72 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 14 



bum. However, this dead area was comparatively small and confined 

 to the extreme apical portion of the leaf. Furthermore, it did not 

 increase in size in spite of the fact that the mid-vein was completely- 

 severed in as many as three distinct places. 



Influence of Environmental Factors — Cage Experiments 



July 29th a second series of tests was conducted in the greenhouse to 

 determine the effect of the potato leafhopper on the plant under different 

 environmental conditions of soil, humidity, and sunlight. These potted 

 potato plants (Early Ohio variety) were all healthy and were caged with 

 an equal ntmiber of leafhoppers, 50 adults and 50 nymphs being added 

 to each cage. Daily observations were made and careful examinations 

 conducted on August 6th and 10th, respectively, the experiment being 

 closed on the latter date. 



One series of plants growing in sand and another in loam were kept on 

 a bench where they were in direct stmlight during the greater part of the 

 da^^ Individual plants in each series were caged with leafhoppers. 

 Burning began within twenty-four hours on those exposed to the insects 

 and increased daily until they were badly injured at the close of the 

 experiment. All check plants remained perfectly normal throughout 

 the test. Potato vines growing in loam burned just as badly as those 

 in sand, the type of soil having little or no influence on the final results. 



In the second phase of the experiment one series of plants growing in 

 loam were placed in pans of water, thus insuring a constant saturation 

 of the soil with moisture; in the second series they were only watered 

 occasionally and enough to keep them from wilting too much; while 

 in the third they were kept in normally moist soil. AH were placed 

 where they were exposed to a maximum amoiint of stmlight. Plants 

 were selected from each series and caged with leafhoppers. Burning 

 developed within twenty-four hours and increased daily on all those 

 exposed to the insects, all these vines being badly injured at the time the 

 experiment was closed. Excess of soil moisture did not retard the 

 biuning nor did lack of it increase the injury, there being no difference 

 in the amount of tipbum on the leaves under these different soil moisture 

 conditions. Check plants were perfectly healthy when the experiment 

 was closed. 



To test the influence of high humidity on the production of tipbum, 

 one series of plants were kept mider glass globes over the top of which 

 panes of glass were placed to prevent evaporation and thus keep the air 

 surrovmding the foliage in a water-saturated condition. Another series 

 were kept as a check and all were placed in sunlight. Individual 

 plants from each were enclosed with leafhoppers. As in the case of the 



