396 



JOURNAL OF ECONOMIC ENTOMOLOGY 



[Vol. 10 



Table IL — E.xperiment in the Infection of Healthy Mallow Plants bt Virulent Specimens of Eutettix 



TENELLA AND THE TRANSFERENCE OP THE CuRLT-fOP INFECTION TO HeALTHT BeETS THROUGH THE AGENCY 

 OF NON-VlRULENT SPECIMENS OF THE InSECT 



Conclusion 



The foregoing experiments show the possibility that common 

 weeds may assist in the perpetuation of the infectious factor which 

 causes the curly-top of sugar beets. That these findings are of the 

 greatest significance can be easily understood. Indeed they introduce 

 a definite field of investigation for the control of this most destructive 

 condition. One fact stands out most prominently from all evidence 

 gathered up to the present time, namely, that the insects must be 

 infected in order to be able to cause the disturbance. From their 

 nature these insects in themselves are unable to produce the disorder. 

 Hence, affected plants are required for reinfection of the insects 

 before a general outbreak can be started. There must be present 

 either some beets affected with curly-top in the fields or weeds which 

 harbor the virulent factor, even in a symbiotic way. Clean cultivation, 

 already extremely desirable from the standpoint of diseases and insect 

 pests, especially during the period that no beets are in the field, is thus 

 necessarily indicated as a means of prevention. Even though all 

 vestiges of affected beets from the previous year have disappeared, 

 and all volunteer beets have been eliminated, there remains the 

 possibility that certain weeds, such as mallow, may harbor the virulent 

 factor during hibernation. After clean cultivation has been put into 

 practice attention must be directed to the surrounding native vegeta- 

 tion. The discovery of the original host plant among this vegetation 

 is the next problem to be considered, and investigations are in progress 

 to determine this point. 



Summary 



Malva rotundijolia, a common weed in the beet fields, has proved 

 to be at least a symbiotic host of the virulent factor of curlj^'-top of 

 sugar beets. 



Individual insects of the curly-top leafhopper {Eutettix tenella 

 Baker) known to be non-virulent were placed on sickly-looking mallow 



