TOXIC GASES AS A CON'TKOL OF THE PEACH-TKEE BOEEE. 9 



ever, and no definite statement can be made as to the exact time 

 required for the gas to kill all the larvse. This, of course, depends 

 upon the concentration of the soil vapor and the condition of the 

 insect. These points are discussed more fully in a later paragraph. 

 Occasionally larvae subjected to the fumigation apparently live for 

 weeks in a comatose condition, their bodies shrinking to one-half 

 or one-third normal size before death. There is no means of know- 

 ing whether this is due to an unequal distribution of the gas or to 

 the greater vitality of the individual so affected. As a rule, after 

 the application of an effective dose in late summer and early fall 

 most of the larvae are killed within a period of about two weeks. 



Use as a Conteol Measure. 



time of application. 



The extent to which an application of p-dichlorobenzene may act 

 as a control measure for the peach-tree borer depends greatly on the 

 season of the year in which it is made. In the central latitudes, 

 where most of the experimental work was done, the hatching period 

 of the insect extends more or less over at least three months — July, 

 August, and September. In exceptional seasons, and perhaps to 

 some extent every season, it may be extended from one to two weeks 

 in either direction from these limits. 



It has been found that a single application of an effective dose 

 made in the early fall gives a very fair degree of control. Ap- 

 parently the ideal time would be from two to three weeks before the 

 end of the hatching period. Applied at that time it kills all the 

 larvse except a few of the more perfectly protected that have already 

 entered the tree, and provides an immunity from the attacks of 

 those which hatch later and appear during the progress of the 

 fumigation. Also the soil temperatures at this time are sufficient 

 to vaporize most of the material before winter, a point discussed 

 in a later paragraph in connection with injury. The seasonal 

 fluctuations in the period of egg deposition prevent the determina- 

 tion of the time of application to any very great degree of exactness. 

 The period for effective application, however, apparently has no 

 arbitrary time limits. Table II gives the summarized results of 

 about 15 experiments carried on in West Virginia, Virginia, and 

 Maryland in 1916, 1917, and 1918. 



125552°— 19 2 



