THE INSECT VIRUSES 387 



The following experiments on cross-transmission with the larvae of Sphinx 

 ligustri, the privet hawk moth (Smith and Xeros, 1953a), give a fairly 

 characteristic picture of what happens with this type of experiment. 



Six similar batches of third and early fourth instar larvae of ligustri, which 

 appeared perfectly healthy, and among which no deaths from polyhedrosis 

 had occurred, were fed on the same day with nuclear polyhedra from the 

 following lepidopterous larvae, chosen entirely at random: Telea polyphemus, 

 Lymantria dispar, Philosamia ricini, Panaxia dominula, and Cycnia mendica. 

 The ligustri larvae infected with virus from mendica and dominula died on 

 the seventh and eighth days after infection with a typical nuclear polyhed- 

 rosis. The polyhedra were nonstaining and square in shape. The larvae 

 infected with the polyphemus virus died on the eighth day with identical 

 symptoms, but the polyhedra in this case were either many-sided or triangular 

 in shape and not square. The deaths were extremely regular, occurring together 

 over two or three days, and the symptoms were classic. One larva only, one 

 of those dying on the ninth day, showed some staining cytoplasmic polyhedra 

 among the nuclear type. Of the twelve ligustri larvae infected with virus 

 from ricini, two died on the tenth day, four on the eleventh, one each on the 

 twelfth and thirteenth days, one on the eighteenth, and one each on the 

 twenty-ninth and thirty-second day after infection. The first six larvae to die 

 had fairly typical symptoms of a nuclear polyhedrosis. Their polyhedra were 

 nonstaining, with a tendency to a square shape. The other six larvae com- 

 menced to die on the eleventh day and subsequently; two of these had cyto- 

 plasmic polyhedroses only, while the other four had both types of infection. 



Those ligustri larvae infected from dispar all died of a cytoplasmic poly- 

 hedrosis, and none developed a nuclear disease. The control larvae for this 

 experiment also died similarly. 



The conclusion we draw from this type of result is that there was genuine 

 cross-transmission of a foreign nuclear polyhedral virus; but, in addition, 

 there was a latent cytoplasmic polyhedrosis. This latter point has been amply 

 confirmed by subsequent observations on S. ligustri. Provided the incubation 

 period of the nuclear virus was short (mendica, 7 days, dominula and poly- 

 phemus, 8 days) the foreign virus was able to establish itself early and kill the 

 larva before any appreciable quantity of gut polyhedra developed at the 

 molt to the fifth instar. In the case of the ricini infections, the incubation 

 period of 11 days was slightly longer and there appeared to be a suppression 

 of any great development of nuclear polyhedra in those older larvae in which 

 the cytoplasmic disease had got a good start before the nuclear one could do 

 so. With the dispar virus one can conclude either that it is not cross-transmis- 

 sible to ligustri or else that the cytoplasmic disease suppressed its getting a 

 foothold. In fact, we can now go further and say that where a latent cyto- 

 plasmic infection is present this is almost invariably stimulated to development 



