388 K. M. SMITH 



by the introduction of a foreign nuclear virus. The reverse phenomena, 

 stimulation of a nuclear virus by introduction of a foreign cytoplasmic virus, 

 does not seem to occur. Some experiments with the larvae of the winter moth, 

 Operophtera brumata, are of interest in this comiection. Failure to find a 

 naturally occurring virus disease of this larva led to attempts to infect it 

 with a nuclear virus from the butterfly larva, Vanessa cardui. This induced a 

 high percentage of mortality in the winter moth larvae, all the controls 

 remaining healthy. The disease which resulted, however, was a cytoplasmic 

 one and, once stimulated into activity could be transmitted indefinitely in 

 series (Smith and Rivers, 1956). 



It may be that the reason for the frequent stimulation of a cytoplasmic 

 virus by a foreign nuclear virus is connected with the location of the cyto- 

 plasmic virus in the cells of the mid-gut. In this position it is likely to come 

 rapidly into contact with the foreign virus swallowed by the larva. There is 

 little doubt that the number of latent infections with cytoplasmic viruses is 

 extremely high and, in the writer's opinion, higher than the number of latent 

 nuclear polyhedroses. 



There are other methods by which latent polyhedral viruses can be 

 stimulated to action, notably by feeding with certain chemicals and by keep- 

 ing the larvae under unsuitable conditions. 



A few other examples of apparently genuine cross-transmission may be 

 quoted. A nuclear polyhedral virus from Vanessa cardui is easily transmissible 

 to Aglais urticae and V. io and is intertransmissible between all three species. 



With the granulosis virus of the large white butterfly, Pieris hrassicae, 

 transmission is easily achieved between this species, P. rapi, and P. napi. 

 It is difficult to say, at this moment, whether it is always the same granulosis 

 virus which is involved, since there is no means of differentiating between 

 them until they can be tested serologically. Steinhaus (1952) has demonstrated 

 a similar cross-transmission of a granulosis virus between species of Colias. 



It is possible that in the case of the three species of white butterfly, Pieris, 

 the granulosis virus may change slightly after passage of one or other of the 

 species. It has been found (Smith and Rivers, unpublished) that a considerable 

 resistance on the part of P. hrassicae can develop to infection with the 

 granulosis virus, but after passage of the virus through P. napi, this resistance 

 breaks down. 



Attempts to stimulate development of the granulosis, which is frequently 

 latent in Pieris spp., by feeding with foreign polyhedral viruses have given 

 negative results. 



VI. Methods of Transmission of Insect Viruses 



There can be little doubt that the two main methods of transmission of 

 insect viruses are by ingestion of contaminated food and by the inheritance of 



