ENHS Nature Trails November 2000 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Of Insects and Plants: A Biological Arms Race

Insects and plants are perennially locked in deadly battle. Each spring a new generation of insects emerge from their winter sleep ready to grow and multiply. To achieve these twin goals, newly emerged insects must first fill their bellies as quickly as possible, usually at the expense of an unsuspecting plant. The voraciousness and sheer number of these six-legged herbivores puts the host plant in severe and sometimes mortal danger. A good example is the cinnabar moth, imported into this country to eradicate another visitor to our shore, tansy ragwort. The tansy plants around our house, abundant and large this year, were gnawed to the ground within days by the mid summer hatch of cinnabar caterpillars. Tansy is an annual and hence doesn't survive long regardless of the magnitude of its local pest population. But what about long-lived plants such as trees. How do they survive the annual attack of arthropod munchers seeking a scrumptious feast?

To fend off attack, trees have evolved a variety of elegant and complex chemical defense systems. The first line of defense are chemicals made by the tree for other reasons which happen to also be noxious to insects. For example, lectins normally act as storage proteins in seeds yet also render the seed unpalatable to hungry insects.

Because these first-line defenses are always present in the plant, some insects have evolved ways of coping with these noxious chemicals. Some develop an insensitivity to the noxious compound while others have evolved clever strategies to isolate the compound before it does any serious damage. The tobacco hawkmoth normally feeds on tobacco plants, which have high enough levels of nicotine to render most insects senseless or worse. The tobacco moth however feeds happily on tobacco leaves because it has a much reduced sensitivity to nicotine. For good measure, it also sequesters nicotine in a special gland before breaking it down into a harmless by-product.

Plants, albeit slow, are not stupid. They have evolved other, more sophisticated defenses to protect themselves from insect attack. One such defense mechanism involves the rapid production of a toxin in response to the first signs of insect chewing. Potato and tobacco are each able to produce toxins throughout the entire plant within hours of even one leaf being attacked by a chewing insect. These toxins make the leaves indigestible and/or unpalatable to the herbivorous insect.

A twist on the above scheme is to release large amounts of a toxic substance in response to an attack by a predator. Conifers, for example, release copious amounts of resin when attacked by beetles. This resin is liberated from attack sites and covers the attackers, embalming them for eternity.

Other plants cope with insects adapting to their chemical weapons by sequentially producing a variety of chemicals over the course of a season. The long lived mountain birch fends off its insect pests by producing 150 different chemicals that fluctuate from June through September. For instance, budding leaves produce gallotanins, phenolic compounds that bind together proteins in the insect gut and make digestion nearly impossible. Growing leaves produce another tannin that makes insect mouth parts quite sticky, requiring the insect to stop feeding and clean itself. Finally, the mature birch tree produces a host of new chemicals in late summer that deter insect larvae from feeding by toughening leaf structures.

My favorite plant defense is the "send-in-the-Marines" strategy. Amazingly some plants appear to call for help in order to attract predators of their predators. For example, some plants release volatile signals in response to predation by insect larvae. These signals attract parasitic wasps who inject one or multiple eggs into the unsuspecting larvae. The eggs hatch within hours of injection, dooming the host to an untimely death as the wasp larvae feed on internal tissues.

Plants may be sedentary but out of necessity they have evolved an amazingly complex array of defense mechanisms. And who says biology is boring?

Nathan Tublitz
Professor of Biology
Institute of Neuroscience
University of Oregon
Eugene OR 97403
Phone: 1-541-346-4510 FAX: 1-541-346-4548

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