262 Journal of Agricultural Research voi. xvu.no. 6 



tubers showed the symptoms later, averaging 44 days after planting, 

 and that they usually came from eyes of the bud end and therefore were 

 probably the first to become exposed to aphid attack. 



• GREENHOUSE EXPERIMENTS WITH INSECT CAGES 



As has been indicated already, plants that appear healthy may pro- 

 duce tubers that develop mottled plants. In studies with potato mosaic, 

 therefore, it is ver>' desirable to grow a second generation if the effects 

 of a given treatment are to be fully disclosed. Under greenhouse con- 

 ditions, especially in Maine, it is necessary to furnish treated plants 

 with as much light as possible if a satisfactory crop of tubers is to be 

 secured. This makes it appear better, in experiments involving the 

 artificial introduction of aphids, to remove as soon as possible any cages 

 that were used. This can be done without compromising the results of 

 the experiments if frequent inspection and fumigation are employed to 

 keep insects reduced to negligible numbers. 



During the winter of 191 8-19 an experiment was performed in the 

 greenhouse at Orono, Me., with Green Mountain potatoes that had been 

 grown in a rogued plot in northern Maine during the season of 191 7 and had 

 been kept for about a year in cold storage. Fifteen tubers were planted, 

 of which 3, or 20 per cent, produced plants which showed mosaic symp- 

 toms when only a few inches tall. The same rogued stock when planted 

 in the field in 191 8 had shown mottling in 11 per cent of the hills. The 

 other 12 tubers, each being divided into 2, 4, or 5 sets, furnished 53 

 plants. Twenty-one plants, i or 2 from each tuber, were kept as untreated 

 controls throughout the experiment and remained healthy. Eighteen 

 plants, I or 2 from each tuber, were fed upon by aphids introduced from 

 mosaic potato plants; and 13 of them, or 72 per cent, eventually devel- 

 oped typical mosaic symptoms. Five plants, from 5 tubers, were fed 

 upon by aphids introduced from a healthy potato plant; 8 plants, from 

 8 tubers, were infested by aphids from radish plants; but all of these 

 remained healthy. 



In this experiment spinach aphids ^ were used and were never found, 

 during frequent inspections, to be parasitized by other insects or by 

 fungi or to be mixed with predatory enemies or with individuals of another 

 aphid species. They were secured from two colonies, one on a mosaic- 

 diseased potato plant and the other on an apparently mosaic-free one. 

 Stock from the former was kept on mosaic-diseased potato plants and 

 that from the latter on healthy ones or on radish plants until ready for 

 use. The aphids were transferred to the plants of the experiment by 

 methods that seemed favorable to the transmitting of mosaic: (i) By 

 laying one or two leaves, bearing feeding aphids, upon the plant so that 



1 Determinations were made by Dr. Editli M. Patch, Entomologist of the Maine Agricultural Experi- 

 ment Station, who informs one of the writers that this species frequently is abundant upon potato 

 plants in Aroostook County and other parts of Maine. 



