, Ann Fowler; Block, Timothy A. The Plants of Pennsylvania.  2007

Illinois Wildflowers

USDA NRCS Plant Database

USDA NRCS Plant Guide – Partridge Pea

[1], [2] Pollination and the Function of Floral Parts in Chamaecrista fasiculata, Andrea D. Wolfe and James R. Estes, 1992.

Natural Selection on Extrafloral Nectar Production in Chamaecrista Fasciculata: The Costs and Benefits of a Mutualism Trait

 

‘Will Work for Food’ – Extrafloral Nectaries

Wild Senna (Senna hebecarpa, S. marylandica) has so many stories to tell!  This tall, herbaceous plant has flowers that are unusual for a member of the Pea (Fabaceae) family.  Rather than curling to form the banner, wings and keel that are common Pea family characteristics,

Everlasting Pea (Lathyrus latifolius) with skipper

Everlasting Pea (Lathyrus latifolius) with skipper

Wild Senna’s petals are open and distinct.

Wild Senna (Senna hebecarpa) with Bumble Bee. Note the pollen on her rear legs.

Wild Senna (Senna hebecarpa) with Bumble Bee. Note the pollen on her rear legs.

Wild Senna’s flowers have another somewhat unusual feature, or more accurately, they lack a feature, nectaries, that many flowers have.  Many plant species have evolved to entice pollinators to their flowers by providing a reward of nectar in exchange for their visits. In spite of the lack of nectar, Wild Senna is pollinated by bees, primarily Bumble Bees but also Sweat Bees (Halictid species).  They visit the flowers for their pollen, a highly nutritious food that contains protein, carbohydrates, fats, vitamins and minerals; plenty of incentive for a bee to visit even without nectar.  Alerted to the possibility of food by the colorful yellow flowers, the adult bees come to dine on pollen and to harvest some to bring back to their nests for their larvae.

Many plant species have evolved to produce chemical compounds whose primary purpose is to protect the plant from being eaten by making it bitter, distasteful or even toxic to potential consumers.  Wild Senna is a species that has adapted to use this defense.  Both the leaves and fruits (seed pods) contain anthraquinones, which are powerful laxatives.  Often people take advantage of the protective chemicals that plants produce by finding medicinal uses for them. In the case of the Senna species, the laxative is used for treating constipation.

These chemicals are a very effective deterrent to many animal species that eat plants (herbivores).  Even in areas where there is severe deer pressure, it’s unusual to see Wild Senna browsed.  But this strategy is not effective against all potential herbivores.  There are some butterflies and moths, including the Cloudless Sulphur,

Cloudless Sulphur caterpillar on Wild Senna (Senna hebecarpa)

Cloudless Sulphur caterpillar on Wild Senna (Senna hebecarpa)

Sleepy Orange,

Sleepy Orange butterfly

Sleepy Orange butterfly

and the Common Tan Wave, whose caterpillars are able to eat the leaves or other parts of Wild Senna.  These insects have evolved to specialize on these and other closely related plants, without being harmed by the chemicals that are toxic to other species.

Which brings us to an interesting back-up strategy Wild Senna employs for protection.  Wild Senna has extrafloral nectaries, a nectar source separate from the flowers. They are positioned on the leaf petioles (stems) near their attachment to the primary plant stem and adjacent to the flower buds.  Why would a plant species offer nectar if it’s not a lure for pollinators?  It takes energy and resources to produce nectar.  What’s in it for the plant to provide this service?  Who feeds here?

The egg-shaped bump is an extrafloral nectary on a leaf stem of Wild Senna (Senna hebecarpa)

The egg-shaped bump is an extrafloral nectary on a leaf stem of Wild Senna (Senna hebecarpa)

Wild Senna’s extrafloral nectaries attract a variety of visitors, many of them beneficial members of the ecosystem.  The Sweat Bee below may have stopped here before or after visiting Wild Senna flowers for their pollen.

Sweat Bee feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

Sweat Bee feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

Many lady beetle species, including the Seven-spotted Lady Beetle pictured here, help to keep the aphid population in check.

Seven-spotted Lady Beetle feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

Seven-spotted Lady Beetle feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

This jewel-like creature is a Perilampid wasp, one of several parasitic wasps that specialize on various insect species as their prey, including some other parasitites.

Perilampid wasp feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

Perilampid wasp feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

For Wild Senna, ants are probably the most beneficial visitor to this nutritious food source.

Ant feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

Ant feeding at an extrafloral nectary on Wild Senna (Senna hebecarpa)

Sugary substances like nectar are important food for ants.  But protein and other nutrients available from insects (including caterpillars) are also an essential part of the diet of most ant species.  Ants that are enticed to visit Wild Senna for its nectar can also hunt for and eat the insects that may be consuming the leaves or buds of the plant.  The placement of the nectaries between the leaf blade and flower buds is an advantageous location for protecting both plant parts.

Ant with caterpillar prey

Ant with caterpillar prey

You might think of the ants as an army of mercenaries paid in nectar to guard the plant, with as many caterpillars and other herbivores as they can catch as a bonus.  Ants will work for food!

Related Posts

Sleepy Orange Butterflies Overwintering in Pennsylvania

Cloudless Sulphurs are on the Move

Resources

USDA NRCS Plant Database http://www.plants.usda.gov/factsheet/pdf/fs_sehe3.pdf

Illinois Wildflowers http://www.illinoiswildflowers.info/savanna/plants/wild_senna.htm

Marshall, Stephen A.  Insects Their Natural History and Diversity.  2006.

Waldbauer, Gilbert.  What Good Are Bugs?.  2003.