ONIONS IN THE CONNECTICUT VALLEY 



25 



Danvers Yellow Globe. The remaining varieties, Ebenezer and Ohio Yellow Globe, 

 proved as susceptible to thrips attack as the Danvers Yellow Globe. 



It would appear likely that the type of growth of the plant has a great deal to 

 do with the abundance of thrips. This theory is supported by the following facts. 

 The susceptible variety Ebenezer exhibits a strong tendency for the leaves to 

 curl, thus allowing protection to the thrips. The varieties Ohio Yellow Globe 

 and Danvers Yellow Globe have tightly closed chits which likewise afford the 

 thrips protection. Thrips-resistant varieties are characterized by upright growth 

 and an open chit. 



In different strains of the same variety, thrips abundance seems to be correlated 

 directly with the presence or absence of a closed chit. This statement is based 

 on a study of comparative resistance of different strains of one variety of onions. 

 Examination of three plots of strain B, which had a pronounced upright type of 

 growth, showed that on two of the plots the plants had open chits, on the third, 

 closed chits. The following data represent the abundance of thrips during the 

 first two weeks in August at the maximum period of thrips abundance. 



Aug. 14 B1+B2 Open chit ' 40 



B3 Closed chit 119 



From the investigations thus far made, it would seem that a type of onion 

 resistant to thrips attack should possess the following characteristics: 



1. Growth upright without any tendency to curl; 



2. An open type of chit; one not tightly closed by the growth of the young 



inner leaves. 



Biological Control ot Thrips 1 



Natural enemies of onion thrips, while prevalent in this region each year, have 

 shown no appreciable value in checking the pest. The predacious insects collected 

 included five species of lady beetles; lace wings, or Chrysopids; several species 

 of Syrphid flies; and a predacious bug. Triphleps insidiosus Say, which has been 

 recorded as feeding almost entirely on different species of thrips. In addition to 

 these are several species of predacious thrips which undoubtedly are among the 

 most effective of all the natural enemies attacking onion thrips. 



While the above list undoubtedly does not include all the species that may 

 prey upon thrips, it indicates that several natural enemies of that pest are well 

 established and generally distributed throughout this region. Unfortunately 

 their numbers are so insignificant as compared with the thrips that they exert 

 little influence upon the infestation. 



Attempts by several growers to liberate large numbers of a species of lady- 

 beetle. Hippodamia convergens, procured frcm dealers in Colorado were un- 



^Credit is given to F. R. Shaw for his observations 

 studies on this disease. 



seasonal abundance and biological 



