No. 129 

UNINJURED TREES--A MEANINGFUL GUIDE 

TO WHITE-PINE WEEVIL CONTROL DECISIONS 

The white-pine weevil, Pissodes strobi, is a particularly insidious forest pest that can render a stand of host 

trees virtually worthless. It rarely, if ever, kills a 

tree; but the crooks, forks, and internal defects that develop in attacked trees over a period of years may reduce 

the merchantable volume and value of the tree at harvest age 

to zero. Dollar losses are especially high in plantations 

where costs of planting and care have been incurred. 

After establishment of the weevil in a forest stand 

or plantation, an increasing number of trees are attacked 

two or more times. Conversely, the number and proportion of 

trees remaining free of weevil damage declines. As the recoverable volume and value of the tree crop is reduced in 

successive years, the owner or manager is confronted with 

the question, "shall I apply control now?" For his decision, he must have sound and meaningful criteria or standards. 

The recent adopt ion of a white-pine weevil control 

program by the New York State Conservation Department on 

selected State reforestation areas brings the problem of 

developing proper criteria for decisions on control sharply 

before us. As with most other insect pests, the weevil 

problem is confounded by the interplay of biological and 

purely economic considerations. Essentially, both boil down 

to this: the plantations selected for control must be at or 
approaching the limit of tolerable injury by the weevil, and 

each must justify by its present or potential value the 

added cost of protection against the weevil. The primary 

entomological factor is the insect population level and the 

corollary damage ; sound decision requires a practicable and 

meaningful measure or index of one or both. What follows is 

concerned with such an index. 

For the New York program, which presently involves 

only white pine plantations, standards have been proposed 

that specify the desired results in terms of number of crop 

trees per acre at maturity or harvest age. General guidelines for white pine management similarly point to production of a given number of high-quality trees per acre. Variation in site quality is recognized, and the stocking specifications change with site class accordingly. Yet the allowable limits of weevil damage are expressed in the form of 

percentage of trees weeviled in the current (or previous) 

- year. This is the generally accepted measure of weevil 

populations for both decision-making and after- control 

evaluations. But for the forester responsible for deciding 

when and where to apply control, considerable omniscience 

is needed to convert this measure into terms of his objectives, which deal with the number of trees remaining undamaged rather than the percentage weeviled in a given year. 

Weevil surveys and population studies in the Northeast have shown that a cumulative damage index based on - the 

number of trees remaining free of weevil injury is both a 

logical and biologically significant criterion for control 

decisions. It puts the factor of allowable damage directly 

in terms that can be interpreted and used by the forest or 

plantation manager. The field data are obtained just as 

easily; and, because no conversion to percentages is involved, there is a statistical advantage too. From the 

biological standpoint the cumulative number of unweeviled 

trees is a far more stable variable than percentage of trees 

weeviled in successive years. Plotted graphically, this 

becomes a smooth curve, while current percentage of weeviling generally becomes a broken line, in which only a general 

trend is apparent at best. 

To illustrate the number-of -undamaged-trees index and 

to compare it with percentage current weeviling, the data 

from six weevil-survey plots in New York for the years 1952- 

57 are shown in Figure 1. The trees in these plots were 1 

to 2 feet tall in 1952 and showed no evidence of weevil 

damage--although some weevils may have been present. In the 
I50 

a 

,, roo 

Figure 1.--The number-of-undamaged-trees index 

(solid line) compared with percentage of current weeviling (broken line) as a criterion 

for control decisions. 

figure, the plots are numbered simply 1 to 6. The scale for 

cumulative number of trees unweeviled is on the left side 

and that for percentage of trees weeviled is on the right. 

The upper set--Plots 1, 2, and 3--represent light, or 

at least slowly developing, infestations. Plot 3 gave evidence of a sharply rising weevil population in 1955, but it 

returned to a low level in 1957. In these plots, the downward curve of unweeviled trees is shallow, or rather flat; 

and the annual percentage values fluctuate at a low level. 

The lower set--Plots 4, 5, and 6--are indicative of 

fast -developing infest at ions. The curve of unweeviled trees 

is more sharp and steep; the upward trend in percentage of 

current weeviling is more pronounced and longer sustained. 
Even in such cases, however, the degree of change is usually 

impossible to predict. In Plot 6, for example, there was a 

more or less regular increase in weeviling each year till 

1956--then it jumped from 13 percent to 41 percent in 1957. 

The percentage of weeviling that might occur in 1958 would 

have been anyone's guess at that time. 

In contrast, the smoother lines of the plotted cumulat ive number of unweevi led trees permit reasonably accurate 

projection and thus prediction of the situation at least 1 

year in advance. And these data are in terms most meaningful to the practicing forester. 

Of added ecological interest is the variation in form 

and shape of the proposed damage-index curves. Analysis of 

these data in conjunction with those of physical and biotic 

factors known or guessed to affect weevil behavior and survival should provide further insight into this complex and, 

possibly, provide a workable basis for characterizing high 

and low hazard areas or stands. 

--WILLIAM E. WATERS, Entomologist 

Forest Insect Laboratory 

Northeastern Forest Experiment Stat ion 

Forest Service, U.S. Dept. Agriculture 

New Haven, Conn.