March, 1936] Agricultural Research in N. H. 19 



Contact Insecticides 



During the year the study of contact insecticides has involved particular 

 attention to the phenomenon "wetting." A new method has been developed 

 for the study of wetting on leaf surfaces. The new method takes into account 

 the volume of spray, the pressure at which it is applied, and the time elapsed 

 after the spray application. The new criterion of wetting is called "the ex- 

 posure period." This is defined as the minimum number of seconds' 

 exposure necessary for a spray applied at a definite pressure and distance to 

 effect complete wetting of a leaf surface. Wetting is indicated by the dark 

 glossy appearance of the leaf when covered by a continuous film of liquid 

 actually in contact with it. The results of this study will appear shortly in 

 the series, "Studies of Contact Insecticides." 



Another phase in the investigation of contact insecticides which has 

 received attention during the year has been a study of the penetration of 

 white arsenic and sodium arsenite through the integument of insects. The 

 arsenicals are generally considered to act chiefly as stomach poisons when 

 used in insect control, but definite evidence of contact action by sodium 

 arsenite has been demonstrated. 



In undertaking this study it was necessary first to develop a standard 

 technique for applying the toxicant. Such a technique was successfully de- 

 veloped. 



Quantitative determinations of the arsenic content of treated insects 

 were made by means of a modified Gutzeit method. Preliminary work in- 

 cluded determinations of the arsenic content of organs and parts of organs 

 of insects treated with either white arsenic or sodium arsenite. Penetration 

 through the insect integument was demonstrated. Arsenic was recovered 

 from all organs or tissues examined. The results of these studies were pub- 

 lished in Technical Bulletin 63, entitled "Penetration of Arsenic into Insects. 

 Studies of Contact Insecticides X." 



These studies are being continued, and an attempt is being made to deter- 

 mine the arsenic content of additional organs and tissues. Separate studies 

 of the effects of arsenic on the blood of insects are in progress. These in- 

 clude effects on total cell count, on cell types, on plasma, etc. Additional 

 studies are being made "in vitro" of the digestive tract to determine which 

 parts are penetrated by arsenic. This phase has an important bearing on the 

 elimination of arsenic from the insect's body. (W. C. O'Kane, J. G. Conk- 

 lin, L. C. Glover, W . A. Westgate — Purnell Fund.) 



Ovicides 



Petroleum oils have been found to penetrate the eggs of insects repre- 

 senting six different orders. The presence of oil was demonstrated in the 

 chorion, underlying the chorion, and surrounding the globular contents of 

 the embryo. A predominance of oil was observed in the micropylar region. 

 The results of this phase of the work are summarized in Technical Bulletin 

 62, entitled "Further Determinations of Oil Penetration into Insect Eggs. 

 Studies of Contact Insecticides IX." It has been found that apparently the 

 extent of oil penetration depends in part on the state of embryological devel- 

 opment. In freshly deposited eggs of the squash bug (Anasa tristis DeG.) 

 oil penetration is rather extensive, while in eggs containing well developed 

 embryos, oil was found to have penetrated only in the chorion. In the case of 



