494 Phylum Arthropoda 



strung on wires and exposed in cages with Macrocentrus adults. The 

 temperature in the parasite cages should be about 75 F. and the humid- 

 ity kept down to 70%. A room held at 76 F. and 60% relative humid- 

 ity is satisfactory. Light is important. This should be neither too 

 strong nor too weak. About 10 foot candles illumination from blue 

 daylight bulbs provides suitable working conditions for the parasites, 

 The cages used are covered with cloth, and moisture for the insects is 

 provided by a wick extending from a pan of water to the top of the cage. 

 Infested apple slices are exposed to parasite action for 24 hours, but if 

 the cage is well stocked they may be removed at shorter intervals. It 

 is not advisable to leave the slices longer than 24 hours. After exposure, 

 the slices are placed in a dry incubator kept at 8o° F. and 50% relative 

 humidity or lower to prevent rot and to allow the larvae to develop. 

 After several days, new apples are placed over the slices to give addi- 

 tional food. If it is necessary to hold the material for any length of 

 time it should be reared in a cool place (6o° F.) instead of the 8o° F. 

 incubator, and as soon as the larvae have spun they should be placed in 

 a still cooler atmosphere, between 32 ° and 45 ° F., humidity 80% to 

 95%. A common storage cellar has been used by us for this purpose, 

 and material may be kept there until June. If removed from the breed- 

 ing rooms and placed at outside temperatures during cold weather 

 (below 20 F.), a large percentage of the larvae and their parasites may 

 be killed. 



In order to maintain a favorable sex ratio, males at the rate of two 

 or three to one female should be used in the cages. Both sexes are put 

 in the cage immediately after emergence and the males removed after 

 24 hours. Large numbers of males kept in the oviposition cages during 

 the oviposition period are undesirable with this insect. The normal 

 ratio appears to be about 60% males and 40% females, and it is possible 

 with care to obtain this ratio in laboratory breeding. 



The main problems that arise in continuous Macrocentrus breeding 

 consist (1) of difficulties in securing abundant host material, as dis- 

 cussed by Mr. Brigham [see p. 348] ; (2) in maintaining a satisfactory 

 sex ratio and ratio of increase; and (3) in hibernating the reared para- 

 sites without excessive loss. It should be mentioned that proper tem- 

 perature, moisture, and light are important items especially in handling 

 the adult parasites, and maintenance of these conditions requires con- 

 siderable equipment and knowledge of air conditioning. Without them 

 satisfactory increases are difficult to obtain. 



(3) A third method of production consists of using strawberry plants 

 artificially infested with strawberry leaf roller larvae. A large outdoor 

 cage placed over a number of growing strawberry plants is employed. 

 Macrocentrus adults are introduced at the proper time and the leaves 



