390 K. M. SMITH 



treated section. In the spring of 1954, from 60 to 80 % of the larval population 

 in the treated block died of polyhedrosis. No evidence of virus was found 

 in the untreated block (Clark, 1956). 



The second most important method of insect virus dissemination is by 

 passage of the virus through the egg; this applies to both types of polyhedral 

 viruses and is probably more frequent with the cytoplasmic polyhedroses. 

 Sudden outbreaks of polyhedroses in areas where the disease was unknown, 

 (Tanada and Beardsley, 1957), and attacks under conditions where external 

 contamination was impossible are certainly due to the stimulation of a latent 

 infection. Geneticists and others who breed large numbers of lepidopterous 

 larvae in captivity are only too familiar with this phenomenon. 



There has been some disagreement on the exact mode of transovarial 

 transmission as to whether the virus is inside the egg or is carried mechanically 

 attached to the outside of the shell. There are two facts which suggest that 

 the virus is not merely an external contamination of the outside of the egg: 

 one is the sudden appearance of a polyhedrosis in caterpillars which have been 

 reared through a number of generations under conditions where external 

 contamination is ruled out. It is unlikely that virus on the outside of an egg 

 would go through several generations in a latent infection. If the virus can 

 remain latent throughout the larval and adult life, there seems no reason why 

 it should not also be latent in the egg stage. The second fact against the theory 

 of external contamination of the egg surface is the discovery that young 

 larvae may die of a polyhedral disease even before they have left the egg shell 

 (Smith et al., 1953). 



There are one or two miscellaneous agents which may possibly help to 

 spread virus infections of insects and these are briefly dealt with. Predacious 

 insects may be instrumental in carrying the polyhedra around, either on their 

 mouthparts or possibly by contamination with their feces. Blowflies and 

 similar insects which are frequently attracted by and feed on the cadavers of 

 insect larvae which have died of virus infections are also potential vectors. 

 The same also probably applies to birds, and, as previously mentioned, the 

 wind and the rain no doubt play a part in distributing the polyhedra. Some 

 recent work by Franz et al. (1955) is relevant in this connection. They state 

 that after passing through the intestinal canal of the predatory bug, Rhinocoris 

 annulatus L., and of the robin, Erithacus rubecula L., the polyhedral viruses of 

 the pine sawfly, Neodijprion sertifer (Geoffr.), proved to be still infectious in 

 experiments carried out with the specific host. 



References 



Acqua, C. (1918-1919). Rend. 1st. bad. Scuola super, agr. Portici 3, 243. 

 Andrewes, C. H. (1957). Nature 180, 788. 

 Bergold, G. H. (1947). Z. Naturforsch. 2b, 122. 



