San Antonio Express News
Sunday, June 22, 2003
By Lynn Rawe
All of us are concerned about the after-effects caused by our use of pesticides. Some plants are naturally pest-resistant and scientists are researching these varieties in hopes of finding new methods of pest control to reduce pesticide use. Some plants defend themselves through morphological and physiological mechanisms. Plant breeders are using this information to develop pest-resistant plant cultivars.
The morphological characteristics that seem to strengthen the resistance of a plant are color, thickness of the cell walls and plant tissue, surface wax, spines and trichomes. These mechanisms can afford a plant some measure of defense by being less attractive visually or by presenting formidable physical barriers to the insects.
Some plants have specific color-related resistance. Most insects are attracted to leaves in the yellow-green color range. Healthy, dark green leaves are less attractive than yellowing plants under stress. In a study performed by Drs. Maxwell and Jennings, 13 pea varieties were observed. They found that the pea aphid preferred yellow-green plants to the dark green ones. Another example is the variety of Brussels sprouts called “Rubine.” The red foliage of the plant is less attractive to the cabbage worm than are the green varieties.
Thick cell walls and plant tissue help a plant to resist the tearing action of chewing mouthparts and the penetration of a proboscis or ovipositor of insects. Thick cuticles, or the tissue that forms bark helps to protect some plants from insect attack.
The cuticles of a lot of vascular plants are covered with a thin layer of largely hydrophobic constituents, or “wax.” The wax functions as a mechanism of water balance within the plant and contains a substance that inhibits and interferes with insect attack. The wax surfaces of raspberries have a highly acidic substance that repels certain beetles.
Hairy leaves or trichomes, act as an effective insect deterrent. The hairs are a collective covering referred to as “pubescence.” The insects experience difficulty feeding on and ingesting plants with these small hairs. African violets, grown in their natural habitat, use these hairs as a defense mechanism. Hollies, with their spiny leaf edges, are able to keep the larvae of foliage feeding moths from devouring whole leaves after eating only the spiny edges.
Physiological defenses have long been studied by scientists. In searching for the reasons why some plants are resistant to insects, scientists have found that certain plant-produced chemicals seemed to deter certain pests. “Secondary metabolites” is the name given to this array of plant compounds. There are at least 30,000 such compounds known to date.
Plants are able to store toxic chemicals for defense against pests without affecting their own health. This can be done by using “storage compartments” located in the cell wall and the vacuole. These areas are located away from metabolic areas of the plant, thus preventing toxins from poisoning it. Latex and resins are stored in cell parts as one of nature’s defenses. Also, specialized glandular hairs may be devoted to the storage and secretion of toxins, such as stinging nettle.
One common compound, known as pyrethrum, is derived from the blooms of an African chrysanthemum. This compound has commercially been used as an insecticide against leafhoppers, aphids, caterpillars, various beetles, and a wide variety of other insects.
Some conifers have resins that resist certain bark beetles and borers. Some plants produce the compound, limonene, which is found in the peel oil of grapefruit and other citrus fruits. It is now common knowledge that citrus oil repels a number of insects including mosquitoes.
Azadirachtin, a limonoid derivative, has become one of the newest and most popular commercial biological insecticides. Azadirachtin is a growth regulator that is extracted from the seed of the Neem tree that grows in Asia. When insect larvae ingest the Neem extract, they stop eating and lose their ability to molt. This disruption of the insect’s life cycle causes death within days.
The ability of some plants to defend themselves against their enemies is amazing. More research is being done on secondary metabolites which will evolve into producing plant cultivars that are more insect resistant. This study may, in the long run, contribute to reduced pesticide use and a safer environment.
This article was written by Lynn Rawe, CEA – Horticulture, with Texas Cooperative Extension in Bexar County.