340 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



Sometimes an artificial condition will permit the survival of an 

 insect far out of its normal range, as seems to be the case in Central 

 Alberta in connection with potato psyllid. An outbreak of psyllid 

 yellows was traced to a greenhouse where the insect had overwintered 

 on tomatoes and moved out into nearby potato fields in the spring. 

 Also there appears to be a greenhouse strain of the onion thrips in 

 England which carries the spotted wilt virus while the ordinary field 

 strain does not. 



CONTROL METHODS 



Many of the complicated interrelationships between insects and the 

 diseases they carry or cause to plants are vitally affected by the con- 

 ditions in their immediate environment and this fact is sometimes 

 useful in control measure^. It will be obvious, however, that such 

 a diverse set of problems as are presented by insects in their trans- 

 mission of plant diseases will require most varied attempt3 at control. 

 Control may be directed at the insects themselves or at the organisms 

 they carry, but there is no standard method of control even in closely 

 related case^. 



Field sanitation helps to remove diseased plants and fruits and 

 may thereby reduce the numbers of disease organisms. Complete 

 eradication of diseased elms is believed to be the only pogsible way 

 to eliminate Dutch elm disease. Diseases such as potato hopperburn, 

 psyllid yellows, and mealybug wilt are all controlled by reducing the 

 number of insects, since damage is directly proportional to the number 

 of insects present and the amount of feeding they do. Control may 

 be by sprays or, in the case of the potato leaf hopper on alfalfa, by 

 timing the cutting schedules so as to disturb the insect's reproductive 

 cycle as much as possible. 



When long experience and study have shown that certain sets of 

 weather conditions are followed by outbreaks, it is sometimes possible 

 to predict these outbreaks in advance. This does not, of course, con- 

 trol the outbreak, but it does help farmers to avoid losses because 

 in years when outbreaks are expected they can plant other nonsus- 

 ceptible crops. This was done in a sugar beet curly top district in 

 Idaho from 1927 to 1933, and in these 7 years the prediction failed 

 only once. The necessity is no longer present as far as sugar beets 

 are concerned, but other susceptible crops are grown for which 

 prediction of outbreaks might prove useful. 



The real control for the sugar beet leafhopper problem, whatever 

 the susceptible host is, has been visualized, but the concept is so mag- 

 nificent as to appear impossible of attainment. The vast breeding 

 grounds of the leafhopper are areas which originally were covered 

 with wild grasses and shrubs. These were disturbed by overgrazing 

 and attempts at dry-land farming, and in their places weeds that are 

 breeding plants for the leafhopper have come in and occupied the 



