— 18 — 



them out and breeding them in large numbers for liberation in infested districts. 

 In addition the "secondary" parasites which were also imported with the 

 "primary" had to be collected and destroyed. 



Notwithstanding all these efforts, the authorities in charge of the work are 

 of the opinion that the present improvement is due to at least three other factors, 

 viz. "the perfection and standardization of the methods for artificial repression, 

 the death of a large proportion of the more susceptible trees or their removal 

 from the infested woodlands, and the development of the "wilt disease". Of 

 course, there is the possibility that the imported parasites will in time become 

 sufficiently numerous to hold the pests under control as they do in Europe. This 

 result will be obtained when 85 or 90 percent of the eggs and caterpillars are 

 killed, as the actual rate of increase of the Gypsy moth is from 6 to 10 fold. 



It will be evident, therefore, that successful control of injurious insects by 

 parasitic and predaceous forms depends upon several favorable factors and sets 

 of conditions. These have been well summarized by H. S. Smith (Journ. Econ. 

 Ent. Aug. 1919) as follows: — 

 1. — The reproductive capacity of available entomophagous insects must be much 



higher than that of the host. This proposition is self-evident, and needs no 



amplification. 

 2. — A complete sequence of parasites affecting the egg, lar\-a and pupa of the 



pest. The importance of this factor was fully shown in the campaign against 



the Gypsy moth. 

 3. — The entomophagous forms must be capable of being reared or obtained in 



sufficient quantities to overcome the pest in the field. This factor is one of 



the most difficult to bring into operation. Lady-birds are readily reared and 



collected. 

 4. — The cost of producing natural enemies must remain well within the bounds of 



profitable crop production. 

 5. — Secondary parasites in the local fauna should limit as little as possible, much 



less entirely prevent, the action of the primary parasites. This factor is also 



one that is most difficult to control. 

 6. — Agricultural practices such as spraying and fumigation which affect adversely 



the breeding of natural enemies should be prevented. It is probable that the 



extermination of the imported Chinese Lady-bird in southern orchards was 



due mainly to spraying operations with lime sulphur carried on as a practice. 

 7. — The relative ability of the pest and its enemies to spread is an important 



factor. If both be good fliers the power of spreading is increased but the 



likelihood of extermination of the pest is lessened. 



The citrus mealy-bug has been controlled in Southern California by the 

 lady-bird Cryptolacnus montroiizicri which was reared in large numbers at the 

 State Insectary and collected in orchards where they had become abundant late 

 in the season. In this instance, as in that of the cottony-cushion scale, the lady- 

 bird is an active insect while the scale insect is fixed to the plant. Moreover, the 

 lady-bird has more generations that the scale insect and is practically free from 

 ■parasites. 



