338 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



Weather, being the day to day expression of climate in that region, 

 vitally affects both numbers and activity of the insects living in the 

 region. But even weather is subject to an almost infinite number of 

 variations due to extremely local conditions, and as far as an insect 

 is concerned, the weather on the outside edges of a bush may be quite 

 unfavorable compared to that within the confines of the same plant. 



With regard to climatic influences, the case of curly top of sugar beet 

 and the beet leafhopper is a well-studied example. This insect is 

 distributed over a vast area bounded roughly by the Rocky Mountains 

 on the east and the dry interior valleys of California on the west. To 

 the north and south it appears to extend as far as dry, semiarid condi- 

 tions hold. The climate is characterized by spring and fall rains and 

 hot, dry summers and by great variation in winter weather from year 

 to year. The insect overwinters near the annual weeds that germinate 

 in the fall, and in the spring breeds freely on these plants. When these 

 short-lived weeds mature, the leafhoppers migrate and disperse over 

 huge areas seeking new feeding grounds. It is this dispersal which 

 brings disaster to cultivated crops for when the uncultivated, unirri- 

 gated lands are dried up, the lush green irrigated sections are oases 

 for the flying leafhoppers which in some years arrive almost overnight 

 in enormous numbers. It is during these great flights that occasionally 

 the leafhopper is carried by wind currents far beyond its normal geo- 

 graphic range and instances have been recorded of finding curly top 

 of sugar beet in the Dakotas. If the climate of the Midwest, with its 

 summer rains and severe winters, would permit the survival of the 

 sugar beet leafhopper, there is no doubt that the insect would long 

 since have established itself there as a serious pest. 



In its home range, the migrations of this insect are greatly affected 

 by weather which shows extreme variations from year to year, and 

 these variations are sufficient to make the difference between a year 

 of early and extensive migration and a year where, except for minor 

 flights, the bulk of the leafhopper population remains on the desert 

 until too late to do serious damage to crops. These differences are 

 governed by the condition of the insect's weed host plants. An early 

 warm and dry season will hasten the maturity of both the new brood 

 of insects and the plants, so that before the plants that normally carry 

 the insect through to summer are growing, the early host plants are 

 dry and unsuitable for food. Variations in the severity of the winter 

 also affect the survival of the leafhoppers to a marked degree, a severe 

 winter materially reducing the numbers that are available for repro- 

 duction in the spring. 



An adequate study of such a problem involves detailed investigation 

 over vast stretches of country. Breeding grounds of the leafhopper 

 must be located and the life history of the insect studied there. Great 

 areas of desert must be traversed to determine the presence of the 



