BUREAU OF ENTOMOLOGY. 245 



and that tliroiigh the application of the percentage principle of con- 

 trol it is entirely practicable to do so at a comparatively small cost. 



In the Southwest, where the mesquite furnishes the only local 

 supply of fuel, fence posts, etc., serious losses are suffered each year 

 from wood-boring insects. Large quantities of mesquite are used 

 for fuel at the Army cantonments, rendering the problem of especial 

 interest in this connection. In order to determine a method of con- 

 serving the resources supplied by the mesquite, a special field station 

 was established near Tucson, Ariz., at which mesquite has been cut 

 every two weeks since October, 1917. The results of this experiment 

 so far have been most gratifying in showing that mesquite cut in 

 November and December and piled in loose ricks is comparatively 

 free from damage, while that cut during the other months is seriously 

 affected and in some cases its value is entirely destroyed. 



The black locust has come into prominence on account of its value 

 in supplying the best pins used in the construction of wooden ships. 

 Heretofore it has not been practicable to grow this tree commercially 

 on account of the damage to the tree and wood by the locust borer, 

 but recent experiments have shown conclusively that it can be pro- 

 tected from the borer by spraying the young trees with a poisoned 

 liquid and by a more practical method of providing a dense shade in 

 the plantations by the use of some quick-growing shade-producing 

 plant between the rows. 



In connection with the pressing need of an increased food supply, 

 Dr. Hopkins volunteered his services for the investigation of periodi- 

 cal farm practice with special reference to the application of his 

 bioclimatic law of latitude, longitude, and altitude as a guide to the 

 best time and period each season to do the work. This law, which 

 has been worked out in its relation to research and practice in ento- 

 mology, was believed by him, as suggested in the report of the Ento- 

 mologist for the fiscal year 1917, to be equally applicable to research 

 and practice in agriculture. Therefore much of his time during the 

 year was devoted to a study of the application of the law to the 

 ipredetermination of the safest and best time to so^y winter wheat 

 at any place in the United States where the conditions are favor- 

 able for its profitable growth. 



The results of the investigations as set forth in his paper, " Pe- 

 riodical Events and Natural Law as Guides to Agricultural Ee- 

 search and Practice," published as Supplement 9 of the Monthly 

 Weather Review, seem to have proved the practicability of the ap- 

 plication and to have introduced a new line ot attack for all problems 

 relating to periodical farm practice, and seem to have a promise of 

 ultimate results of special value toward increasing the food supply 

 and the general advancement of agriculture. 



TROPICAL AND SUBTROPICAL FRUIT INSECT INVESTIGATIONS. 



This branch of the bureau's investigations is under the charge of 

 the assistant chief of the bureau, Mr. C. L. Marlatt. 



Investigations of insects affecting citrus fruits in Cali- 

 fornia. — Work in control of the two important mealybug enemies of 

 citrus trees has been continued during the year and has resulted in 

 notable success. The means now recommended by the department are 



