INSECTS AND PLANT DISEASES — CARTER 341 



land. If, by control of grazing, the original plant species could be 

 restored, then the vast area at present devoted to weeds would once 

 more be restored to usefulness and at the same time the source of most 

 of the leafhoppers would be elimmated. 



If the sugar beet industry in the West had had to rely on this pro- 

 gram, however, it would have continued to decline. Why then has 

 it increased in size and productiveness? The answer is because dis- 

 ease-resistant beets have been sought and found (pi. 6, fig. 1). This 

 has been the work of many men in many places, but the net result 

 is one of the most successful in the history of science in agriculture. 

 Many other efforts are being made to produce plants resistant to virus 

 diseases, because in these cases merely reducing the numbers of insects 

 does not seem to help very much. An insect pauses for a moment to 

 feed on a plant and that plant becomes diseased whether the insect 

 stays there or not. If the disease-bearing insect can be kept from 

 reaching the plant at all, then control is possible. Aster growers, for 

 instance, can get a considerable measure of relief by protecting their 

 aster beds with a low screen fence. The aster yellows leafhopper is 

 not a migrant on the grand scale like the sugar beet leafhopper, but 

 rather a low-flying species which can be kept out by means of screens 

 which would not affect the sugar beet leafhopper in the least. 



A somewhat similar control method is used in Java against the 

 mosquito bug on tea. This insect belongs to the group that could be 

 controlled by sprays, but a Java tea plantation looks like a huge 

 forest with the tea plantings as a solid undergrowth, and spraying 

 is not practicable. The control used there is based on the fact that 

 the insect is shy and retiring and does not cross open spaces willingly. 

 When the tea plant is pruned, therefore, the pruning is done in alter- 

 nate strips so that the insect tends to remain in the unpruned portion 

 instead of spreading over the whole plantation. Although this is 

 avoidance rather than control, the method has proved its usefulness 

 in reducing damage (pi. 6, fig. 2), 



The most recent method of control is being used in connection with 

 some tobacco viruses. It has been found that some viruses which do 

 little damage to plants inoculated with them, nevertheless protect 

 the plant against other more damaging viruses. This protective 

 inoculation offers much promise, but there is one apparent danger in 

 that some viruses, when found in plants with other viruses, cause 

 damage far more severe than when they attack the plants singly. 



THE OUTLOOK FOR THE FUTURE 



Certain trends may be recognized which may indicate what future 

 developments will be. First there is a definite move toward compila- 

 tion of scattered data into summaries and textbooks; a part of this 

 trend is the attempt to set up formal classifications. Once a beginning 



