Method for estimating density of Cryphalus piceae (Ratz.) brood galleries using a regression model

Estimation of the severity of infestation by the bark beetle (Cryphalus piceae Ratz.) on silver fir (Abies alba Mill.) is commonly done using methods that lack satisfactory statistical foundations. The object of this study was to develop a statistical method for estimating stem infestation density which permits calculation of the estimation error and which also does not require debarking of the entire stem. The study was carried out in the near-natural forests of the Swietokrzyski National Park in central Poland. Two general patterns (A and B) of distribution of C. piceae brood galleries on stems of A. alba were observed. In pattern A, brood galleries occurred on the 1st 1-m length of stem (measured from the base), and the probability of infestation on successive 1-m lengths further up the tree was also very high (above 90%). In pattern B, brood galleries were absent from the 1st 1-m length but the probability of infestation in successive 1-m lengths increased as the distance from the base increased. In a statistical analysis of linear correlations between the numbers of C. piceae brood galleries in selected 1-m-long stem lengths and the total density of infestation, the most significant correlations for pattern A were found for the 1st, 2nd, 6th and 7th 1-m lengths (counting from the base). Here, mean relative errors of estimation did not exceed 35%. For pattern B, the most significant correlations were found for the 8th, 10th, 11th, 12th, and 13th 1-m lengths, where mean relative errors of estimation did not exceed 42%. The method described is only mildly invasive and therefore may be used in strictly protected areas of reserves and national parks.