192 
JOURNAL OF ECONOMIC ENTOMOLOGY 
fVol. 17 
The probable error of the product of two quantities, A db a and 
B =t b is E p = ± V(Ab) 2 + (Ba) 2 . 
B ±b, . 
The probable error of the quotient , —- is 
A A 9- 
Should the results be desired in the form of a percentage, 100 
B ± b 
A ± a 
the probable error is 
ESTIMATING THE ABUNDANCE AND DAMAGE BY THE 
SAN JOSE SCALE 
By John J. Davis, Purdue University , Lafayette, Indiana 
Summary 
The gradual increase of the San Jose scale, Aspidiotus perniciosus Comst., is less 
likely to be recognized before it becomes a serious factor than many insects which 
appear suddenly or which cause immediately visible injury, nor is the relative im¬ 
portance of the loss of trees in comparison with an annual crop usually realized. 
The various types and degrees of injury must be taken into consideration in estimat¬ 
ing abundance and damage, which may include a weakening or killing of the tree, 
spotting of fruit so as to make it unmarketable, and subjection of the trees to further 
insect and disease troubles following the weakening effect of scale. Other factors 
entering into the problem are the vigor and resistance of individual trees and indi¬ 
vidual varieties to scale injury. The degree of infestation and method of making 
counts are reported. To reach a basis for estimating monetary losses figures are 
given which enables the author to estimate the crop loss to an acre of apples, if allowed 
to become badly infested, to be SI,008.00 and this even if the scale is eventually 
controlled and the trees brought back into a thrifty condition. 
Injury by a scale insect is less conspicuous than that by chinch bugs, 
army worms, and similar insects which appear suddenly, usually without 
advance warning. The San Jose Scale, for example, gradually saps the 
life of the tree and often the orchardist does not recognize its harmful¬ 
ness or the impending danger until the branches are dying, although 
sometimes the infestation may be recognized earlier by a spotting of the 
fruit. Neither does the orchardist always realize the after-effect follow¬ 
ing even practical eradication of the insect. The fanner may immediate- 
