25 MASS. EXPERIMENT STATION BULLETIN 318 



successful, largely because the beetles upon liberation immediately spread over a 

 wide area and did net confine themselves to the fields where they were released. 



In the late summer of 1932 a parasitic fungus was discovered to be attacking 

 onion thrips throughcut onion fields in the Connecticut Valley. This appeared in 

 considerable abundance about August 1 and continued to be prevalent up to the 

 close of the growing season. The first evidence of its presence was a slowing up in 

 activity and general sluggishness on the part of the infected thrips. Subsequent 

 examination revealed the presence of a fungus in these dead and dying thrips, and 

 microscopic examination in the laboratory showed the bodies to be completely 

 filled with the mycelium of the fungus. During the remainder of the season it was 

 fcund that the fungus was well distributed throughout the onion growing section 

 of the Connecticut Valley. Studies on the thrips abundance for the season re- 

 vealed that frcm approximately the middle of August the decline of numbers of 

 thrips, instead of following the usual curve established by life history studies over 

 a period of years, dropped off very rapidly. It was evident that the presence of 

 the fungus had much to do with this, for it was noted in many fields that the 

 appearance and increase in abundance of this fungus were coincident with the 

 sharp decline in thrips abundance. 



During the season of 1933 the fungus was observed to be present over approx- 

 imately the same area but was not as abundant, and did not cause such a reduc- 

 tion in the numbers of thrips as was noted during the previous season, undoubtedly 

 because prevailing weather conditions were adverse during midsummer. 



The fungus was again abundant in 1934 and very generally distributed 

 throughout the Valley. Unfavorable weather conditions did not allow it to develop 

 as early or as rapidly as in 1932 so that its presence did not coincide as accurately 

 with the peak of thrips abundance. It is worthy of note that in the season of 

 1932, when the disease was present in greatest abundance, large numbers of lady 

 beetles were found present in the fields. These may have served to distribute the 

 fungus since several observers have noted that a large part of the food of certain 

 lady beetles consists of fungus spores. 



Value of Insecticides in Thrips Control 



On the onions, thrips tend to congregate in the narrow space deep in the axils 

 of t he inner leaves, the so-called "chits". Fully 80 to 90 per cent of the thrips on a 

 plant are usually clustered there. This habit presents one of the most baffling 

 problems in thrips control and largely explains the comparative failure thus far of 

 dusts (nicotine, cyanide, pyrethrum or rotenone) to control this pest. Insecti- 

 cides applied as a spray, however, have proved more successful. 



The peculiar type of leaf and its smooth waxy surface make the onion a difficult 

 plant to cover thoroughly. Any spray to be effective must possess superior 

 adhesive and spreading qualities as well as a satisfactory killing action on the 

 insects. 



In general, the various oil sprays proved very toxic to the thrips, but showed 

 a great tendency to form into drops and roll off the leaves, resulting in uneven 

 coverage. This allowed many insects to be untouched by the spray and so escape. 



Nicotine sprays alone did not have the necessary physical qualities. The addi- 

 tion of soaps improved the adhesive and spreading qualities, and this combined 

 spray has proved very successful. It showed excellent spreading and flowing 

 qualities and allowed a thorough and uniform coverage of the smooth, waxy 



