GO Bulletin 332 



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

 Ill other words, h)w humidity, high temperatuie and j)robal)ly air 

 velocity undoubtedly bring about a greater evaporation of the 

 water content (if the embryo, and thus destroy the liviny foi-ni. 



The iiuier pigmented (black) layer of the egg is not an 

 et^cient ])rotector against evaporation. Numerous ami varied ex 

 periments at the laboratory and observations made on the per- 

 centage of hatched eggs of A. auout during the past two totally 

 ditt'erent seasons, 1917 and 1918, show conclusively the pervious 

 nature of this layer. 



The eggs are most susceptible to evaporating factors and con- 

 tact insecticides during the latter part of .March, or in other words, 

 when the greatest number show a split outer layer, and this occurs 

 when the first nymphs start to emerge (fig. 7 and 10). 



Experiments conducted in the laboratory under controlled 

 percentages of moisture and also experiments where similar eggs 

 of A. aveiuv were kept out-of-doors during the critical period 

 (February 15 to March 31, and especiall}' important March 15 to 

 March 31) in 1917 which was wet, while in 1918 this period was 

 dry, show cpiite conclusively that the percentage of hatched eggs 

 is nuich higher in a low evaporating environment than in a high 

 evaporating medium. 



Contact insecticides prol)al)ly prevent the egg from hatching 

 in several ways. From a physical viewpoint some sul)stanees tend 

 to harden the outer semi-transparent shell (lime-sulfur), and this 

 makes it impossible for the nymphs to split the hardened layer. 

 This hardening effect may be due to dessication. Dessicating sub- 

 stances may also remove the water content of the embryo within, 

 especially if applied after the outer layer has split. Other sub- 

 stances soften and disintegrate the outer impervious layer (crude 

 carbolic acid and cresols) and thus expose the inner pigmented 

 layer to evaporating factors. The above physical reaction of con- 

 tact insecticides on eggs of aphides may be important, but it is 

 l)robable that the toxic effect upon the embr^'o of various contact 

 insecticides is more important. So far, no technic has been found 

 which will determine the penetrative ability of the various chemicals 

 used. 



Control IMeasures 



In conclusion to the foregoing experiments and observations 

 we can safely reconnnend as a c(mtrol measure for aphids a delayed 

 dormant spray of lime-sulfur, 1-8 or 1-9, combined with nicotine 

 (" Black-leaf 40"), 1-500. The combined spray kills 98-100 per 

 cent of all the eggs that are coated and will also kill all the newly- 

 hatched nymphs, ])fovided they are hit with the spray. Dormant 



