April, '12] COOLEY: ORTHO ARSENITE OF ZINC 145 



clisulphicl, arsenic trisulphide, calcium arsenite, lead arsenate (a mix- 

 ture of ortho and acid arsenates) Paris green (from Ansbacher & Co.,) 

 sodium arsenite and zinc ortho arsenite from the California Spray- 

 Chemical Company. These were applied to branches varying from 

 one-fourth to two and one-half inches in diameter and to crowns of 

 from three to five inches in diameter. Weighed amounts were spread 

 on small squares of absorbent cotton, which were wrapped about the 

 limbs in such a way that the chemicals were applied to the bark in a 

 fairly even layer, completely encircling the limb for a distance of about 

 four inches. These were then kept wet with distilled water during 

 the term of the test. These bandages on the limbs were intended 

 to roughly simulate the physical conditions, especially as to moisture, 

 found naturally about the crown of the trees where the chemicals are 

 knoA\Ti to accumulate from spraying with arsenical insecticides. In 

 treating crowns, the earth was removed to form a shallow trench 

 close about the tree, the chemicals, suspended in water, were poured 

 in and the earth replaced. A part of the limbs and crowns used were 

 wounded in various ways and to various extents. 



The experiments and the results here discussed were published 

 during the past year and you are referred to the original paper for 

 further details. See, "A preliminary report on the effects of arsenical 

 compounds upon apple trees." D. B. Swingle and H. E. Morris, 

 J^hyto pathology Vol. 1 No. 3, June 1911, pp. 79-93. 



Of the various chemicals used, ortho arsenite of zinc is the only 

 one which, up to the present time, has caused no visible injurj^ under 

 the various conditions of the tests. In a number of instances wounds 

 which had been made under the bandages and in the crowns for the 

 purpose of allowing access into the circulation of the trees, thereby 

 making the tests more severe, healed completely while continually 

 covered by and in contact with the poison. 



In strict truthfulness, it should be stated that in a single instance 

 there was a superficial discoloration of the bark about three mm. in 

 diameter under a lenticel. We believe this to be of no practical sig- 

 nificance, however, when we consider that in all cases where the bark 

 was opened by the removal of a twig, a tangential slash to the 

 cambium, or by peeling to the wood, no injury whatever was inflicted. 



Killing Power 



Comparative tests have been made on potatoes for the Colorado 

 potato beetle and on cabbages for larvae of the European cabbage 

 butterfly and of the diamond back moth, (Plutella maculipennis Curt.) 



On potatoes, ortho arsenite of zinc was used at the rate of one pound 

 to 50 gallons of water applied by a field sprayer on the Experiment 



