"The injury and resulting losses caused by the potato leafhopper to alfalfa and red clover in the eastern half of the United States apparently have been far more extensive than has been realized," F. W. Poos and H. W. Johnson wrote in 1936. The statement is still true. They and other workers have found considerable differences in the degree to which the insect injures different varieties of alfalfa, red clover, sweetclover, and soybeans. In red clover and soybeans, the differences are associated somewhat with the hairiness of the different varieties. The possibility of finding and making use of resistance to the potato leafhopper as a pest of leguminous crops needs further investigation.

Resistance in potatoes to several kinds of insects has been reported. Sequoia is resistant to the potato leafhopper and the potato flea beetle. Some other varieties, individual plants within varieties, and other species of plants closely related to the potato are said to be resistant to the leafhopper or moderately so. Extreme differences have been found among potato varieties in resistance to the green peach aphid, which attacks potatoes as well as peaches. Some varieties are less injured by the potato psyllid than are others, and the tubers of some varieties are less subject to injury by wireworms than are those of others.

Cotton has several pests, and varietal resistance to some of them has been observed. Severe damage by leafhoppers was prevalent in South Africa, India, and Australia until varieties highly resistant to them were developed. Resistance to the pink bollworm in certain wild species of cotton has been bred into some cultivated varieties by hybridization and selection. Red-leaved varieties less subject to boll weevil infestation than some of the green-leaved varieties and hairless varieties less infested by plant-lice than hairy varieties have been reported. The available evidence on the relation between hairiness and plant-louse infestation has been conflicting, however. In studies of thrips, some varieties of cotton have shown less injury than others. Early-maturing varieties or the early plantings are more likely to escape severe infestation by the boll weevil and some other insects than late varieties or plantings. Resistance to several other insect pests of cotton has been recorded.

Considerable resistance to the sugarcane borer has been discovered in several varieties of sugarcane. A few of them are in commercial use. Some 25,000 pedigreed sugarcane seedlings, representing more than 225 parent varieties, have been tested for borer resistance in Louisiana or Florida since 1940. Of the commercial varieties, C. P. 34/120, C. P. 34/92, F. 31/962, C. P. 36/18, Cl. 38/32, F. 40/96, and C. P. 28/19 are resistant. C. P. 34/79, which has been released for commercial production in Florida, is highly resistant. A number of promising unreleased varieties have been classified as resistant. Depending on the variety, only about 25 to 80 percent as many of the joints are bored in the resistant varieties as in the moderately susceptible variety Co. 281. Working on Mauritius Island, which is heavily infested with a white grub, W. F. Jepson and H. Evans developed a variety by hybridization that tolerated injury and yielded twice as much as certain more susceptible varieties. One of the helpful measures for the control of the sugarcane beetle and the sugarcane weevil in Louisiana is the planting of vigorous varieties that give the best stands of cane and recover well from beetle injury.

We might cite many instances to show the presence of resistance of one kind or another to insects that attack such crops as fruit, nut and forest trees, small fruits, vegetables, and ornamental plants. The breeding and selection of insect-resistant varieties of those crops, many of which are long-lived perennials, involve greater difficulties than are encountered in the improvement of annual crops. Nevertheless, with the growing interest in insect resistance and recognition of its possibilities, more rapid progress very likely will be made in reducing insect damage to perennial as well as annual crops by this means.

C. M. PACKARD is an entomologist. He was in the division of cereal and forage insect investigations, Bureau of Entomology and Plant Quarantine, for 37 years and retired in 1950. Until 1937, when he was put in charge of that division with headquarters in Washington, he worked at various field stations on the biology and control of cereal and forage insects.

JOHN H. MARTIN is an agronomist with 37 years of service in the Bureau of Plant Industry, Soils, and Agricultural Engineering. His work has included research on the culture, utilization, adaptation, genetics, economics, and improvement of small grains and sorghums throughout the States.