ANNUAL REPORT, 1931 219 



irrigation system to maintain plenty of moisture during oviposition resulted in 

 only a 2 per cent greater injury to carrots by the first generation but caused 

 13 per cent greater injury by the second generation. 



The field infestation at Waltham was again so light that date of planting and 

 variety susceptibility records were of little value, although a 38 per cent infesta- 

 tion in White Belgian by the first generation indicated that this variety is sus- 

 ceptible. 



Studies of control by insecticides were greatly handicapped by the light 

 infestation. Under the existing conditions Scotch Soot and tobacco dust were 

 the most satisfactory, both giving nearly perfect control with a slight stimula- 

 tion to the plants. These materials were applied to the crown of the plant and 

 adjacent soD by hand at the rate of about 5 pounds to 180 feet of row. 



Napthalene and Similar Materials as Fumigants for the Control of 

 Greenhouse Insect Pests. (\V. D. Whitconib, Waltham). Fumigation with 

 vaporized naphthalene is an effective treatment for the control of most injurious 

 insects and niites on greenhouse plants, and with a few exceptions can be used 

 safely when the proper conditions are maintained. In a tight chamber, the fumes 

 from 13^ ounces of melted naphthalene per 1000 cubic feet of air killed 93 per 

 cent of the red spiders in one fumigation of 6 hours; but 2 or 3 ounces per 1000 

 cubic feet have been found necessary to counteract the leakage and adsorption 

 in commercial greenhouses. An exposure of 4 hours is necessary to cause any 

 appreciable kill of red spiders. Under ideal conditions, two or more fumigations 

 using 3 ounces per ICOO cubic feet killed all red spider eggs; 12.5 per cent hatched 

 after one fumigation, and 89 per cent of the unfumigated eggs hatched. Eggs 

 which were 4 or 5 days old were killed more easily than newly laid eggs. The 

 protonymphal stage of the red spider was most easily killed and the deuto- 

 nymphs and adult males appeared the most resistant. Three or four fumigations 

 are often necessary to obtain a satisfactory kill of red spider, and it was found 

 that an interval of 1 day between treatments was more effective than an interval 

 of 2 or 3 days, and that an interv^al of 3 days was 5 per cent more effective than 

 an interval of 2 days. 



Fumigation under controlled atmo.spheric conditions, ranging from 60 per 

 cent to 90 per cent relative humidity and including 75° and 80° F., .showed a 

 slight but regular decline in effectiveness from the higher to the lower humidity, 

 and a slightly increased kill at 80° F. over 75° F. However, there was a tendency 

 toward plant injury at 90 per cent relative humidity, and the most favorable 

 condition appeared to be a relative humidity of 80 per cent and a temperature 

 of 80° F. 



The addition of 15 per cent paradichlorobenzene to the naphthalene tended 

 to increase the effectiveness of the fumigation with one treatment, but this 

 formula and naphthalene alone were equally effective in two or three fumigations. 



Thrips are more easily killed by this treatment than other insects although 

 aphids and white flies are easih^ controlled by two or more good fumigations. 

 Mealy bugs are very resistant, and the fumes will not penetrate the soil suffi- 

 ciently to kill bulb mites. Exposed cyclamen mites are killed by 3 or more fumi- 

 gations, and treatments at regular intervals keep this pest in check satisfactorily. 



Carnation, begonia and cyclamen tolerate naphthalene at strong concentra- 

 tions and can be fumigated safely in any greenhouse in which the necessary 

 temperature can be maintained. Freesia, oxalis, violet and fuchsia are very sus- 

 ceptible to injury and should not be fumigated unless the temperature, humidity 

 and concentration can be controlled absolutely. Chrysanthemums appear to 



