Master Naturalists: Plants to the defense
By Paul and Mary Meredith
Aug. 15, 2013 at 3:15 a.m.
Animals faced with a predator go through known responses. Adrenaline enters their blood stream and surface blood vessels dilate to reduce blood loss if injury occurs. Respiration increases as does heartbeat.
This is often referred to as a "fight or flight" survival reaction. If a plant is attacked, it cannot see or feel and can neither run nor fight back to prevent being eaten by an insect or herbivore.
Flight is impossible, but it turns out that plants have developed a formidable range of fighting capabilities, both physical and chemical.
We've all seen prickly pear with its spines to ward off herbivores like cattle. Agarita, a highly heat-tolerant plant, is a rounded shrub with holly-like foliage and clusters of fragrant yellow spring flowers producing beautiful, sweet red berries that birds love and pioneers used in jellies.
But woe to those who pick them because their twisted leaflets are armed with sharp spikes that discourage even browsing deer. Citrus trees are also great spike/thorn producers. Our orange produces lethal two- to three-inch thorns to ward off browsing animals.
Some plants produce chemicals to make themselves taste bad. Others produce toxic chemicals that kill or impact an insect's development, like tobacco does.
Monarch butterfly larvae eat milkweed leaves, which contain cardenolides that taste really bad and are toxic, impacting heart function in most animals. Monarchs are different, storing the toxins to sicken birds attempting to eat them.
Plus, one type of grasshopper, Poekilocerus bulonius, is immune to milkweed cardenolides. It stores them in a gland and sprays them on rodents and other predators when attacked.
The final part of the milkweed-and-monarch story is found in Mexico where the butterflies winter. Most birds there leave them alone.
However, two birds, black-backed orioles and black-headed grosbeaks, account for about 60 percent of monarch mortality in Mexico. Why? Adaptation.
Grosbeaks have developed insensitivity to the toxins and eat the entire monarch. Orioles, who are sensitive, delicately disassemble the Monarch body, eating only the muscles and abdominal contents but not the wings or cuticle (the exoskeleton) where toxins concentrate.
Immediate chemical defenses
As a plant, how do you defend when attacked? Nature has developed some neat tools. Plants do not feel. So how do they know when to defend? Some produce chemicals that a caterpillar eats and digests.
Those same plants have chemical receptors that can detect those chemicals, on a predator's mouthparts, prompting the plant to produce yucky-tasting stuff to drive off the insect or give it indigestion. Tomatoes do this, and those same chemicals make the fruit taste better to humans.
Plants can even mimic insect chemicals. Aphids, when attacked, exude a chemical that signals other aphids to flee. Some plants, when attacked, produce the same chemical, fooling the aphids into thinking they are under attack.
Other plants produce chemicals that impact maturation. One, in fact, causes molting caterpillars to develop three head sections, thus causing the insect to starve because its mouth isn't connected to its gut.
In movies - when attacked, call for the cavalry. Detect you're being attacked by a caterpillar? Exude a chemical that attracts a carnivore like a wasp or a sucking bug (also known as an assassin bug) that eats that caterpillar.
Corn farmers that have army worms in their fields report that their corn fields smell "sweet." That defensive odor attracts natural predators to control the worms.
The United States Department of Agriculture researchers are studying this as a natural control for pests versus using pesticides on crops.
Texas Native Plant Database,
The Chemical Defense of Higher Plants, Gerald A. Rosenthal, www.uky.edu/~garose/link100.htm
Plants Under Attack: Plant Biologists Discover Plant Defenses Against Insects, http://www.sciencedaily.com/videos/2007/0906-plants_under_attack.htm
Constitutive plant toxins and their role in defense against herbivores and pathogens; Wittstock U; Gershenzon J.; Current Opinion in Plant Biology.
Excellent background material and references:
Plant defense against herbivory, http://en.wikipedia.org/wiki/Plant_defense_against_herbivory
Paul and Mary Meredith are master naturalists. Contact them at email@example.com.