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,
Wild Senna’s petals are open and distinct.
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.
Wild Senna has two sets of brown anthers, the male reproductive parts that disperse pollen. One set of anthers is in the center of the flower, when the bee in the photo above is dining. Those anthers have sterile pollen, there just to pay off the bee for visiting, not capable of fertilizing an ovule. While eating, the bee uses a technique called ‘buzz pollination’ or sonication that disperses fertile pollen from the lower anthers like salt from a shaker, dusting the bee’s lower abdomen and legs. When the bee visits another flower, she will likely touch the stigma at the tip of the curved green appendage in the photo above. This is where pollen must be placed to achieve pollination.
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,
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?
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.
Many lady beetle species, including the Seven-spotted Lady Beetle pictured here, help to keep the aphid population in check.
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.
For Wild Senna, ants are probably the most beneficial visitor to this nutritious food source.
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.
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!
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.