406 BIOLOGICAL CONTROL 



Conservation of Parasites. The principle involved in parasite 

 conservation consists of either altering the host-parasite ratio 

 by the adoption of methods which allow of more hosts than 

 parasites being destroyed ; or, the abandonment of measures 

 which tend to reduce the existing parasite population. The 

 earliest application of any method directly aimed at conserving 

 indigenous parasites appears to have been made by F. Decaux 

 in 1880 with regard to the insect enemies of the apple blossom 

 weevil (Anthonomus pomorum) in Picardy. By collecting infested 

 or " capped " blossoms from about 800 trees and placing them in 

 gauze-covered boxes, he claimed to have evolved a procedure far 

 superior to destruction by burning. All that was necessary 

 afterwards, he stated, was to raise the gauze from time to time 

 to allow of the parasites escaping. This process was repeated 

 for a second year, and since the orchards were isolated in the 

 middle of cultivated land all serious damage from the weevil 

 was stated to have been held in abeyance for ten years. A 

 similar procedure was advocated by Berlese in 1902, with 

 reference to the conservation of the parasites of the grape-vine 

 Cochylis. 



With regard to the Hessian fly, Marchal (1907) pointed out 

 that the destruction of wheat stubble, if carried out a little late 

 after the harvest, results in the annihilation of great numbers of 

 parasites, since the Hessian flies emerge before their parasites. 

 Similarly, Kieffer has shown that one of the measures for con- 

 trolling the wheat midge, viz., burning the debris after threshing, 

 has only an injurious effect, for the reason that the healthy pupae 

 occur in the soil, while those found in the debris are parasitised. 

 In Louisiana it has been pointed out by T. E. Holloway and 

 others (1928) that the practice of burning the trash or debris left 

 in the fields after the cutting of sugar cane is not an effective 

 method of decreasing infestation by the moth-borer Diatrcea 

 saccharalis Fab. The trash, it appears, is the hibernating place 

 for large numbers of parasitic insects. Following this contention 

 the collected trash was left unburned at the sugar experiment 

 station near New Orleans for a period of years, while the State 

 planters, as a whole, burned their trash. A comparison of losses 



