EXPERIMENT STATION BULLETINS. 481 



and were checked in such a manner, it is believed, as to make the appar- 

 ent conclusion seem undoubted. In the dormant condition, the insects 

 absorbed less gasoline vapor and less of the vapor of ether than did 

 the same insects when active. The difference in solubility of the insecti- 

 cide vapors in the dormant and in the active insects, was so great that 

 ordinary charges of the insecticides used did not bring the concentration 

 of the insecticide, within the body tissue of the dormant insects, to the 

 danger point. Thus, the lowered absorption capacity, which accom- 

 panied the dormant condition, may easily furnish the chief explanation 

 of the fact that insects, dormant from cold, are harder to kill by ordinary 

 fumigants and b}' ordinarj' miscible oil, contact sprays. 



Evidence has been given that the tissue fats and lipoids of living in- 

 sects. should be considered as the principal solvents of such insecticides 

 as gasoline and kerosene vapors, ether and the like. The suggestion made 

 in connection with egg-yolk absorptions, therefore, seems proper here. 

 Namely, the changes, which accompanied respectively an active or a 

 dormant state, occasioning an increase or decrease in the capacity for 

 absorption of insecticide vapors, was probably a change in the condition 

 of the tissue fats and lipoids — or, in their relation to the other cell 

 constituents.. 



IN REGARD TO THE MANNER IN WHICH BORAX AND SOME OTHER 

 POWDERS (SOLID CONTACT INSECTICIDES) BECOME 

 EFFECTIVE AGAINST CERTAIN INSECTS. 



Mention has already been made (Technical Bulletin 11) of certain 

 dry powdered solids that may be applied as contact insecticides. In the 

 case of pyrethrum powder, attention was called to the fact that, as ap- 

 plied in most cases, the principal influence of this insecticide is due to 

 the volatile bodies held in the powder. This is very clear when the 

 insect is confined in a warm air-space with some of the powder. How- 

 ever, certain insects (for example diptera and grasshoppers) may some- 

 times be overcome — for a time, at least — by dusting their bodies with 

 the powder, even in the open air. When such insects are examined, one 

 may often observe with a binocular microscope that some of the powder 

 becomes lodged in the dry vestibule of the tracheae within the spiracles, 

 and no doubt the volatile bodies from the powder charge the air in the 

 tracheal system. The poison in such cases, therefore, really becomes 

 absorbed as a gas or vapor. Nevertheless, it is possible in the case of 

 some insects for certain dry non-volatile powders to become effective as 

 contact insecticides. White hellebore (although it contains a slightly 

 volatile body) does not kill insects confined in the air with it at ordinary 

 temperatures, provided they do not come in contact with the poison — 

 or do not eat it. It is Avell known that the fresh hellebore powder is a 

 stomach poison to insects; it may be observed to be effective as a con- 

 tact insecticide also, with moist rose slugs. I have watched many young 

 rose slugs that did not attempt to eat after their bodies had been pow- 

 dered with fresh hellebore. If they attempted to crawl, the body seemed 

 to be clumsy as if the hypodernial sense organs had been numbed ; and 

 usually, the slugs would fall from the leaf. Slugs, in this condition, were 

 kept for varying lengths of time (up to nearly two days) until they died. 

 61 



