Observe the arms race between animals and plants

By Gary Raham
Writer and Illustrator

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As you tend the domesticated custom-designed vegetables of your garden, it's easy to forget that plants come armed with potent defenses. They are veritable chemical factories, producing repellents, poisons and even contraceptives of sufficient variety to impact the behavior of insects, birds, mammals, worms and mollusks.

While humans may breed tasty plant "wimps" with few toxins and irritants, or cook plants until their noxious qualities are disarmed, our animal comrades have discovered equally clever "work-arounds," even if it may have taken them a few million years longer to do so. This pitched and ongoing battle between producers and consumers can provide great entertainment every season while you're pulling out weeds, fertilizing your plants of choice or turning soil in the garden.

Consider milkweed for a moment. Here is a plant with loads of defenses, both mechanical and chemical: It "bleeds" gooey white latex from a tube network that parallels leaf veins. Careless insects that sever a latex tube can get their mouthparts so gummed up that they are effectively muzzled.

The red and black-spotted longhorn leaf beetle solves this problem by severing the large latex tube near the leaf's midvein. Latex pours out and she quickly cleans her mandibles. Now she can move to the leaf tip and munch all she wants, as the tributary tubule network has been cut off from the source of goo. If you find a milkweed leaf tip with a notch out of it, look along the midvein to discover where the beetle cut the supply line. Some ladybird beetles, both adults and larvae, perform similar operations by trenching their way across a series of smaller veins, then dining farther out along the leaf.

The milkweed's chemical defenses are also formidable. Their sap contains alkaloids, most notably cardiac glycosides similar to digitalis, that are bitter and also speed up the heart rate of birds and mammals. The bitterness repels many creatures that attempt to eat the plant.

Monarch butterflies and a few other creatures have digestive enzymes that can handle the compounds. (Monarch caterpillars also practice the vein-cutting trick.) The monarch goes one step further by appropriating the glycosides to use for its own defensive measures. The glycosides are transferred from larvae to adults and end up concentrated in the monarch's showy wings. Birds that nip a wing usually stop with one taste and avoid monarchs (and other butterfly look-a-likes) thereafter.

Trees, ferns and other long-lived plants with a venerable heritage often produce bitter-tasting compounds called tannins and repellents called terpenoids. Over the course of Earth's long history, some animals like pigs, squirrels and certain woodpeckers have developed cast iron digestive systems seemingly impervious to tannin-laden acorns and other delectables. Humans either soak raw nuts to wash out tannins or actually seek herbal teas, dry red wines and certain ciders for their briskness and bite.

As every murder mystery writer knows, plants can produce some potent poisons like strychnine, cyanide, atropine and curare. This strategy seems to be the technique of choice among soft-stemmed and herbaceous flowering plants--recently evolved and fast-growing plant types that need to discourage their animal antagonists quickly. Even raw parsnips and soybeans contain nasty compounds that would not be part of our diets without cooking.

Many insects can eat or even specialize on plants with poisonous defenses when they have evolved the correct oxidases and epoxidases in their digestive enzyme mix. Polyphagous caterpillars (eaters-of-many) have a greater variety of such enzymes than oligophagous ones (eaters-of-few). Painted lady butterflies, for example, which fall into the first group of eaters, can consume thistle, sunflower, alfalfa and other plants. Checkerspot butterflies, whose enzyme arsenal is more limited, restrict their diet to turtlehead plants and close relatives.

Some mammals like the North American moose have picked up bacterial symbionts in their gut that detoxify alkaloids and other compounds in their diet. Moose have even learned to shun the leaves of plants that contain methyl salicylate, a compound that is harmful to their bacterial buddies.

It almost seems as if plants have gone too far when they mess around with an animal's sex life, but scientists have found examples of the practice that impact both insects and mammals. Balsam fir, for example, releases an insect juvenile hormone, so named because it keeps the insects from maturing and thus reproducing. Other compounds called precocenes can force insects into a premature (and fatal) adulthood.

Some time ago it was found that certain clovers produced an estrogen mimic that could induce sterility in sheep who grazed on them. In the western United State, certain grasses produce phytoestrogens--especially in late summer and fall - that effectively provide birth control to meadow voles during the season when the plant's seeds are most at risk. The phytoestrogen concentrations wane in spring and vole populations increase in step with the most vigorous growth of potential food plants.

And the beat goes on, as Sonny and Cher were fond of pointing out. The plants mix up new chemical brews to discourage those who would eat them, and the hungry eaters find new ways to get a meal. Try and observe some of this animal/plant arms race as you work in the garden this season. I won't blame you if you glance over your shoulder at the parsnips or sneak carefully away from the tomatoes on the way back to the kitchen.

References:

Eisner, Thomas. "For the Love of Insects." Cambridge, Mass.: Harvard University Press. 2003

Grant, Susan. "Beauty and the Beast. The Coevolution of Plants and Animals." New York: Charles Scribner's Sons, 1984.