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Morphological Mechanisms of Crop Resistance to Insects

By Dan Stein

Statement "Organic farmers tend to pick plants that are insect resistant and so higher in natural pesticides. Most celery has about nine hundred parts per billion of two natural carcinogens. But one variety might have two thousand parts per billion, another two hundred parts. Someone just introduced a new celery into the United States with nine thousand parts per billion. The organic farmers love it"

Response. Farmers, regardless of whether or not they use synthetic pesticides, have made use since agriculture 's beginnings of crop varieties that are resistant to insects and diseases. These resistant varieties do not necessarily rely on the production of "natural carcinogens" to achieve this resistance. Many morphological characteristics have been used in plant breeding to reduce pest abundance and damage. --Ed.

The relationship between plants and insects is an ancient and fascinating one. Over the years- plants have developed numerous strategies to make them resistant to predation by insect herbivores. Insects, at the same time, have figured out ways of circumventing or detoxifying the defense mechanisms in plants. This interrelationship is dynamic and ongoing.

The study of the interrelationships between the various insect and plant species was labeled "coevolution" by Ehrlich and Raven2 in 1964 and this name took. One model used to look at coevolutionary dynamics is think of it as an interspecies arms race" where each insect and plant species tries to develop a weapons system that the other cannot match. The weapons in this case are ecological, biochemical, and morphological adaptations.

This article gives some brief examples of morphological characteristics that have been used in crop plants to meet the demands of this "arms race" and to breed for resistance to insect pests.

Examples of Morphological Traits that Confer Crop Resistance

All plants must assume forms that allow certain metabolic functions to take place, including photosynthesis, and orderly growth. But in addition, some plants have acquired morphological adaptations that make them at least partially resistant to insect predation. These adaptations can be broken down into the following categories. Keep in mind that there are no resistance adaptations that hold true for all plants and specific examples are hard to point to with certainty.

Color

Certain colors are less attractive to certain insects. For example, imported cabbage worm is less attracted to red colored Brassica species (cabbages, broccoli, and related species). Cucumber beetles do less damage on reddish colored varieties of leaf lettuce and are attracted to certain hues of yellow. Some birds prefer red plums to green plums.3

Shape:

While it is impossible to generalize what shapes resist predation better, shape does play a role in avoiding predation. For example, one study noted that thick rooted turnips were less damaged by turnip maggots.3 Another study showed that onions with leaves having a narrow angle of contact are more attractive to thrips than onion varieties with looser leaves.4

Thickening of cell walls and/or rapid growth:

In response to predation, some plants create tougher leaves or callous tissue. Both corn and soybeans have been noted to increase growth in response to feeding by certain aphids.3

Hairiness:

Many plants utilize trichomes (plant hairs) to protect themselves against predation. A few examples of insects that are at least partially suppressed by plant hairiness include bean aphid and potato leaf hopper on beans, two spotted spider mite on strawberry, and whitefly on tomatoes, peppers, and potatoes.3

Surface waxes:

Waxy leaf surfaces provide protection against some insects, cabbage flea beetle on Brassicas, for example, but may encourage other insects (i.e., cabbage aphids).3 i

Anatomical adaptations:

For example, corn with very tight husks is somewhat resistant to corn ear worm.3 Corn varieties with tough, resilient stalks can tolerate burrowing by corn borers with breaking and causing yield loss. A variety of wheat with a solid stem does no~t allow sawfly larvae to bore through the stems and reach their feeding sites.4

Toxicology of Plant Defensive Compounds

It is certainly of concern whether the various substances that protect plants from insects may be harmful to humans. Most have not been tested for either acute or chronic toxicity in humans, including tests of their ability to cause cancer.

As our knowledge of secondary plant substances increases, we may choose to exclude certain foods from our diet, especially if we have allergies or certain diseases. We may also wish to eat more foods containing certain secondary plant substances as we learn that they contribute to good health. But given the large number of ways in which plants are resistant to insects and diseases, it is an oversimplification to state that resistant varieties must contain higher concentrations or a greater number of toxic compounds.

References

1. Moseley, Bruce. 1991. Interview: Bruce Ames. Omni 13(5):7~80, 103,106, February.

2. Ehrlich, P. and R Raven. 1964. Butterflies and plants: a study in coevolution. Evolution 18:586-608.

3. Norris, Dale, and Marcos Kogan. 1980. Biochemical and morphological bases of resistance. In F.C. Maxwell and P.R Jennings. (eds.) Breeding plants resistant to insects. New York, NY: John Wiley and Sons.

4. Yepsen, Roger B. (ed.). 1984. The encyclopedia of natural insect and disease control Emmaus, PA Rodale Press.

Citation for this article: Stein, Dan, 1991, "Morphological mechanisms of crop resistance to insects", Vol. 11, No. 2, Summer 1991, pp. 15

Copyright 1991 Northwest Coalition for Alternatives to Pesticides.

Reprinted with permission.


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