22 THE CURLY-TOP OF BEETS. 



count of its influence on the leafhoppers. Everything considered, 

 there is good reason to urge early sowing of beets in order to get 

 the plants well established before leafhoppers become numerous. 



NATURAL AND ARTIFICIAL BARRIERS TO LEAFHOPPER 



INVASION. 



The writer's observations as to the disposition and manner of beet 

 leafhopper invasions lead him to think that certain natural and arti- 

 ficial barriers check or prevent the invasions. For example, the 

 intervention of a considerable area of land occupied by crops not 

 known to act as food plant* to the beet leafhopper probably acts as 

 a more or less effective barrier to an invasion, while land immediately 

 contiguous to a wide stretch of sagebrush country is likely to be 

 invaded any season when the hoppers are abundant. It is thought 

 that the intervention of a mountain or a large body of water will 

 act as a barrier and that the direction of the prevailing winds has an 

 influence. Until late in the season of 1900 the writer had not ob- 

 served the symptoms of curly-top on other plants than beets, but in 

 northwest Idaho he noted numerous leafhoppers on cabbages grownng 

 between two badly infested beet fields. The outer leaves of a few 

 of these cabbages exhibited unmistakable vein symptoms and mar- 

 ginal leaf-curl (PL IX. fig. 2). 



HOST PLANTS OF EUTETTIX TENELLA. 



"While there is still some doubt as to the original host plants of 

 Eutettix t( nella Baker, it is rather probable that the principal ones 

 are greasewood {Sarcdbatus sp.), sea-blite (Dondea sp.), Atriplex sp., 

 and Russian thistle (4. 5. 6). The writer has found the insect on 

 Sarcdbatus sp. and Atriplex sp. This insect seems to be native to 

 the southwestern part of the United States (4. G). 



DESCRIPTION OF THE BEET LEAFHOPPER. 



The beet leafhopper {Eutettix tenella) (PI. I. fig. 1) is very gen- 

 erally, though erroneously, known in the Western States as the 

 " white fly." The white fly. properly so called (Aleyrodes citri R. 

 and H.). is a quite different insect and a common pest of citrus trees. 

 The name " white fly " is doubtless somewhat descriptive of the 

 general appearance of the insect, which is of a light yellowish green 

 tint, looking white at a little distance. In size it is almost exactly 

 one-eighth of an inch long and about one-thirtieth of an inch wide. 

 The nymphs vary in size from but little larger than the egg to almost 

 adult size, according to age; their range of color is almost as great 



181 



