124 



Journal of Agricultural Research 



Vol. IV, No. 2 



For the sake of uniformity the range of temperature is considered 

 as extending from 71° to 82° F. Naturally the ranges of temperature 

 are not always the same, for one experiment might extend over a greater 

 number of days than another, thus perhaps including 

 a few temperatures different from those in the latter 

 experiment. Taking the midway point in the tem- 

 perature range as 77°, the number of wilt deaths 

 occurring at and above 77° is found to be 14, while 

 there are 10 occurring below that temperature. There 

 seems to be in this third series of experiments the 

 same correlation between high temperature and wilt 

 as in the first series. 



Fig. 12. — Curve show- 

 ing the mortality 

 among 25 gipsy-moth 

 caterpillars fed with 

 unfiltered wilt virus. 



DISCUSSION OF ETIOLOGICAL EXPERIMENTS 



Fig. 13. — Curve showing 

 the mortaUty among 

 25 control gipsy-moth 

 caterpillars. Compare 

 with figure 12. 



In the three series of experiments, out of the entire 

 number of controls, comprising 78 

 caterpillars, only 4 died of the wilt 

 disease. This is equivalent to about 2.25 per cent, 

 almost a negligible error when compared with about 

 39 per cent mortality in the infected animals. 



It will be seen, on examining the diagrams, that the 

 wilt incubation period varies considerably — from 2 to 

 27 days. The differences in the individual constitu- 

 tions of separate caterpillars undoubtedly account for 

 much of this variation, and slight differences in the 

 amount of the infectious material 

 ingested or in the virulence of the 

 virus are important also; but such 

 factors as heat, humidity, and food 

 play a role in determining the length of the period of 

 incubation. 



In spite of apparent infection, a number of moths 

 emerged in each series of experiments. Out of no cat- 

 erpillars fed with the Berkefeld filtrate, 42 adults 

 emerged, while out of 85 individuals fed with the unfil- 

 tered material 15 imagoes were obtained. All individ- 

 uals used in the infection experiments really partook of 

 the material administered, so one can not very well 

 account for the moths on the basis that the caterpil- 

 lars escaped infection. It is possible that a genetic 

 immunity towards wilt exists among certain members of the gipsy- 

 moth race and that others can also be actively immunized with sublethal 

 doses of fully virulent material. 



Fig. 14. — Curve show- 

 ing the mortality 

 among 40 gipsy-moth 

 caterpillars fed with 

 wilt virus after filtra- 

 tion through Berke- 

 feld filter. 



