Resistance to Insecticides

Firefly.

Insects Are Harder To Kill

B. A. Porter.

Some people have discovered that "insects are getting harder and harder to kill." They are partly right. A few of the hundreds of pests that farmers must control have developed resistance to insecticides.

That means that an insect can survive and thrive in the presence of a chemical that is supposed to kill it. It does not mean that every insect of the kind involved will survive the application of the insecticide. If a sizable proportion can survive the insecticide at a practical strength, the pest is said to be resistant. Sometimes insects from one source are harder to kill than those of the same kind from another source. The group harder to kill usually is referred to as a resistant strain.

The development of resistance by an insect can be explained simply in a general way, but the actual details of the process in a given kind of insect, with respect to a given insecticide, are complex and not understood fully.

A few principles are basic. No two living creatures are exactly alike. Among people there are differences in color of hair and eyes, height, weight, health, and many other details. People vary also in the effect that diseases have on them. An epidemic of disease in a community may attack some persons seriously but not touch others.

So it is with insects. A speck of an insecticide may kill one insect but leave another of the same kind unaffected. If the amount of the insecticide to which the insects are exposed is great enough, all will be killed, of course, but often the amount applied is insufficient to kill all the insects present. The least resistant are killed at once. The resistant ones survive.

Apparently what has happened when insects have developed increased resistance is that the offspring of the resistant survivors have a similar degree of resistance. After that selective process has gone on for several generations, most of the insects that are easily killed have been eliminated and only the resistant ones are left. More applications or greater quantities of the insecticides are needed then for adequate control. The point finally is reached where the particular insecticide becomes so ineffective that some other material or method of control must be developed.

The men who have made a special study of the development of resistance do not agree entirely on the technical details of just what happens. Exact information on the subject is essential from a scientific standpoint but is not needed for our understanding of the general subject as discussed in this article.

THE POSSIBILITY of differences in the susceptibility of insects of the same kind to insecticides has been recognized for more than a half century. As early as 1897, John B. Smith, entomologist of the New Jersey Agricultural Experiment Station, mentioned variations in results in the control of the San Jose scale and other insects. He commented on "an outstanding difference in the amount of resistance to poisons, either external or internal."

The possibility of growing resistance to insecticides was first pointed out in 1914 by A. L. Melander, then professor of entomology in Washington State College. His studies were made on the San Jose scale, which for some years apparently had been well controlled by spraying with a strong solution of lime-sulfur. At the time, the San Jose scale was much harder to kill with lime-sulfur in the Clarkston area of Washington than it was in the Wenatchee and Yakima valleys or elsewhere.

A similar situation developed in the early 1920's in southern Illinois, southern Indiana, northwestern Arkansas, and elsewhere in the Midwest. Lime-sulfur suddenly seemed to have little effect on the San Jose scale in many orchards although previously it had given good control. The insect killed the trees in several thousand acres of fine orchards despite careful and liberal applications of lime-sulfur.

The work of Melander and others suggested that the San Jose scale might have developed resistance in some localities, but their studies did not entirely rule out the possibility that differences in resistance were caused by seasonal or local conditions. The development of resistance among other insects was demonstrated within a few years, however.

Perhaps the first clear-cut demonstration of strains of insects differing in resistance and the possibility that their average resistance could increase was reported by H. J. Quayle, of the California Citrus Experiment Station at Riverside. In 1916 he published an article, "Are Scales Becoming Resistant to Fumigation?"

A standard method of controlling scale insects on citrus in California for many years was to place canvas tents over the trees and fumigate them with hydrocyanic acid. Quayle brought to Riverside scale insects from Corona, Riverside County, Calif., where there had been serious difficulty in getting control by fumigation, and other scale insects from Orange County, where control was easy. Both lots of insects were fumigated under the same tent over an artificial tree. The results paralleled those obtained at the places where the stocks of scale originated. Five percent of the scale insects from Corona survived, but fewer than 1 percent of those from Orange County survived. Quayle and his associates studied the problem at least 25 years.

Since 1930 the Department's citrus insect laboratory at Whittier, Calif., has also carried on studies of the resistance of the California red scale to hydrocyanic acid. Several strains of the scale insect have been reared in the laboratory through many generations and have been fumigated in various ways. Their resistance has been determined from time to time.