Parallel studies, but much less extensive, have been made with the black scale and the citricola scale, species that seem also to have developed resistance to fumigation with hydrocyanic acid.

One practical result in California citrus orchards has been a marked reduction in use of fumigation against the California red scale and other scale insects. Growers have found it necessary to resort to spraying with oil, sometimes supplemented with ground derris or cube root, or with extracts from them.

THE NEXT MAJOR INSECT pest known to have developed resistance was the codling moth, or appleworm, against which lead arsenate was the chief insecticide for more than 40 years. It has always been hard to control in many of the drier western regions and in some eastern localities where the growing season is long and hot. In other places in the East, control was not especially difficult in the early decades of the century, although even there the number of applications of lead arsenate needed for adequate worm control had steadily increased. The differences in the efforts required to control the worms generally were attributed to differences in climate and other conditions.

W. S. Hough, of the Virginia Agricultural Experiment Station, was the first to attack the problem. He started in the late 1920's. He carried on his studies in an insectary a screened shelter where conditions were like those in the shade of an apple tree. He brought together strains of codling moth from areas that needed different degrees of spraying. Apples from unsprayed trees were thoroughly sprayed with lead arsenate. Then he allowed newly hatched worms to try to chew their way in, as they do in the orchard. In the experiments Hough eliminated any differences due to locality, spray practices, or abundance of codling moth. Differences in the proportion of the worms that could get through the spray covering without being killed would indicate differences in resistance to the insecticide.

Hough compared worms from Virginia orchards, in which three or four sprays gave almost complete control, with worms from near Grand Junction, Colo., where the insect was notoriously hard to control. In the first season's tests, 31 to 39 percent of the Colorado worms got through the poison successfully, but only 5 to 7 percent of the Virginia worms survived. Hough raised both strains of worms in the insectary through 14 or more generations and continued to find the same differences.

He later found that appleworms from Virginia orchards that had been regularly well sprayed with lead arsenate entered sprayed fruit in much greater numbers than those from unsprayed or poorly sprayed orchards. Strains from various Virginia orchards fed through successive generations in the insectary on sprayed fruit became more and more resistant to lead arsenate and were able to enter sprayed fruit in increasing numbers.

L. F. Steiner and associates at the Department's fruit insect laboratory at Vincennes, Ind., later made a similar study and reached similar general conclusions. They found that codling moth stocks from different orchards in the Ohio Valley differed greatly in ability to enter sprayed fruit. The greatest resistance was in codling moth worms from an orchard that had been heavily sprayed with lead arsenate the preceding 5 years. Worms from a similar orchard that had been unsprayed for 5 years were much more readily killed.

It is not always easy to prove that an insect has developed resistance. Two or more strains of the insects have to be kept under the same conditions, although separated from each other in such a way that the strains cannot become mixed, before differences in the results with an insecticide can be said to be caused by differences in the insect itself. The control results in one year may be quite different from those obtained in another. Such differences do not necessarily mean resistance. They could as well be caused by seasonal or local factors.

On the other hand, the development of resistance has sometimes gone unrecognized or has been minimized by skeptics who have felt that some other factor was responsible: Even after Hough's first results were published, some workers in other areas averred that the trouble was that Colorado growers did not know how to spray.

Resistance is most likely to develop when all insects in a given situation are exposed to the insecticide and there is little reinfestation by insects not exposed. For instance, when a citrus grove is fumigated for the California red scale, the cyanide gas reaches all parts of the trees, and there is little movement of scales from untreated to the treated area. When an orchard is sprayed for the codling moth, an effort is made to spray every tree thoroughly. Repeated applications are made the same season. That means that almost all of the California red scales or codling moth worms in an orchard are exposed to the action of the insecticides. But with many of our common insects, only a small part of the total number present in an area may be exposed to insecticides. For example, only a small portion of the food plants attacked by the Japanese beetle in a given area are usually covered with an insecticide. Many home gardeners in cities do little spraying; shade trees on private property and many on public land rarely are well sprayed. Often they are not sprayed at all. The area surrounding the city usually has many beetles in wasteland or in other situations where spraying is impractical or unprofitable. Up to 1952 there had been little or no indication that the Japanese beetle had built up resistance to DDT, the insecticide most commonly used in its control.

The development of resistance to standard insecticides has practical significance. Such would be the testimony of growers in Colorado and elsewhere who gave up trying to raise apples because they could not get rid of the worms with lead arsenate. Such developments have made it necessary for entomologists to develop new insecticides and alternate methods of control.

B. A. PORTER is in charge of the division of fruit insect investigations in the Bureau of Entomology and Plant Quarantine. He joined the Department in 1917 and for many years conducted field investigations of various orchard insect problems in Connecticut and Indiana. He was graduated from Massachusetts Agricultural College in 1914.