DISTRIBUTIONS OF SCOLYTUS VENTRALIS ATTACKS, EMERGENCE, AND PARASITES IN GRAND FIR

Abstract

The distributions of Scolytus ventralis LeConte attacks, emergence, and parasites were studied by stratified sampling of 20 grand fir. Of the theoretical distributions tested the negative binomial gave the best fit to all three sets of data. Three sample unit sizes were analyzed (1/4, 1/2, and 1 sq. ft) and all gave similar estimates of the mean, although the smallest sample unit influenced the distribution by increasing the frequency of the zero class. Approximate normality was achieved by applying logarithmic transformation to the attack data and the Taylor power transformation to the emergence and parasite data. Analysis of variance showed that the vertical position of the sample in the tree was die most consistent factor influencing the distribution of attacks, emergence, and parasitism, while cardinal aspect had little effect. Considerable variation also occurred among trees within a group. The density of attacks was extremely variable both within and between trees, although it was slightly higher on rough bark. Emergence was generally greater in the basal half of the tree, being reduced in the upper levels by competition with other insects. Emergence increased with attack density up to about 13 attacks per square foot. Thereafter survival was probably limited by intraspecific competition. Parasitism appeared to be reduced near the base of the tree by thick bark and at the top by lack of host larvae. Sampling designs for the fir engraver will have to consider variations between trees and vertical strata within the tree.