THE INSECT VIRUSES 389 



infection through the parent. This statement should perhaps be modified 

 slightly in so far as it is known to apply to the polyhidroses and granuloses; 

 there is not yet sufficient evidence to say definitely that the viruses without 

 intracellular inclusions are similarly transmitted. We know that TIV is 

 spread by ingestion orally, in the laboratory and presumably also in the field; 

 there is a slight amount of evidence that there may be latent infection with 

 the same virus. 



Transmission of the polyhedral viruses and those of the granuloses through 

 a given population of insect larvae during the current season is undoubtedly 

 by the ingestion of contaminated foliage. So far as the nuclear polyhedroses 

 are concerned the disease itself greatly facilitates the spread of the polyhedra. 

 The skin, being one of the organs attacked, quickly becomes extremely 

 fragile and easily ruptures, scattering the polyhedra in the liquefied body 

 contents over the foliage. This process is helped by the wind and the rain 

 which carry the polyhedra still further afield; the fact that the polyhedra are 

 quite resistant to the weather is very important in the spread and over- 

 wintering of the virus. The same principles probably hold in the spread of the 

 granuloses, since the skin is also attacked and the liquefied body contents are 

 similarly spread abroad. There is some evidence, however, that the longevity 

 of the granules is much less than that of the polyhedra; the granules from 

 Pieris spp., for example, seem to lose much of their infectivity after storage 

 for one winter. 



An interesting fact in regard to the granulosis disease of the large white 

 butterfly, Pieris brassicae, is the apparent attraction of the liquefied cadavers 

 for the healthy caterpillars, which can often be seen feeding greedily on the 

 bodies with, of course, disastrous results. 



The situation regarding the cytoplasmic polyhedroses is slightly different, 

 since the skin of the affected larvae is not attacked and in consequence there 

 is no general liberation of polyhedra. In this case, the polyhedra are excreted 

 in large numbers with the feces and the food plant in consequence is heavily 

 contaminated. This fact is easily demonstrated by examining smears of the 

 droppings of larvae with cytoplasmic polyhedroses, as in Operophtera bruma'a, 

 the winter moth, for instance, or by examining sections of the gut of infected 

 larvae (see Fig. 8). 



Since the polyhedra are capable of retaining viable virus within them for 

 many years, fifteen years in the case of the silkworm, Bombyx mori, it is fair 

 to assume that infective material will remain over winter on contaminated 

 food plants. This has been proved on more than one occasion, for example, 

 with the Great Basin tent caterpillar, Malacosoma fragile (Stretch). In 1953, 

 a number of trees in an abandoned orchard in California was sprayed with a 

 polyhedral virus from this species and a second block of trees was left un- 

 treated. It was observed that a number of larvae died of the disease on the 



