620 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



and other poisons made up especially by certain manufacturers in 

 the hope of discovering a cheaper compound and one which at the 

 same time will posssibly have less direct injurious effects upon the soil. 



TOBACCO-INSECT INVESTIGATIONS. 



The section of the southern field-crop insect investigations dealing 

 with tobacco insects had its headquarters, as last year, in Clarksville, 

 Tenn., and the work dealt largely with the control of the two species 

 of horn worms of tobacco. An important addition was made to the 

 knowledge of the life history of the hornworms. It was found that 

 the emergence of the moths from the ground in the spring extends 

 over a long period — in fact, until August. It has been supposed that 

 the moths appearing in August were of the second generation, and 

 hence it was argued that large numbers of the first generation of the 

 worms must have escaped destruction by the poisons used. It was 

 thought by planters that it would do little good to attempt to destroy 

 the insects in hibernation, because a large number of worms would 

 escape on young tobacco and in the next generation cause the so-called 

 "August shower of worms." It was found that a comparatively 

 small percentage of the overwintering insects emerge before the end 

 of June — in fact, in 1909 onh^ 4 per cent issued before this time. 

 The last individuals emerging produce a generation of such numbers 

 as to injure tobacco seriously by August 1. During the eleven days 

 from July 30 to August 9, 50 per cent of the hibernating pupre be- 

 came adult, and about 8 per cent became adult later. This em- 

 phasizes greatly the importance of taking all possible means toward 

 the destruction of the hibernating forms — a move which the planters 

 have hitherto been disinclined to make. 



The usual means of controlling the hornworm has been the use of 

 Paris green, but this poison is objectionable on account of injury to 

 the leaf. Arsenate of lead, which does not injure the leaf, kills the 

 insects so slowly that it is objectionable. A form of arsenate of lead 

 has been found, however, which has not this disadvantage, and ex- 

 periments were conducted with a number of other poisons which 

 promise to be of practical value. 



Experimental work against the tobacco flea-beetle was continued, 

 in the way of application of sprays and dust in the plant beds, the 

 dipping of the plants at the time of transplanting, and the applica- 

 tion of various poisons both in dust and spray form to plants in the 

 field. The improvement of the seed beds to avoid attack was also 

 studied. 



Further investigations on the so-called wireworm of tobacco were 

 carried on at Appomattox, Va. It was found that the insect occurs 

 in great numbers on several wild plants, but the great majority on 

 two species which grow in fallow fields and waste places. It seems 

 obvious that the elimination of these two weeds by cultural process or 

 otherwise would go far toward reducing the injury to tobacco. The 

 greatest damage was found where tobacco was planted on fields in 

 which these weeds had occurred in the greatest numbers. Rotation, 

 therefore, and the keeping down of weeds promise good results. 



Further work has been done upon the tobacco thrips and the 

 splitworm in Florida, but it was impossible to conduct this work 

 continuously. 



