254 EXPERIMENT STATION ItECORD. LVol. 41 



coveriugs were soon on i'cbruury 15, and when tlio lirst nymphs emerged, on 

 March 21, approximately 95 per cent of the normal live eggs (45-50 per cent 

 of the eggs were dead) had split their outer semitransparent covering. 



" These observations on the morphology and behavior of the egg coverings 

 show conclusively that the egg is not a hard resistant body, and that it goes 

 through a critical change previous to the emergence of the nymph. It is in the 

 midst of this critical period that the egg is most susceptible t;) evaporating 

 factors and certain contact insecticides. 



"The outer semitransparent layer of the egg is somewhat impervious to water; 

 consequently, the water content of the embryo does not undergo very much 

 evaporation in moist weather, or in other words, when low evaporating fac- 

 tors exist, such as high humidity, low temperature, and probably small wind 

 velocity. The outer layer, however, is not entirely impervious, for extreme 

 drought will cause the vast majority of the eggs to shrivel and never hatch. 

 In other words, low humidity, high temperature, and probably air velocity 

 undoubtedly bring about a greater evaporation of the water content of the 

 embryo, and tlius destroy the living form. The inner pigmented (black) lay- 

 er of the egg is not an efficient protector against evaporation. Numerous and 

 varied experiments at the laboratory and observations made on the per- 

 centage of hatched eggs, of A. avencc during the past two totally different 

 seasons, 1917 and 1918, show conclusively the pervious nature of this layer. 

 The eggs are most susceptible to evaporating factors and contact insecticides 

 the latter part of March, or in other words, when the greatest number shows 

 a split outer layer, and this occurs when the first nymphs start to emerge. 



"Experiments conducted in the laboratory under controlled percentages of 

 moisture and also experiments where similar <\ggs of A. arcnw were kept 

 out-of-doors during the critical period (February 15 to March 31, and espe- 

 cially important March 15 to March 31) in 1917 which was wet, while in 

 1918 this period was dry, show quite conclusively that the percentage of 

 hatched eggs is much higher in a low evaporating environment than in a 

 high evaporating medium. 



"Contact insecticides probably jjrevent the egg from hatching in several 

 ways. From a physical viewpoint some substances tend to harden the outer 

 semitransparent shell (lime-sulphur) and this makes it impossible for the 

 nymphs to split the hardened layer. This hardening effect may be due to 

 desiccation. Desiccating substances may also remove the W'ater content of 

 the embryo within, especially if applied after the outer layer has split. Other 

 substances soften and disintegrate the outer imperAious layer (crude car- 

 bolic acid and cresols) and thus expose the inner pigmented layer to evapo- 

 rating factors. The above physical reaction of contact insecticides on eggs of 

 aphids may be important, but it Is probable that the toxic effect upon the 

 embryo of various contact insecticides is more important. So far, no tech- 

 nique has been found which will determine the penetrative ability of the 

 various chemicals used. . . . 



" We can safely reconunend as a control measure for aphids a delayed dor- 

 mant spray of lime-suliihur (1:8 or 1:9) combined with nicotin (blackleaf 

 40) 1:500. The combined spray kills 98 to 100 per cent of all the eggs that 

 are coated and will also kill all tlie newly hatched nymplis, provided they are 

 hit with the spray. Dormant lime-sulphur (1:9) by itself will kill a large 

 percentage (90 per cent or better) of the eggs, but not enough to rely upon it 

 alone. Furthermore, lime-sulphur alone will kill only a small percentage of the 

 newly hatched nymphs if they have made their appearance. Therefore, a com- 



