February, '16] PADDOCK: TURNIP LOUSE 69 



found on turnips and radish, whereas the cabbage louse is usually- 

 found on cabbage. The cabbage louse seemed to be more hardy than 

 the turnip louse. The garden aphis was found abundant on turnip 

 and radish at all times. This species was very closely associated with 

 the turnip louse. It was much more hardy and consequently bears 

 very interesting biological relationships. The garden aphis proved 

 to have a far greater adaptive capacity and so it could withstand far 

 more unfavorable climatic conditions. In times when the turnip 

 louse was absent from the fields this species served as a host for the 

 natural factors of control. The parasites would not work on the 

 garden aphis until the turnip louse was very scarce, but in this way 

 the parasites could maintain themselves in numbers until the turnip 

 lice might appear again. The garden aphis seemed to be free from 

 the attacks of the predaceous enemies as long as an ample supply of 

 turnip lice was present. The most wonderful 6i the seemingly 

 selective power was that of the fungous disease. This disease proved 

 destructive to the turnip louse when the garden aphis flourished. It 

 was only when the numbers of turnip lice were very reduced that the 

 garden aphis died from the disease. The garden aphis did not seem 

 to be affected as much by the low temperatures as did the turnip 

 louse. 



The natural factors of control, which were present to a greater or 

 less extent over the state, exerted a very marked effect on the turnip 

 louse. In some sections the turnip louse was held in check so effec- 

 tively that it was not feared as a pest. Observations were made on 

 two species of parasites, three species of coccinellids, two species of 

 syrphid flies, one species of chrysopa, and a fungous disease. 



The most widely distributed parasite was Lysiphlehus testaceipes 

 Cress. This species was always found in numbers in the northern 

 part of the state in those localities where the "green-bug" has been 

 destructive. This parasite always proves very effective in the control 

 of the "green-bug." The other parasite, Diccretus rapce Curt., was 

 present in great numbers at College Station, and to a limited extent 

 in the southern sections of the state. From the observations made 

 it is evident that the life-history and habits of these lice parasites are 

 quite similar. 



Some interesting results were noted of the effect of temperature on 

 the development and activities of Diccretus. The last week of Decem- 

 ber 1913 was very cold but the first week of January 1914 was quite 

 mild. On January 9, adult Diceretus were taken in the outdoor cages. 

 This brood continued to emerge for a period of over one week. On 

 January 17, dead parasitized lice were again observed in the outdoor 

 cages. During these first seventeen days of January the average 



