A SAMPLING TECHNIQUE TO ESTIMATE WITHIN-TREE POPULATIONS OF PRE-EMERGENT LPS TYPOGRAPHUS (COL, SCOLYTIDAE)

A sampling method was designed to assess within-tree pre-emergent adul t populations of Ips typographus in Belgium. In a first series of 6 st anding, attacked trees, a large number of 1 dm(2) samples (56-171 samp les, according to tree) were collected and analysed. Sets of samples w ere selected at random with successive increments of one unit among th e samples from each tree, and the mean numbers of beetles and standard deviations in each of these random sets were used to calculate change s in sampling precision as the number of samples in a set increased. F or each tree, there was an excellent fit with a power function linking sampling error and number of samples in each set. Fifty random sample s per tree allowed estimates of the mean with an error of 9-24%. Furth er analysis of sample variation along and around the trunks showed tha t beetle density did not vary around but along the trunks, suggesting that sampling should not be totally random but should take into accoun t longitudinal changes in beetle populations on attacked trees. At eac h level of each tree, sets of samples were selected at random with suc cessive increments of one unit to calculate changes in sampling precis ion. At any level along the trunks, taking four 1 dm(2) samples yielde d estimates with an error inferior to 10%. To validate the conclusions drawn from these results, eight other trees were then sampled at regu lar 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 t he basal portions of the infested stems (15% of the infested stem), re main constant along the following 65% of the infested stems, and decre ase linearly along the terminal 20%. Taking all this information into account, the total pre-emergent beetle population in each tree was est imated. Total beetle production per tree ranged from 35 000-72 000.