ONIONS IN THE CONNECTICUT VALLEY 23 



caused by a parasite. On the basis of work done at this Experiment Station, 1 

 it is not recommended that onions be sprayed every year with a copper fungicide 

 alone, for down}' mildew and blast are not important here every year. It is 

 suggested, however, that growers spraying onions for protection against thrips 

 include Bordeaux mixture 4:4:50 in the combination spray. 



ONION THRIPS AND ITS CONTROL 



The onion thrips is an outstanding pest in practically every region where the 

 crop is grown on any commercial scale. It is always present and in seasons fav- 

 orable for its development causes serious losses to the crop. In regions similar 

 to the Connecticut Valley, the situation is aggravated by the very common 

 practice of growing seed onions close to fields of sets. Set onions, because of their 

 early start and rapid growth, are able to withstand thrips, the attack developing 

 so near the time of maturity that the yield is seldom, if ever, seriously affected. 

 Sets, however, offer a ready source of food and excellent breeding grounds for 

 thrips. It is not unusual to find from 500 to 600 thrips on a single plant. As the 

 set onions reach maturity, the insects migrate to young, tender plants of seed 

 onions near by, where more serious damage is caused. Also this migration usually 

 occurs at the period when the rate of reproduction of thrips is at its height. 



Laboratory experiments with plants cleared of thrips and screened to prevent 

 infestation from outside have shown that 150 to 200 young thrips may emerge 

 from eggs imbedded in the leaf tissue during the space of five to six days. This 

 accounts for the fact noted by many observers that, despite a high percentage of 

 kill by an insecticide, fields often show a heavy reinfestation within a few days 

 after the application. Very often this leads the grower to believe that the spray 

 has been ineffective. 



Effect of Environment on Abundance of Thrips 



The two most important sources of infestation of seed onions are grassland and 

 sets. Set onions offer excellent opportunity for the rapid multiplication of thrips 

 during the early season, while grassland furnishes excellent hibernation quarters 

 for the insects. 



Repeated observations on the seasonal abundance of thrips in commercial 

 plantings of seed onions exposed to either or both of these environments have 

 invariably indicated the danger of early and heavy infestation from both sources. 

 One such field, bordered on the one hand by set onions and on the other by 

 grassland, showed at the time the sets were maturing and just after the field of 

 grass was mowed an average of 1900 thrips per 100 plants in the rows next the 

 sets and 1760 thrips in the rows adjoining grassland. In the center of the plot 

 the average infestation was less than 600 thrips per 100 plants. 



Studies throughout the season of a field of seed onions isolated from sets showed 

 the number of thrips to be negligible up to the middle of July. From July 21 to 

 28 the number per plant averaged 25 to 40 and the infestation reached its peak 

 during the week of July 28 to August 4, with an average of 42 thrips per plant. 

 The plants in this field showed little or no injury and matured a good crop. 



'Doran, William L., and Bourne, Arthur I. Onion spraying and dusting experiments. Mass. 

 Agr. Expt. Sta. Bui. 279. 1931. 



