INSECT ENEMIES OF INSECTS — CLAUSEN 359 



may not even maintain itself in the new locality, even though condi- 

 tions are ideal for the host and very nearly duplicate those to which 

 the parasite species has previously been accustomed. We are thus 

 forced to adopt the empirical method and test all of the parasites in 

 turn in the hope that some one or more of them will find the new 

 environment favorable to its development and be able to subdue the 

 pest. 



The introduction of beneficial insects into the United States from 

 other parts of the world is only one phase of the attempt to keep 

 the injurious species sufficiently under control so that our various 

 crops may be produced in sufficient volume and at a profit to the 

 grower. The customary methods of control involve the application 

 of various poisons which kill by contact or when eaten. In the 

 citrus groves the great majority of pests are controlled by fumigation 

 with hydrocyanic acid gas. Many of these control methods are very 

 effective and the insect pests are kept at a very low level by their 

 use. It is necessary, however, to employ this artificial control year 

 after year, or several times each year, and consequently such work 

 involves a constantly recurring expense, and one which the margin 

 of profit on many crops does not justify. The control of the pest 

 by the importation of insect parasites and predators, if it is success- 

 ful, has this one great advantage: The initial cost is usually the 

 total cost. For this reason a successful introduction may result in 

 greatly reducing the damage and at the same time may render 

 unnecessary any further annual expenditure. This complete control 

 unfortunately is seldom effected, and in a great majority of cases 

 we secure only partial success. The customary control by other 

 means is still necessarj^, though possibly on a reduced scale. 



The United States Department of Agriculture has been engaged 

 for a great many years in the importation of the natural enemies 

 of a considerable number of our principal crop pests. Rather 

 curiously, the first attempt along this line, namely the introduction 

 of the Vedalia beetle, was a complete success and served to focus 

 attention upon the possibilities of this method of control. 



The cottony-cushion scale became established upon citrus in Cali- 

 fornia in 1872 and within 10 3^ears was generally distributed through- 

 out the State and seriously threatened the commercial production 

 of citrus fruits. Entire orchards were being abandoned as un- 

 profitable and the outlook was exceedingly gloomy. At this stage 

 the department delegated one of its representatives to undertake 

 the search for parasites and predators in Australia. This is the 

 native home of the scale and consequently offered the best oppor- 

 tunity for finding its enemies. A short search revealed that the 

 scale in Australia was kept in continued subjugation by a small 

 beetle, Vedalia card'malis by name. A quantity of these beetles were 



