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The Garden Scoop

Ants and the Partridge Pea

This past week, I visited several Central Illinois prairies to catch a last glimpse of waning flowers and look for pollinators.  I was pleasantly surprised to see an old favorite in full bloom as the beautiful and minute, yet brightly yellow flowers of Partridge pea (Chamaecrista fasticulata) filled the prairie edges, adding a speckling of color.   Partridge pea is an annual, native plant that frequents prairies along with a wide range of other locations such as, abandoned fields, railroads, roadside ditches and other disturbed areas.

In a prairie setting, partridge pea is an early invader, as is the case with many other annual plants, which capitalizes on some kind of disturbance (fire, mowing, soil disturbance, etc.) to explode from seed and occupy a space in the plant community.  It is a legume and a nitrogen fixer, meaning its roots have the ability to add nitrogen to the soil, with important soil building attributes.  As other, more perennial prairie plants become established and compete with partridge pea for light, it is fairly quickly shaded out of the mix, fading away until the next disturbance stimulates its seedbank.

Partridge pea has been found to occur naturally in nearly every county in Illinois, with exception of a few northern counties.  Similar to its behavior in prairie, it invades human-dominated areas in response to disturbance, giving it a wide variety of potential habitats and making many consider it as a weedy plant.  However, I have always found it to be a bit a late summer beauty as its yellow flowers add interest to otherwise weedy, disturbed areas or newly established prairie.

Earlier this week in the prairie, as I examined the plant closer to look for insect life that may be helping with pollination, I was reminded of a very unique and interesting aspect of this plant.  One of the reasons why I’ve observed and admired it in the past.  The plant has specially adapted nectar producing glands which are located at the base of each leaf near the stem.

In most cases, nectar glands (called nectaries) are located within flower structures to attract insect and animal pollinators who use the nectar as food, and in turn, pollinate flowers for the plant.  It is a mutually beneficial relationship that drives pollination across many plant communities.  However, nectaries physically apart from the flower, called extrafloral nectaries, serve a different and fascinating function in plant ecosystems.

Worldwide, over 2,000 plant species across 64 families have been identified with extra floral nectaries.  In Illinois, these plants occupy a wide range of habitats, from herbaceous prairie plants, like partridge pea, to native trees and shrubs like catalpa (Catalpa speciosa), wild black cherry (Prunus serotina) and elderberry (Sambucus canadensis).

Similar to nectaries in flowers, extrafloral nectaries attract insects to benefit the plant.   The benefit to the insects in both cases is food, although the plant reward is quite different.  Nectar secreted from extrafloral nectaries differs slightly from floral nectar and has been shown to attract beneficial insects for plant defense.  These beneficial insects, such as wasps or ants, defend the plant against herbivorous insects. 

Nectar feeding ants are attracted to extrafloral glands for their sweet reward, frequenting the plant during times of nectar flow.  These ants defend against leaf herbivores and seed predators through their aggressive, defensive behavior or by removing herbivore eggs directly from the plant.

On partridge pea, extrafloral nectaries are active throughout the growing season, attracting nectar feeding ants to leaves.  Research has shown that visitation from these ants reduces herbivore numbers which decreases leaf area loss to herbivory, increasing plant growth. 

Interestingly, the peak ant feeding activity on partridge pea coincides directly with peak herbivore activity during the growing season.  Although unmeasured in studies I reviewed, I have to wonder if nectar production in these extrafloral glands is timed directly with the peak occurrence of insect herbivores. 

In catalpa trees, nectar production from extrafloral nectaries on leaves has been shown to peak when herbivore larvae hatch.  In addition, nectar production increases as leaves are damaged from feeding insects.  What a remarkable and dynamic plant-animal relationship!

I am constantly amazed and impressed by the complex relationships between plants and animals in nature.  It is truly awe-inspiring to think about all the vital interactions that occur each year between insects as small as ants or as large and showy as pollinating butterflies to perpetuate our native ecosystems.