14 THE CUBLY-TOP OF BEETS. 



Among the standard varieties there was apparently little difference 

 in susceptibility. Several species of leafhopper were noted as being 

 generally, though not abundantly, distributed over the field. A small 

 lot of seed was sown in July; quite late in the season some of the seed- 

 lings were inclosed in a cheesecloth-covered cage to exclude leaf- 

 hoppers and other insects. 



No curly-top developed among the caged plants; some cases ap- 

 peared in the open row. This experiment was of little value, because 

 all of the plants had been exposed to insects for some time before any 

 of the plants were isolated. Plans were then made to observe the 

 beets with special reference to this disease from the outset of the 

 following season. 



With the beginning of the season of 1908 various experiments were 

 started looking to the solution of this problem. These covered, in a 

 preliminary way, almost all the ground suggested by the theories 

 heretofore mentioned, with negative results so far as curly-top is 

 concerned. The ground being thus cleared, the experiments were 

 focused on the study of the effects of the work of certain sucking 

 insects, notably thrips, red spiders, aphides, and leafhoppers, all of 

 which were numerous in our field at Garland. 



It was observed that each of these insects produced characteristic 

 effects on beet leaves. At the outset it was deemed desirable to 

 ascertain the specific effects of the attacks of each kind of insect 

 upon beets. This was desirable not only to determine conclusively 

 whether any of these might produce the curly-top symptoms, but, 

 in case they did not, to know just what the effects of each might be. 

 Since all of these insects abound on beets, it was necessary to be 

 able to discriminate the work of each one from that of the others. 

 Then, in case it was found impracticable entirely to exclude the 

 tiny insects from beets isolated with leafhoppers, it might be easy 

 to distinguish effects that could not have been produced by thrips, 

 red spiders, or aphides. In practice it was possible to exclude all 

 except a very few thrips, which entered the cages through the 

 soil. 



ISOLATION CAGES. 



The construction of some of these cages will be understood on 

 referring to figures 4 and 5. A foundation frame of $ by 6 inch 

 lumber was used. This was sunk in the soil about 2 inches to pre- 

 vent the entrance or egress of insects beneath the cage. Upon this, 

 foundation a frame of laths, stout wire, or ^-inch soft iron rods, 

 bent into hoops was made (fig. 5). Over the frame was stretched 



181 



