174 . Maine Agricultural Experiment Station. 1920. 



crease in the seasonal abundance of aphids and by greater prox- 

 imity to diseased stocks, and therefore by conditions that seem 

 the spread of mosaic was increased apparently both by an in- 

 to favor aphid dispersal from diseased to healthy plots. 



During 1918, the season of great abundance of aphids, three 

 lots of tubers were harvested at progressively later dates during 

 the increase in numbers of the plant lice. From each of 78 

 healthy hills — mostly Green Mountain — growing near to mosaic 

 hills, one tuber was removed on August 15 and another on 

 August 26, the remainder being harvested on September 12. 

 Meanwhile aphids, which became noticeable on potato plants the 

 latter half of July, had become very numerous about the middle 

 of August and were more excessively abundant as the end of the 

 month was approached, seeming when migrating to be as abun- 

 dant as the flakes in an ordinary snowstorm. The three sets of 

 tubers in 1919 produced plants mosaic in 6, 14, and 50 per cent 

 respectively. Apparently some of the transmission occurred 

 before August 15 but most of it took place after August 26. 

 Such late infection of the tubers is apparently due to the late 

 development of the chief cause of the spread of mosaic, namely, 

 abundant dispersing aphids. 



FLEA BEETLES: 



Five caged groups of healthy Green Mountain hills were 

 fed upon by flea beetles (Epitrix cucumeris Harris) 14 for a 

 week, several hundred from all-mosaic potato plants being in- 

 troduced into each cage. As controls, four similar groups were 

 treated likewise except that the beetles were taken from plots of 

 mostly healthy potatoes or from bushes. All the plants in this 

 experiment remained healthy until harvested. This result at 

 least indicates that flea beetles are not important in spreading 

 potato mosaic, since the artificial infestation was much more 

 severe than any ordinary natural one while the contemporary 

 artificial infestations with aphids, described as causing mosaic 

 transmission, were not as heavy as natural ones. 



"This insect is the subject of Bulletin 21f of the Maine Agricultural 

 Experiment Station. 



