Pests and Diseases 195 



another species {Tycfiea brevicorni-s Hart.) has done con- 

 siderable damage. This species also works on corn roots. 

 No direct method of control is known for either of these 

 insects. Sprays are impractical since the insects work 

 under the ground. Prevention, the only known method 

 of coping witli the pest, can be practiced, however, in crop 

 rotation and clean cultivation. Thorough tillage early 

 in the spring is thought to help in controlling aphids. 



Sugar-beet nematode {Heterodera schachtii Schmidt). 



One of the pests that has done most damage to sugar- 

 beets during the last few years is the nematode. This 

 is not a true insect, but is an exceedingly fine, threadlike, 

 colorless worm, so small that it is difficult to see with the 

 naked eye. When these worms hatch from the egg, they 

 enter the nearest rootlet and feed on the plant juices. 

 This results in the formation of a dense mass of rootlets 

 which cling to the beet when it is pulled up (Plate XXII). 

 This has resulted in calling the trouble "bearded roots," 

 "hairy roots," and other similar names. 



The first evidence of the pest is a change in the color 

 of the foliage, which takes on a lighter tint when the beet 

 is injured. The outer leaves gradually wilt and finally 

 die. The inner ones are small and do not thrive. Often 

 the plant dies and the infected land is left bare. Usually 

 this condition appears as a spot in the field which gradu- 

 ally enlarges. Since the pest is readily carried about in 

 the soil, when it once becomes established in a district, 

 it is likely to extend to all the fields unless its spread is 

 checked. 



Rotation of crops seems to be the best method of 



