ANNUAL REPORT, 1946-47 41 



DDT in water suspension remained at the surface of the bark to a greater extent » 

 and so permitted more adults to tunnel out to the surface. 



It was interesting to note, however, that as the concentration of DDT and 

 DDD in the sprays was increased, usually a higher percentage of adults was 

 found dead in exit holes in the autumn, a phenomenon which has not been ob- 

 served to result from the types ot sprays tried previously. This suggests that 

 some emerging adults were killed by the DDT and DDD spray material they 

 encountered very close to or at the surface of the bark. No one type of DDT 

 spray proved to be consistently superior, in this respect, to the others at the 

 several concentrations used in these tests. 



To what extent DDT caused mortality among the adults which completely 

 emerged from the bark was not determined. It would no doubt be influenqpd by 

 the amount of DDT residue remaining effective on the surface of the bark when 

 they emerged and by the extent to which the adults crawled around on the bark 

 immediately after emerging. The extent of this crawling may be influenced by 

 the environmental conditions on the surface of a log. 



Preliminary Spraying Expeiiments to Prevent Twig Feeding by the Smaller 

 Euiopean Elm Bark Beetle. (W. B. Becker.) Feeding on live elm twigs by the 

 smaller European elm bark beetle {Scolytiis mnltistriatus Marsham) after it 

 emerges from dead elm bark is at present believed to be one of the more impor- 

 tant methods by which the fungus which causes Dutch elm disease, Ceratostomella 

 ulmi (Schwarz) Buisman, is introduced into living elm trees. Since twig feeding 

 may occur from spring to aul umn, it is desirable that any preventive spray should 

 give protection for as long as possible, to keep the number of spray applications 

 at a minimum. Some of the newer types of spray materials which leave a deposit 

 having a relatively long residual effect were used in attempts to reduce the 

 c-mount of such twig feeding. Commercially prepared DDT and DDD sprays 

 were used in concentrations between 0.0625 percent (which is slightly less than 

 the strength used to combat^defoliating insects on living plants) and 5.0 percent 

 (which is the strength commonly used for some household pests where spray 

 injury to plants is not a problem. 



Sprays were applied directly to low branches of living American elms, Ulmus 

 americana L., until or almost until the spray began to run off. Three series of 

 applications were made, each on different elms in full foliage, two series in June 

 and one in August. A three-gallon compressed air sprayer was used with a 

 Mohawk adjustable nozzle which gave a cone-shaped spray pattern. Feeding 

 tests were run in the laboratory with freshly cut twigs from sprayed elms placed 

 in gallon size glass jars with newly emerged beetles. Control estimates are based 

 on a comparison between the number of punctures which reached the cambium 

 of sprayed and unsprayed twigs kept in separate jars. 



At spray concentrations near those now used for defoliating insects, prevention 

 of 5. multistriatus twig feeding did not last as long as would seem desirable. As 

 the concentrations were increased, much more lasting protection was obtained, 

 but unfortunately the foliage injury to elms caused by the DDT emulsions in- 

 creased to an undesirable point, and injury to hard maples and some other plants 

 on which the spray drifted was even greater. While the DDT wettable powders 

 caused no injury worth mentioning, the spray deposit left by the higher dosages 

 was quite noticeable and the nozzle frequently clogged. Mites also caused some 

 very noticeable browning of the sprayed leaves, as other workers with DDT have 

 reported. Such high spray concentrations would cost correspondingly more 

 than the lower strengths commonly used for defoliating insects. 



