14 CIRCULAR 295, U.S. DEPARTMENT OF AGRICULTURE 



what dilution of oil will give the best control with a reasonable degree 

 of safety to the trees. 



As in previous tests, 4 percent oil sprays gave a good control of the 

 scales developing unprotected by old scale covers whereas they gave 

 only a fair degree of control of those beneath the old covers. At the 

 same time they did not injure dormant pecan trees in the tests in 

 which they were used. Sprays containing 5 percent of oil, not pre- 

 viously tested, gave a decided increase in the percentage of control 

 (table 3) over those containing 4 percent, but they were slightly more 

 dangerous to use (table 4), indicating that they should not be recom- 

 mended for use without some reservations. As in previous tests, 

 sprays containing 6 percent of oil gave a reasonably satisfactory con- 

 trol of both light and heavily encrusted infestations and proved the 

 most satisfactory ones to use from the standpoint of control obtained, 

 but, as in the previous year, they caused no injury in some and serious 

 injury in other instances. For this reason, this strength cannot be 

 recommended for use. It should be noted that oils of very low vis- 

 cosity (oils 14 and 17, table 6) were practically as effective as those of 

 higher viscosity and caused less injury (table 4) to the trees in the 

 orchards in which they were tested. It is desirable that these oils 

 be tested further before they are recommended for general use. 

 With the exception of oil 19, which gave somewhat lower results than 

 other oils, there were no consistent differences in the percentage of 

 control given by the oils used in tests reported on in table 2. This 

 indicates that, in general, oils having the physical characteristics of 

 those usually recommended for use in dormant oil sprays will prove to 

 be about equally satisfactory from the standpoint of scale control, with 

 the added possibility, as previously mentioned, that oils lower in 

 viscosity and higher in volatility (oils 14 and 17, table 6) may prove 

 effective and safer. 



The data in table 4 indicate that pecan trees are unable to with- 

 stand as strong concentrations of oil sprays as are many other decidu- 

 ous trees. The injury which in some cases did result did not vary 

 in degree according to the viscosity, volatility, or unsulphonated 

 residue of the oils used, although some oils (16 and 18) appeared 

 slightly more likely to cause injury than others. Also, oils which 

 caused injury in one orchard were used with perfect safety in others, 

 and vice versa. Therefore it seems likely that some condition or 

 factor other than the oils limits or controls the quantity of oils that 

 pecan trees are able to stand without suffering injury. Tests not 

 listed in table 4, in which oils 15 and 16 were applied to young, vigor- 

 ous trees on the laboratory grounds at dilutions as high as 25 percent 

 of oil, showed that no injury resulted when the applications were 

 made during the strictly dormant period and but little injury when 

 they were made at the delayed-dormant period. In other tests, oils 

 18 and 19 caused no injury when applied at a dilution containing 3 

 percent of oil during the dormant period, but when they were applied 

 at the delayed-dormant period they caused injury in some, but not 

 all, instances to the extent of setting back bud opening and develop- 

 ment for 4 or 5 days. This setback was soon overcome and appar- 

 ently no permanent injury resulted. Even so, it is evident that the 

 application of dormant types and strengths of oil sprays should be 

 confined to the strictly dormant period. 



