48 MASS. EXPERIMENT STATION BULLETIN 227. 



A STUDY OF THE LIFE HISTORY AND CONTROL OF THE 



ONION THRTPS 



By A. I. BOURNE 



The onion thrips is, without question, the outstanding insect pest of the 

 onion. In cases of severe outbreaks, the losses it has caused liave often 

 been rated as high as 25 to 50 per cent of the crop. The injury, variously 

 termed blast, white blight, and silver top, results from these minute insects 

 extracting the plant fluids, by means of their rasping and sucking mouth parts. 



Throughout the Connecticut Valley, the main onion growing section of 

 Massachusetts, this pest has, during late years, become very generally es- 

 tablished and appears in large numbers annually. Previously, its ravages 

 were chiefly confined to occasional outbreaks, many times of only local 

 importance. With the increased acreage devoted to set onions, the thrips has 

 come to be an annually recurring pest, and gives every evidence of in- 

 creasing abundance. 



Relation of Set Onions to Thrips Injur ij 



The very close correlation of the set onion industry to the problem of 

 thrips injury to onions grown from seed is generally recognized. Set 

 onions on the average develop several weeks earlier than the crop of seed 

 onions. Sets, therefore, serve as ideal nurseries for the colonization and 

 multiplication of the thrips. The insects are thus supplied with an abun- 

 dance of their favorite food plant, and consequently are able to reach large 

 numbers comparatively early in the season. From actual counts made in 

 the field, of the thrips colonized on nearly mature -plants of set onions, it 

 was found that two to three hundred thrips per plant were about an 

 average, while five to six hundred were by no means uncommon. The sets 

 themselves do not as a rule sufl'er .severely from the attack of the thrips, 

 since, by the time the insects have developed a heavy infestation, the plants 

 have advanced well toward maturity and accumulated enough leaf surface 

 to withstand the attack. 



Areas given over to sets are often located close to, or even alongside, 

 fields later planted to seed onions, so that transfer over onto the younger 

 and smaller plants is easy. This practically assures the later development 

 of a heavy infestation in the fields of seed onions. Observations have 

 shown that while there is more or less of a spread from the sets before 

 they are mature, the great movement takes place when the sets are ready 

 to be pulled. From our studies of the life history of the insect, it was 

 learned that the time of this general transfer usually coincides very closely 

 with the period of greatest reproductive activity of the insect, thus in- 

 creasing the danger to the fields of seed onions. 



There are other sources of possible infestation — refuse and screenings, 

 weeds, grasses, etc. The list of plants upon which the thrips has been found 

 to feed includes many garden and field crops, ornamentals, and greenhouse 

 plants as well as various grasses. While these are of comparatively little 

 importance as compared M'ith set onions, they do offer the insect ample 

 opportunity for hibernation and early spring colonization, and explain 

 why onion fields are often invaded from grass or weedy borders. For this 

 reason, it is recommended to burn over, in late fall or early spring, grass 

 and weedy areas bordering on onion fields. 



