THE PLUM CURCULIO IN APPLES 43 



tion with an extension disc harrow reaching close to the tree trunks kills 

 many of these pupae. In heavy packed soil this cultivation will be more 

 effective than in sandy soil. Many fruit growers object to cultivation at this 

 time because of the danger of stimulating later growth and consequent winter 

 injury. Clean cultivated orchards usually suffer less injury by the plum 

 curculio than sod orchards. 



Summary of Cultural Practices. 



In orchards where the plum curculio continues to inflict severe injury, 

 all of the above cultural operations in addition to careful spraying may be 

 necessary to reduce the injury to a commercial minimum. Where injury is 

 slight, some or all of the cultural practices can be eliminated and satisfac- 

 tory control obtained by spraying alone. The cultural operations are valued 

 in the following order: (1) destruction of larvae in dropped apples, (2) burn- 

 mg hibernating quarters, and (3) cultivation while the iusecte are in the soil. 

 Burning of hibernating quarters reduces the curculio population during the 

 following spring, but the effect of the other operations is not apparent until 

 the second season. 



Treatment of Soil with Insecticides to Kill Larvae and Pupae. 



In laboratory experiments using 200 larvae and pupae of the plum cur- 

 culio under natural conditions in exposed soil cages, carbon disulfide emul- 

 sion (7), 1 cjuart in 49 ^ gallons of water {yi per cent), applied to the soil 

 at the rate of three pints per square foot, killed 99.5 per cent of the in- 

 sects, and when applied at the rate of one quart per square foot, 90 per 

 cent succumbed. This emulsion which is extensively used in the control of 

 Japanese Beetle larvae is prepared by dissolving one part (liquid measure) 

 resin fish oil soap in three parts of water to which is added slowly with vigor- 

 ous stirring ten parts of carbon disulfide. 



In the orchard this treatment is practical only in cases of severe infesta- 

 tion and under a few selected trees. It should be applied between July 25 

 and August 5, and is more effective when the soil is moist so that the emul- 

 sion easily penetrates at least three inches. The materials cost about twenty 

 cents per tree of average size. 



When apples containing living eggs and larvae were immersed in this emul- 

 sion for ten minutes, the number killed was only slightly greater than the 

 natural mortality in untreated apples. 



Spraying 



With a normal crop of fruit, efficient spraying will give commercial control 

 of the plum curculio when the number of insects averages less than 25 per 

 tree of average size. When there are more than 25 beetles per tree, the 

 number of apples punctured by the beetles while obtaining a fatal dose of 

 poison usually represents a commercial loss, even though the fruit is protected 

 by the most effective spraying practices now available. A small crop of fruit 

 suffers more injury than a large crop. 



Laboratory experiments have shown that the majority of beetles feeding 

 on apples sprayed with the most effective formulae are killed in less than eight 

 days, and that during the most active period of their life they make from 

 two to five punctures per beetle in the poisoned fruit before dying. 



