C. M. Packard, John H. Martin.
Growing resistant varieties is the ideal way to protect crops from damage by insects. When a good resistant variety is available, a farmer may not have to change his desirable cultural practices or pay the costs of insecticides.
Farmers, entomologists, and agronomists have been seeking good insect-resistant varieties of crops for many years. The resistance of the Mediterranean variety of wheat to hessian fly was reported shortly after 1819, when it was brought into the United States. Its resistance has been maintained up to now. In recent years exhaustive searches for resistant varieties and the breeding of resistant characters into otherwise desirable varieties have been important phases of over-all crop-improvement programs; a line of research that has been productive and promising.
Records of insect resistance in nearly 100 species of plants were listed in 1941 by Ralph O. Snelling. They included such diverse crops as beans, cabbage, corn, wheat, alfalfa, cane fruits, tree fruits, and forest trees. He also listed examples of resistance to more than 100 species of crop pests, including aphids, beetles, borers, caterpillars, flies, grasshoppers, leafhoppers, saw-flies, scales, and wireworms.
The word "resistance" is ordinarily used to mean the ability of a variety to avoid, tolerate, or recover from the attack of an insect to a greater degree than do certain other varieties. Resistance may be due to one or more characteristics in one variety or to entirely different characteristics in other varieties of the same crop. It may range from practical immunity down to moderate susceptibility, and the expression of resistance shown by any plant or variety may be modified by the environmental conditions under which it is grown.
Freedom from insect infestation or injury may be due to inherent characteristics of a variety such as hardness or toughness of tissues, hairiness of leaves and stems, or to lack of nutritive value for the insect in its tissues or sap. Hessian fly maggots, for instance, cannot attain normal size or grow at all in certain varieties of wheat even though large numbers of them reach their normal feeding position under the leaf sheaths. Varieties of corn with long, tight husks suffer less injury to the ears by the corn earworm than varieties with short, loose husks. Hairiness of soybeans makes them unfavorable to attack by leafhoppers. The granary weevil destroys the grains of soft varieties of wheat much more rapidly than it does the grains of hard wheat. These are examples of what properly may be considered true resistance.
The ability of vigorous corn hybrids to produce satisfactory yields when attacked severely by corn borers is a good example of insect tolerance. Certain strains of maize grow new roots readily to replace those cut off by the corn rootworm. Some varieties of sugarcane may do likewise after attack by white grubs. The ability to outgrow injury by an insect is a type of tolerance rather than inherent resistance. Nevertheless it is of value in reducing crop losses caused by insects.
The freedom of a variety from infestation may be due merely to the fact that it is in an unattractive stage of growth at the time the insect is laying its eggs or doing its feeding. Early varieties of cotton, or the earliest bolls formed thereon, for example, may escape serious infestation by the boll weevil and pink bollworm. Moderately late planted corn may be much less infested by the European corn borer than very early or very late plantings. The ability to escape insect infestation may be fully as important as actual resistance.
INSECT-RESISTANT VARIETIES are obtained by four general procedures :
1. Introduction of varieties, from foreign countries or elsewhere, which might provide a source of greater resistance than exists locally.
2. Selection of resistant strains from existing varieties.
3. Crossing resistant species or varieties with those not resistant but possessing otherwise desirable characters, followed by the selection of desired recombinations.
4. Grafting otherwise desirable but susceptible species or varieties on the rootstocks of resistant varieties.
Ladak alfalfa, which was introduced from northern India, proved to be resistant to the pea aphid when it was tested in the United States an example of how resistance to certain insects may be obtained by plant introduction, even though the resistance of the variety may be unknown in its original habitat.
The Kawvale variety of wheat is an example of selection as a method of obtaining insect resistance. A selection from Valley (Indiana Swamp) wheat, resistant to leaf rust, made by the Kansas Agricultural Experiment Station and the United States Department of Agriculture in 1918, was found to be resistant to hessian fly. The selection was named Kawvale and was distributed in 1932.
The breeding of the Rescue variety of hard red spring wheat in Canada and its introduction into the United States illustrate the use of crossing, selection, and introduction in the development of resistant varieties. It is resistant to the wheat stem sawfly, a pest of wheat in western Canada, Montana, and North Dakota. Rescue was developed after many years of intensive breeding by Canadian workers. A small lot of seed obtained from them was increased rapidly by the cooperative efforts of agronomists, entomologists, and growers in the United States and released to farmers. An interesting feature was the production of an extra crop during the winter in Arizona in time to make the seed available for spring seeding in Montana the same year. Rescue is widely grown in the infested areas of Montana, although its susceptibility to some forms of rust makes it less suitable for North Dakota.
The resistance of Rescue is ascribed in part to the fact that its stems are nearly filled with pith. Susceptible varieties have hollow stems that do not retard the activities of the sawfly larvae that develop inside the stems.
The grafting of European varieties of grapes on rootstocks of American varieties is an example of how resistance to a destructive insect may be gained by grafting. A plant-louse called the grape phylloxera, native to the eastern United States, attacks the leaves and roots of grapes. The varieties of grapes native to the East are nearly immune to the root-infesting form, but European varieties are killed or badly injured by it. After the insect was accidentally introduced into France about 1860, it nearly ruined the important grape-growing industry there. The industry was saved, however, by grafting the European grapes on resistant American rootstocks.
A special type of resistance, induced resistance, is listed by R. H. Painter in his book, Insect Resistance in Crop Plants. Induced resistance is obtained by applying to the soil fertilizers or insecticidal chemicals, which are taken up by plants through their roots. The ordinary conception of resistance does not include this means of reducing insect infestation, but the effects produced are similar to those resulting from natural resistance. R. G. Dahms, working in Oklahoma, reported that sorghum plants growing in soils low in nitrogen and high in phosphorus or chlorine are less favorable to chinch bug multiplication than soils high in nitrogen and low in phosphorus. Other workers have made similar studies on various crops and insects, but the information we now have does not indicate that a high degree of insect control can be obtained through the use of fertilizers. Several investigators have reported that the addition of parathion or certain other complex organic compounds to the soil makes corn, cotton, and other plants grown thereon practically immune to certain insects for several weeks. Insect control by means of insecticides that are taken up by the plants through their roots is still in the experimental stage, however, and cannot yet be recommended for practical use.
It has long been believed that vigorously growing plants well supplied with all needed food constituents are less subject to infestation or better able to withstand and outgrow injury than poorly nourished plants, but more evidence is needed on the subject. For certain insects the contrary may be true. The European corn borer, for instance, usually chooses the largest and most vigorous plants on which to deposit its eggs.
The discovery and improvement of insect-resistant crop varieties usually is not a simple procedure that the farmer can carry out for himself. Neither is it one that can be expected to solve all crop-pest problems and do away with all need of cultural and insecticidal methods of control. With certain crops and insects where little or no resistance is available, one has to continue using other control methods. The search for the desired resistance may require the testing of thousands of varieties, always with the possibility that none of them will prove to be resistant. Even after resistant varieties have been discovered, they are likely to have some undesirable qualities. In that case it is necessary to improve the resistant variety in other respects or to transfer the resistance to more desirable varieties by the tedious process of artificial crossing and selection. Fortunately, true insect resistance, when found, is transmissible from parent to progeny in accordance with the laws of heredity.
The process of crossing and selection ordinarily requires many years of cooperative teamwork by entomologists, agronomists, and geneticists in order to produce a commercially satisfactory insect-resistant variety. That would be a simple procedure so far as the insect resistance is concerned if readily visible plant characters responsible for or closely associated with it could be used in making the crosses and selections. In most instances, however, it has not yet been possible to find such characters. In an attempt to determine the nature of resistance to the chinch bug in sorghum, for example, the plants have been subjected to 30 or more chemical determinations, numerous studies of their structure, and hundreds of measurements of the growth, survival, and reproduction of the insects when feeding on different varieties. Despite this intensive research over a period of 30 years, it is still impossible to ascribe resistance in sorghum to chinch bugs to any specific plant character or constituent. It is therefore usually necessary to develop special methods of subjecting the varieties or selections to infestation by the insect in order to separate the resistant ones from the susceptible ones.