February, T8] hartzell: graphic illustration 35 
and precautions must be taken to maintain the same degree of precision 
throughout the survey. The results will be more accurate if a moder¬ 
ately precise method be used to determine the amount of injury and 
the survey be completed before any marked changes occur, than if a 
more exact method be used but one which reduces the speed to such 
an extent that decided changes occur before the determinations are 
completed. 
The size of the subdivisions is important. In a vineyard or an 
orchard, the individual vine or tree may be taken as the unit. These 
are easily located and can be combined into larger subdivisions if 
necessary. With field crops, the size of the area should be such as will 
give a sufficient number of points to determine the isofags to the degree 
of accuracy required. If undecided as to the size of the subdivisions, it 
is better to err in making the plots too small rather than too large for, if 
necessary the small plots can be combined into larger plots, whereas, 
if made too large, one has no opportunity of correcting the error. 
The plotting of the isofags is accomplished in the office by methods 
of interpolation. In this portion of the work one should use a suffi¬ 
cient number of points to determine the contours so as to show varia¬ 
tion in considerable detail but excess of detail should be avoided, as 
this is apt to make the chart confusing. For the same reason the 
contour interval should not be too small. We are interested in aver¬ 
ages, so too great detail tends 1 o obscure the general trend of the facts 
and, moreover, requires an amount of labor in plotting that is not 
justified by the problem. 
A Practical Example 
On June 4, 1914, our attention was called to a vineyard that was 
severely injured by the grapevine flea-beetle (Haltica chalybea). This 
insect injures the grape by destroying the swelling buds in the spring. 
The vineyard was near Sheridan, N. Y., and consisted of about 6.5 
acres having 4,097 vines (77 vines missing). It was situated just east 
of a woodland and northeast of a section of waste land containing 
approximately fifty acres in which latter were growing great masses of 
wild grapes, practically all belonging to the species Vitis bicolor. The 
beetles, which hibernate as adults—evidently had greatly multiplied 
in this area the previous season and during the spring of 1914 invaded 
this vineyard. Although grapes were cultivated to the southeast, 
south and west of the waste land, the injured vineyard was somewhat 
isolated. 
On our first visit, it was evident that the injury was not uniform 
throughout the vineyard and, as it seemed to present a problem that 
needed further study, we made a map which showed the location and 
