Thursday, August 22, 2013

Plants Play Defense

Slugs and snails and puppy-dogs' tails, 
That's what little boys are made of.

Snail- an armor coated slug- Wikimedia
Bob Korpella posted an story on slugs, snails and plants on his always interesting website.  Some plants seem to wait to release their predator defenses until there is a threat of attack.

Plants aren't able to run away from predators but have evolved a number of defensive mechanisms to fend off assaults, such as thorns and toxins.  Animals likewise can evolve ways of evading these defenses.  The whole process reminds me of when radar detectors were the rage.  The companies that made them also developed new forms of radar, then sold the heavy footed driver the latest new detector as they developed and even stealthier radar gun.

All lifeforms are deeply involved in the production of energy to reproduce.  Fighting off predators, running away or even producing a toxic chemical diverts precious energy from growth and reproduction.  A plant with a defense chemical that it can make of demand could save some of its energy.

Snails and slugs, which are snails without the hardware covering, leave a trail of slime as they crawl around, a source of fascination to little boys.  These are generalist herbivores, eating a wide variety of plants but they are especially fond of mustard plants so common in nature.  John Orrock of the University of Wisconsin–Madison* has shown that pre-treating mustard plant seedlings or seeds with slime produced plants that were less palatable to the snails and slugs.  The plant seems to hold its chemical defenses in reserve until it senses danger.

C. Barnhart
Plants response to animal stimulus is well known.  They can release volatile chemicals which repel potential predators but can also serve as an attractant to others.**  Black swallowtail butterflies tapping their feet, equipped with olfactory sensors, elicit a chemical response from plants in the carrot family, telling them that it is a plant necessary for their caterpillars growth.  As long as there are a number of plants available, a little caterpillar damage on each plant is tolerable.  Too many cats on just a few plants in your garden may be a problem.

Milkweeds and caterpillars are a good example of this evolutionary battle.  The plant has hairs on the leaves to discourage browsing bugs and their milky latex fluid has cardenolide toxins to help protect it from attack.  The monarch butterfly caterpillar is adapted to the toxin and even absorbs and concentrates it to protect itself from other predators.  Now studies indicate that "more recently evolved milkweed species use less of these preventative strategies, but grow faster than older species, potentially regrowing faster than caterpillars can consume them." Wikipedia

Species that are closely associated like milkweed and monarch are said to be co-evolved.  Monarchs require milkweed for their caterpillars to eat and grow.  They would disappear in a world without milkweed.  They have reached a balance of powers and are unlikely to ever completely destroy each other.  We, on the other hand have developed our own defensive chemicals as well as monoculture crops, capable of wiping out enough milkweed to end the monarch migration.  Now if  we could only learn to use our chemicals sparingly, like some plants can.

*    Eavesdropping Plants Prepare to be Attacked
**  Extensive information is here on Plant Volatiles as a Defense against Insect Herbivores

Like butterflies?  Try planting some fennel and let it grow.  You may be able to watch black swallowtail caterpillars go through their growth stages and develop into the next generation.  They eat a little of your fennel, dill or golden Alexander, birds and other insects eat a few of the caterpillars and the cycle of life goes on right in your backyard.
Black Swallowtail- C. Barnhart

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