A CIRCULAR 3 4 6, U. S. DEPARTMENT OF AGRICULTURE 



more common types encountered by plant growers are discussed, 

 and general information is presented regarding their habits, the 

 insects upon which they feed, and general descriptions by means of 

 which they may be identified. For purpose of ready" reference, 

 these topics are dealt with under the primary headings of the insects 

 that are attacked. 



CONTROL OF INSECT PESTS BY PARASITES AND PREDATORS 



In most cases when insect pests have gained entry into the United 

 States they have not been accompanied by the parasites and preda- 

 tors which attack them in their native home. This being so, it is 

 quite understandable that the pests have been able to increase to 

 enormous numbers and to inflict great damage to crops. The im- 

 portation of the parasites and other insect enemies is an effort to 

 duplicate here the natural balance which exists in the former home 

 of the insect. 



The first attempt at the control of an insect pest by the impor- 

 tation of its predators from abroad was the importation of the 

 vedalia beetle (Rodolia cardinulis Muls.) from Australia in 1888-89 

 to work against the cottony-cushion scale (leery a purchasi Mask.) 

 on citrus in California. Within a few years the scale was com- 

 jDletely subjugated. This striking success led to further efforts 

 along the same line, but it was many years before so decisive a 

 result was again obtained. The most extensive program for para- 

 site importation was that against the gypsy moth (Porthetrm dis- 

 par L.) and the brown-tail moth (Nygmia phaeorrhoea Don.). The 

 first shipments of parasites of these pests were received in 1905, and 

 the work was continued until 1912 and then resumed during the 

 period from 1922 to 1932. At the present time importations are 

 being made of the parasites of a number of insects, such as the 

 European corn borer [Pyrausta nub'dalis Hbn.), the Japanese beetle 

 (PopRlia japonica Newm.), the oriental fruit moth (Grapholitha 

 molesta Busck), and several forest insects. In general, it may be 

 said that the importation of parasites from abroad is one phase in 

 the attempted control of each of the most important foreign insect 

 pests that have become established in this country. 



There are certain definite limitations to the possibilities of con- 

 trol of insect pests by the use of parasites. Only in exceptional cases 

 can complete control be achieved by this means. In the great 

 majority of cases different degrees of partial control are secured. 

 For this reason the introduction of parasites should be considered as 

 supplementary to the usual methods of mechanical and chemical con- 

 trol rather than as a alternative method. The reduction in numbers 

 of the insect pest by the use of parasites renders outbreaks less fre- 

 quent, and control is then more readily accomplished by the use of 

 sprays or other mechanical or chemical methods. 



There are two distinct phases in the attempted control of an insect 

 pest by the importation of its parasites. The first phase comprises 

 the search for the parasites in foreign countries, the study of these to 

 determine their usefulness, and finally their shipment to the United 

 States; the second phase includes the rearing of the parasites and 

 their distribution throughout the entire area infested by the pest in 

 this country. It is obviously impossible to provide colonies of para- 



