INSECTS AND PLANT DISEASES — CARTER 339 



insect and the appearance of the new brood, for it is when this brood 

 matures that the danger period is at hand and flights into cuKivated 

 fields are likely to occur. 



In the beet leafhopper problem, the weather conditions over indi- 

 vidual areas perhaps 200 miles long are significant. In Africa, where 

 a virus disease of the peanut is sometimes serious, weather is also 

 important but in an extremely localized way. There is a story told 

 which may be apocryphal, but which has a substantial basis as far as 

 the scientific data are concerned. A director of agriculture in an 

 African colony, wishing to improve the native methods of culture of 

 peanuts, arranged to have demonstration plots planted to show the 

 natives that their method of crowding the plants together was wrong. 

 When the crops were grown, however, the nice, orderly, well-separated 

 official plants were badly diseased, while the native plantings escaped 

 with only minor damage. The explanation, which seems to be well 

 authenticated, is that the more open plantings permitted freer air 

 movement, which increased temperature and light and reduced mois- 

 ture around the plants. These changed conditions were enough to 

 favor the activity of the insect carrier of the peanut mosaic, which is 

 an aphid. The native knew through long trial and error that his 

 method worked, but the investigator, looking for the reason, had to 

 have recourse to delicate measurements of extremely local weather in 

 a very limited environment. 



The old cartoon concept of an entomologist was of a bespectacled 

 individual waving a huge collecting net, but Welsh and Cornish 

 farmers have recently been treated to the sight of men tramping 

 through their fields, swinging not nets but thermometers, measuring 

 humidities and wind velocities and collecting not butterflies but plant 

 lice; and doing all these things because the potato crop depends on 

 good disease-free seed. 



The seed-potato business in the British I^les has been almost a 

 monopoly of certain Scottish districts where potatoes have enjoyed 

 freedom from the virus diseases. In recent years, however, these dis- 

 eases have become established there and the desirability of finding 

 new districts where disease-free seed could be grown has led to a 

 careful study of potato-growing areas to determine the reasons why 

 potato phmts are free from the disease in some locations and severely 

 affected in others in the same district. It was found that the princi- 

 pal plant lice species that carry potato viruses are very sensitive to 

 a combination of wind movement and air humidity and that fields 

 subject to the right combination of these two factors were likely to be 

 afl^ected only to a very slight degree. This is one of the most recent 

 and best illustrations of how a knowledge of an insect's reactions to 

 extremely small variations in the weather has led to important 

 practical results. 



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