Lecture on Plants and Arthropods by Kimberly Grace

Did you know that the “fresh grass smell” so many people know and love is actually a call for help?

I’ll explain, but first let’s look at a bit of history.

The Bad
Many arthropods are well known for their eating habits.  They dine on many of our important crops, and, in fact, because of them we experience between 10 to 30 percent crop losses every year. You have probably heard of many examples of plant eating insects.  Aphids, mites, nematodes, and various caterpillars are all problematic to our plant friends.

These guys are just the first chapter to a very interesting and complex story, because, you see, the plants fight back.

The Fight is On!

Plants fight back in one of two ways, and sometimes a combination of both.  The first way is with a physical defense system.  For example, plants like cacti have prickly spindles that help deter the pests that may attempt to eat them.  The second defense is a chemical response.

When an organism eats a plant, they often have a chemical compound in their saliva that the plant responds to.  Two common ones are volicitin and inceptin.  The plant detects this compound that comes from the herbivore and responds with its own chemical defense system.  These defense compounds work both directly and indirectly.

The direct method involves an induction of defenses that produce alkaloids and other toxins that are harmful to the herbivore.  Chemicals such as nicotine and cyanide are made by plants specifically to help ward off pests.  The indirect method is a “call for help” to other arthropods that prey on the herbivore.  This is what causes the “fresh cut grass” smell that we love.  It is called an induced indirect defense.

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Sometimes this alliance between arthropods and plants takes on a more long term agreement.  Creatures like ants will oftentimes reside on plants such as an acacia.  The plant provides shelter or some other kind of benefit for the animal, and, in turn, the animal protects the plant.

Mutually Beneficial

Continuing with the theme of the animal/plant alliance, the relationship between arthropods such as bees and butterflies, the relationship goes a bit further; the plant and the pollinator.  Plants attract pollinators with their flowers through fragrance, color, shape, and patterns.  Bees are more drawn to colors like blue and green due to their photo-receptors being more sensitive to UV light than humans.  Plants reflect or absorb that UV light to make themselves more visible to their pollinators.

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Pollinators are important to plants because they depend on them for reproduction.  In fact, approximately 84% of commercial crops depend on them.  As a reward for their hard work, pollinators get a nice bit of nectar or protein-rich pollen.

“The Plant that Cried Wolf”

Sometimes organisms are naughty.  Some of them are con artists.  The large earth bumblebee (Bombus terrestris) is one of these organisms. These bees will tear a hole in the corolla of the plant and steal the nectar without collecting the pollen to share with another plant.  As of yet scientists don’t know why they do this, but it certainly breaks the mutualistic agreement.

Another organism that undermines the symbiotic relationship is the cabbage plant.  Normally plants send out chemical signals that is a match to the number of herbivores that is invading it.  Not so with the cabbage.  Whether it is one cabbage- eating caterpillar or 10, the response is the same – large amounts of volatiles that call in a large number of parasitic wasp cavalry.  If 10 wasps show up and there is only one worm, nine wasps just wasted a lot of precious energy.

But, nature is smart.  The wasps have learned to adapt to this deception and now only respond to low levels volatiles.

Seeing it First Hand

On our last trip to the UAB Gardens, we were able to see this arthropod and plant interaction first hand.  We came across a bean plant that was alive with ants and aphids.  The aphids were eating the plant, and, using a chemical signal, called the ants to come in for aid.  In turn, the ants appeared and preyed upon the aphids.

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We also saw a great many examples of the symbiotic relationship between pollinators and plants.

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There are many strange and wonderful relationships on this earth, but the ones concerning plants and their arthropod companions are some of the most interesting.

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