42 BULLETIN 100, U. S. DEPARTMENT OF AGRICULTURE. 



Experiment No. 5. — Distillate-oil emulsion, 2 per cent. Orchard of Mr. F. Leib, 

 near San Jose, Cal. A block of six badly infested walnut trees was sprayed under 

 110 pounds pressure, July 31, 1912. A count made on August 6 showed that 74 per 

 cent of the "lice" had been destroyed. 



Experiment No. 6. — Distillate-oil emulsion, 2 per cent, combined with commercial 

 tobacco extract No. 2 (1-2,000). Orchard of Mr. F. Leib, near San Jose, Gal. (PI. 

 IV, figs. 1, 2). Six walnut trees, badly infested, were sprayed under a pressure of 

 110 pounds. A count, made August 6, showed that 85 per cent of the "lice" had 

 been killed by the spray. 



Experiment No. 7. — Whale-oil soap, 1 pound; water, 5 gallons. Orchard of Mr. 

 E. I. Hutchinson, Concord, Cal. A block of 12 moderately infested European walnut 

 trees was sprayed under 150 pounds pressure, May 10, 1913. A count made two days 

 later showed that out of 473 "lice" counted, 263, or 55.6 per cent, had been destroyed. 

 A thorough drenching had been applied and the trees were in full leaf. It was noticed 

 that the great majority of the "lice" that escaped were situated close to the base of 

 the midrib. In this position they were partly protected by the projecting rib, and 

 it is to be supposed that the wash lacked the pressure nece-'iry to reach these 

 individuals. 



All the foregoing experiments were undertaken on trees on which 

 the foliage was fully developed. It was noticeable that on thickly- 

 foliated trees the percentage of plant lice killed was the smallest, 

 while on thinly foliated trees the greatest mortality resulted. Much 

 of the leaf surface on thickly foliated trees is almost inaccessible 

 to spray. 



A comparison of the results of the foregoing tests favors distillate- 

 oil emulsion and tobacco. The most desirable combination for spring 

 and summer spraying is a 2 per cent distillate-oil emulsion, commer- 

 cial or homemade, combined with commercial tobacco extract No. 

 2, 1 to 1,500. High pressure (150 pounds or over) is desirable, 

 although not absolutely necessary unless the spraying be done before 

 the walnut leaflets have flattened out in spring. 



In timing the application for the aphides on the leaves it is desirable 

 to spray as early as possible in order to reduce the amount of leaf 

 surface to be covered by the wash and to destroy the plant lice before 

 they attack the nuts. On the other hand it will be found very hard 

 to destroy the plant lice before the leaflets flatten out, for the young 

 leaflets are pressed against one another in a manner that affords 

 very good protection to the insects from a spray. Moreover at this 

 period all the stem-mother plant lice will not have hatched from 

 the winter eggs. The time most preferable for the application is 

 just as soon as the growing leaflets shall have flattened out and before 

 they have attained their full size. At this time the "lice" have all 

 hatched and are all exposed on the underside of the leaves. Should 

 an oil spray be applied care should be taken that there is no free oil 

 in the emulsion, as the young nuts are susceptible to burning. No 

 stronger than a 2 per cent distillate-oil emulsion should be used for 

 this early application. The spray should, be directed to the underside 

 of the leaves, and angle nozzles used. A round nozzle is to be pre- 



