Defenses, Physical

Angiosperms and seed-bearing conifers provide a source of nutrients and habitat for large grazing mammals and birds as well as numerous species of insects and related arthropods. For many insect herbivores , the entire life cycle takes place on or within plant tissue. The evolutionary success of these plant taxa , the most conspicuous plants of our geological age, is due, in part, to their ability to adapt to a broad range of environments as well as their development of defenses against vertebrate and arthropod herbivores. Physical adaptations evolved by these vascular plants to defend against herbivores range from simple structural barriers to complex changes in anatomical form. Plant organs at which major defensive interaction with herbivores occurs include stems, leaves, and reproductive structures.

External Defenses

The leaves, stems, and aerial reproductive organs of vascular plants provide a large surface area and many opportunities for use as food or habitat by herbivores. Physical defensive adaptations are associated with plant color (controlled by tissue pigments, internal leaf structure, and the nature of the leaf surface), surface waxes, pubescence , the presence (or absence) of specialized glands, and anatomical adaptations involving plant shape and form.

Plant Color.

Herbivorous insects sometimes use leaf and flower color as an aid in finding suitable host plants for food or for egg laying. Red foliage of cotton (Gossypium ), cabbage (Brassica oleracea ), and Brussels sprouts (Brassica oleracea, variety gemmifera ) is discriminated against by the cotton boll weevil, imported cabbage worm, and cabbage aphid, respectively.

Surface Waxes.

The epicuticle of plant tissue is composed of surface waxes that protect against desiccation and often provide defense against insect attack and disease pathogens . Defensive waxes on plants of the mustard family (Cruciferae), such as cabbage, contain chemical compounds that repel pests such as flea beetles. The spatial orientation of waxy plates and rods on the leaves of resistant Brussels sprouts interferes with locomotion and adhesion by flea beetles. In some species of raspberry (Rubus ), thick secretions of surface waxes act as a physical barrier by restricting aphids from successfully reaching the phloem with their sucking mouthparts.

Pubescence.

Pubescence, the specialized epidermal hairs (trichomes) of vascular plants, plays an important physiological role in water conservation but also forms a physical barrier to use by herbivores and constitutes the last line of external defense. The sharp spines of cacti, the thorns of Acacia, wild rose (Rosa ), raspberry, and the irritating hairs of stinging nettles (Urtica ) create a painful barrier to grazing mammals and human interference. Trichomes that defend against insects and related arthropods also provide mechanical barrier protection, but some have evolved to entrap and immobilize these pests. Trichomes can interfere with insect adhesion and locomotion on the plant surface; trichomes rich in indigestible silica and lignin are nutritionally harmful and may lacerate the gut following ingestion. Highly specialized glandular trichomes can secrete substances toxic or repellent to insects through contact, ingestion, or inhalation. Although not defensive in the strict sense, glandular trichomes and extrafloral nectaries of carnivorous plants such as sundew (Drosera ) secrete substances that not only entrap but aid in the digestion of prey arthropods.

Specialized Glands and Anatomical Adaptations.

In many plant species leaves and flower buds bear nectar-secreting glands, probably to encourage visitation by ants or to attract pollinators such as bees. In cultivated cotton, adult moths of major pest species such as the cotton bollworm and pink bollworm are also attracted to the sugar-rich nectar. A wild species of cotton that lacks these glands has been used to breed nectarless varieties resistant to pest attack.

Some grain-producing plants such as corn (Zea ) protect their seed by completely surrounding it with long and tight wrapper leaves (the husk) that interfere with penetration through the silk channel by corn earworm larvae. In cotton the flower bud is normally enclosed by three overlapping modified leaves (bracts), creating a moist, enclosed environment favored by the female cotton boll weevil for feeding and egg laying. In contrast, cotton varieties with the Frego bract condition are resistant because the narrow, twisted bracts create an exposed, unattractive environment for the weevil. An added benefit of the Frego bract trait is heightened weevil mortality resulting from greater exposure to weather extremes and natural enemies as well as improved penetration of pesticides applied by cotton farmers.

Internal Defenses

While the bark of woody perennials serves as a line of defense to most potential herbivores, some insects such as bark and ambrosia beetles can readily penetrate this barrier; their larvae feed just under the bark within the phloem tissue producing a distinctive pattern of tunnels often diagnostic for the species, for example, Dutch elm beetle. Feeding tunnels interfere with the normal transport of nutrients; disease pathogens introduced by the initial entry of the adult beetle further weaken the tree. Some species of conifers (Pinus ) have evolved a form of hypersensitive response to bark beetle attack by increasing their production of oleoresin, effectively drowning or entrapping the insects within their brood chambers. Resin of resistant conifers is also characterized by high viscosity and rapid crystallization that further enhances its entrapping ability.

Another form of internal defense involves accelerated growth or replacement of damaged tissues to physically crush the invading pest (as in the case of cotton varieties resistant to the pink bollworm), or to restrict the passage of the invader through plant tissue, as occurs with galls produced by some plants in response to the entry of insects and mites.

Tissue strength and toughness are important internal defenses against the wheat stem sawfly, a significant pest of wheat (Triticum ) in North America. Larvae feed within the semihollow stems of the wheat plant creating tunnels that cause the stem to break or lodge, effectively reducing grain production. Some species of wild wheat have solid stems that impede the feeding and tunneling of sawfly larvae. This characteristic has been bred into modern wheat varieties to provide resistance to the sawfly. Other examples of plant breeders using wild relatives of crop plants to create barriers against tunneling insects by increasing the stiffness or toughness of stems include sugarcane (Saccharum ) resistant to the sugarcane borer, zucchini squash (Curcubita ) resistant to the squash vine borer, and rice (Oryza ) resistant to the rice stem borer. In the case of rice, stems of resistant varieties have high levels of abrasive silica that causes accelerated wear of insect mouthparts, reducing feeding efficiency and limiting the number and length of tunnels within the stem.

see also Cacti; Cotton; Defenses, Chemical; Dutch Elm Disease; Interactions, Plant-Insect; Interactions, Plant-Vertebrate; Trichomes.

Ward M. Tingey

Bibliography

Norris, Dale M., and Marcos Kogan. "Biochemical and Morphological Bases of Resistance." In Breeding Plants Resistant to Insects, eds. Fowden G. Maxwell and Peter R. Jennings. New York: John Wiley & Sons, 1980.

Tingey, Ward M., and John C. Steffens. "The Environmental Control of Insects Using Host Plant Resistance." In Handbook of Pest Management in Agriculture, 2nd ed., Vol. 1, ed. David Pimentel. Boca Raton, FL: CRC Press, 1991.