A sampling technique to estimate within-tree populations of pre-emergent Ips typographus (Col., Scolytidae)

Abstract: A sampling method was designed to assess within-tree pre-emergent adult populations of Ips typographus in Belgium. In a first series of 6 standing, attacked trees, a large number of 1 dm2 samples (56–171 samples, according to tree) were collected and analysed. Sets of samples were selected at random with successive increments of one unit among the samples from each tree, and the mean numbers of beetles and standard deviations in each of these random sets were used to calculate changes in sampling precision as the number of samples in a set increased. For each tree, there was an excellent fit with a power function linking sampling error and number of samples in each set. Fifty random samples per tree allowed estimates of the mean with an error of 9–24\%. Further analysis of sample variation along and around the trunks showed that beetle density did not vary around but along the trunks, suggesting that sampling should not be totally random but should take into account longitudinal changes in beetle populations on attacked trees. At each level of each tree, sets of samples were selected at random with successive increments of one unit to calculate changes in sampling precision. At any level along the trunks, taking four 1 dm2 samples yielded estimates with an error inferior to 10\%. To validate the conclusions drawn from these results, eight other trees were then sampled at regular intervals along the trunk. A total of 43–56 samples were taken per tree, yielding estimates with a computed error of 9–13\% of the mean. Beetle density along the trunks was found to increase linearly along the basal portions of the infested stems (15\% of the infested stem), remain constant along the following 65\% of the infested stems, and decrease linearly along the terminal 20\%. Taking all this information into account, the total pre-emergent beetle population in each tree was estimated. Total beetle production per tree ranged from 35 000–72000.