The degree of control obtained for various insect populations under treatment with 

 insecticides or similar methods obviously varies, depending on the insect, the control 

 measure employed, the schedule of treatments, and the manner in which the treatments 

 are applied, as well as other factors. For some insects, the available materials and 

 methods may be more effective than 90 percent, and for others, less effective. In some 

 cases, currently available control measures properly applied may be capable of 

 completely eliminating the portions of the insect population actually exposed to the 

 treatment. 



Failure to achieve complete elimination of a pest population may be the result of 

 neglect by some growers, the presence of abandoned hosts, presence of wild hosts, lack 

 of coordination in control efforts, presence of some of the population in hibernation, and 

 other reasons. It should be mentioned that the model assumes 90-percent control per 

 generation. For insects that are developing and emerging over a period of time, which 

 is usual for most insects, ^t may take several applications during a generation to achieve 

 the 90-percent level of control for that generation. 



Even though for many insects the level of control, or rate of increase, may differ 

 substantially, for the purposes of this report the model will serve to explain the 

 principles involved in controlling insect populations by the use of insecticides. Any 

 reasonably high level of control, if sustained over a period of several generations, has 

 a drastic effect on an insect population, in relation to an uncontrolled population. Note 

 comparative trends in models 1 and 2. At the same time, it is difficult to reach the zero 

 level in an insect population by following conventional methods of control if the initial 

 population is fairly high. If the calculation in model 2 were continued for subsequent 

 generations , we would find that a zero population level would not be reached until 

 treatments had continued for 18 generations. If the level of control were 99 percent for 

 each generation, it would take treatments for five generations to achieve theoretical 

 zero. The most significant point to note in the model is that the rate of decline in the 

 population, theoretically, will remain constant for each generation regardless of the 

 number of insects present. In actual practice, however, the rate of decline might 

 become less and less each generation. Certain factors regulating the survival rate of 

 the organisms may cease to function after the application of insecticides begins. 



Parasites and predators generally play a vital role in keeping the normal rate of 

 increase down to a reasonable level. If most of the parasites and predators are destroyed, 

 we might then expect a ten-fold increase per generation for those insects that are not 

 killed by the insecticides. In such event, after a few treatments with insecticides, the 

 population would show no further decline. For the purpose of this report, however, we 

 will assume that each insecticide treatment requires the same dosage rate but has a 

 constant effect on the total population, and in such case the rate of decline in the 

 population remains constant. 



Another point of great importance to note in model 2 is that the insecticide 

 treatments are highly efficient in terms of numbers of insects killed when the natural 

 population is high, but they are inefficient when the population is low. In the first 



- 5 - 



