344 GROWTH OF PLANTS 



penetration in most cases was not complete, especially with aqueous solu- 

 tions; therefore, the view that death is due entirely to suffocation is unten- 

 able. The tracheae serve to conduct the active ingredient of the spray into 

 close proximity to certain tissues, especially the nerves. 



The importance of surface forces in the performance of contact insecti- 

 cides had long been recognized, but few careful measurements had been 

 made. Preliminary observations had shown that spray solutions wet insects 

 poorly because of the nature of the integument and did not spread and form 

 a film unless a suitable spreading agent was present. Drops of water that 

 had been sprayed on an aphid, for example, retained their spherical shape 

 and evaporated before spreading to a film. By adding various spreading 

 agents, such as calcium caseinate, saponin, and gelatin to each of a series of 

 0.1 per cent nicotine solutions, it was noted that only in the case of soap 

 solutions containing 0.5 per cent or more of soap were many of the drops 

 observed to flatten out and spread to a thin film.*'- 



Spray solutions were rated both with respect to their tendency to 

 spread on insect integument, as evidenced by their spreading coefficients, 

 and as to their toxicity to the bean aphid (Aphis rumicis L.). As a result 

 of this work it was demonstrated that spreading is correlated directly with 

 toxicity. An aqueous solution of nicotine (0.1 per cent), for example, gave 

 a kill of approximately 60 per cent of Aphis rumicis "wdthout a spreading 

 agent. When 0.5 per cent of sodium oleate was added, the kill was increased 

 to 97 per cent. 



The effectiveness of contact insecticides is greatly increased if the spray 

 solution penetrates the tracheae of the insect. WTien excised portions of 

 the tracheae from the common tomato worm (Phlegethontius quinquemacu- 

 lata Haw.) were submerged at one end in drops of spray under a microscope, 

 it was observed that water and several other spray solutions did not enter 

 the tracheae, the surface forces between the liquid and the tracheal wall 

 evidently being opposed by hydrostatic pressure tending to cause entrance. 

 This is quite different from the behavior of such solutions in capillary glass 

 tubes. 



Soap solutions containing 0.5 per cent or more of soap entered the 

 tracheae and exhibited a contact angle within the tracheal walls of slightly 

 less than 90°. This large contact angle indicates that even in the case of 

 soap solutions the capillary forces tending to cause entrance are relatively 

 feeble. It mil be remembered that when the contact angle is 90° the capil- 

 lary force is zero. 



A study was made of the actual penetration of the spray solutions into 

 the tracheae of the common tomato worm by immersing the insect in the 

 spray solution, removing and immediately dissecting the larva without the 

 use of water or other liquid. Photomicrographs were taken of these dis- 

 sections (Fig. 138). Nicotine-soap solutions were plainly visible in the 

 tracheae. Even with the use of soap as a spreader the solutions did not 

 penetrate the tracheal system of a tomato worm that had been previously 



