INSECTS AFFECTING FIELD CROPS 495 



choose the right date for sowing. After conducting long series 

 of experiments the United States Bureau of Entomology has 

 suggested the following dates as safe for sowing wheat in aver- 

 age seasons: in northern Michigan soon after the first of Sep- 

 tember; in southern Michigan and northern Ohio, about 

 September 20; in southern Ohio, after the first week in October; 

 in Kentucky and Tennessee, October 10 to 20; in Georgia and 

 South Carolina, October 20 to November 15. The exact 

 time will also depend upon altitude as well as latitude. Crop 

 rotation, thorough cultivation, clean culture and the use of 

 good fertilizers are all important in regions where the Hessian- 

 fly is a bad pest. No varieties of wheat thus far found are 

 wholly exempt from the attacks of the fly but some, on account 

 of their habit of growth, withstand the injury better than others. 



The larvae of certain other flies may also often be found 

 working in the wheat in much the same way as the Hessian-fly, 

 but they rarely occur in sufficient numbers to do serious 

 damage. 



The Wheat Joint-worm (Isosoma tritici}. The joint- worms 

 occur in the stems of the wheat causing them to swell slightly 

 and become hard and woody so that the grain is not well 

 nourished. The adults are small black insects belonging to 

 the family Chalcidida, nearly all the other members of which 

 are parasitic. Crop rotation is the best means of control. 

 Infected straw and stubble should be burned before the adult 

 insects, which are overwintering in such places, issue. 



The Wheat Straw-worm (Isosoma grandi}. This insect is 

 closely related to the preceding species, but has somewhat 

 different habits. Many of the members of the spring brood 

 are wingless and look much like small ants. The eggs are laid 

 on the young wheat, and the larva? frequently destroy the whole 

 plant by eating out the crown. The adults of the next genera- 

 tion are larger and provided with wings so that they may fly 

 considerable distances. The young that hatch from the eggs 

 of this brood work in much the same way as the wheat joint- 

 worms. They pass the winter in the pupal stage in the straw. 



The control measures are the same as for the wheat joint- 

 worm. 



