May 15, 191S 



Wilt of Gipsy-Moth Caterpillars 



125 



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Fig. 15. — Curve showing the 

 mortality among 40 gipsy- 

 moth caterpillars fed with 

 unfiltered wilt virus. 



How can we, however, account for the numerical difference existing 

 between the moths obtained from the Berkef eld-filtrate infections and 

 those obtained from the unfiltered-virus infections? In the experi- 

 ments with the former, 38 per cent transformed 

 as compared with 18 per cent in the latter case. 

 As the experiments show, the virus is filterable, 

 but with difficulty; consequently naturally un- 

 filtered material would be more deadly than 

 filtered material, for the reason that the former 

 contains a greater number of micro-organisms 

 than the latter, and caterpillars in order to con- 

 tract wilt would have to ingest more of the 

 Berkefeld filtrate than of the unfiltered virus 

 so as to obtain the lethal dose. This will 

 account for the greater number of moths ob- 

 tained in the Berkefeld experiments as com- 

 pared with those obtained in the other infections. In 

 the case of Berkefeld infections the virus was less 

 concentrated and more caterpillars escaped eating 

 the lethal dose, thus perhaps acquiring an immunity 

 toward a second infection, for it must be remem- 

 bered that most of the individuals were infected a 

 number of times. 



All the experimental males and females were 

 mated and eggs were obtained which, it is hoped^ 

 will hatch so that they can be used the coming season 

 (1914), in order to determine whether this apparent 

 immunity is racial or merely acquired. If racial, some 

 interesting Alendelian ratios may be obtained; if ac- 

 quired, the whole of the next generation will proba- 

 bly be susceptible to the disease, unless certain of 

 its members become actively immunized. If racial 

 or acquired immunity does not exist among cer- 

 tain individuals of the gipsy moth, it is difficult 

 to understand how any of these insects escape 

 death under certain conditions in the field. The 

 writer has often seen dozens of caterpillars congre- 

 gating on trees under burlap and has seen them 

 dying of wilt in great numbers in such places; Fig. 17— Curve showing the 



J. .1 1- • i. i- u J" „ fl mortality among so gipsy- 



yet, m spite of the dismtegratmg bodies flow- ^^^^ caterpillars under 

 ing out over other individuals in the immediate conditions approximating 



.„ 1 ii J J. c those in the field. 



proximity, many will escape death and transiorm. 



To repeat this field observation in the laboratory, 50 caterpillars were 

 gathered from a locality where wilt had been raging for several weeks. 

 The entire lot was placed in a single tray so that the dying individuals 



Fig. 16. — Curve showing 

 the mortaUty among 

 20 control gipsy-moth 

 caterpillars. 



