20 BULLETIN 438, U. S. DEPARTMENT OP AGRICULTURE. 



To ascertain how far the control of the pear pests just mentioned 

 could be relied upon to keep the pear leaf-worm in check, control 

 experiments were made in the spring of 1913 in California and in 

 1915 in the State of Washington, and are shown in Table XVI. 



Actual count was made 24 hours after spraying, and the larvae 

 found on the leaves only were taken into consideration. Leaves 

 with holes eaten in them, but with no larvae present, were not made 

 part of the record. This spray was applied with pressure, the force 

 of which, when hitting the leaves at close range, more than likely 

 caused larvae to loosen their hold and fall to the ground. It is also 

 more than probable that some sick larvae likewise fell before the 

 count was made. At that time leaves were noticed with holes in 

 them smaller than those which would have been produced by larvae 

 remaining on the leaf until their full development had been attained. 

 The mortality therefore would be greater than is recorded in these 

 tables, and this accounts in a measure for the difference in the results 

 found for the same formula applied in California and in the State of 

 Washington, because in the latter instance the experiment was made 

 under laboratory conditions which would afford opportunity for closer 

 observations and would yield more precise results. 



Field conditions prevailed in the California experiments, because 

 common every-day spraying, as ordinarily practiced in orchards for 

 other pests, was the only object in view as a control at the same 

 time for the pear leaf-worm. 



In the control table (Table XVI) the experiments with tree No. 3 

 and tree No. 7, in which a contact spray was used containing 

 fish-oil soap and extract of nicotine, indicate a comparatively small 

 percentage of mortality compared to that in which the material con- 

 tained in adchtion arsenate of lead, as in the experiment with tree 

 No. 2. But it must be mentioned that in the case both of tree No. 

 3 and of tree No. 7, the absence of larvae on leaves with holes when 

 the count was taken was very conspicuous and the larvae that sur- 

 vived were all large. 



A contact spray, whether with or without the addition of dis- 

 tillate oil, is a mechanical emulsion or mixture, which, to be effective, 

 requires application with greater pressure than does a poison spray. 

 Because of this, the liquid strikes the leaves with enough force to 

 dislodge many of the worms, which drop to the ground, where death 

 ensues, caused by the spray adhering to them. 



The addition of fish-oil soap to a mixture of water and nicotine 

 extract increases the efficiency of the spray by imparting to the 

 liquid more penetration and better spreading and adhering properties. 



