No abstract

Until recently, the management of spruce bark beetles in Belgium has relied upon felling, debarking and removal of susceptible or infested material, and in a number of cases, on the use of pheromone traps. Gradually, due to the cost of manpower, on-site debarking has been replaced by lindane spraying. Experimental work to design new management techniques was started in 1989, when plans were developed by the Regional Government of Wallonie to ban lindane from the forest. Later on, in February 1990, severe storms causing 2.6 millions cubic meters of spruce windfalls considerably increased the need for improved control methods. As a result, a co-operative research project was setup to investigate the possibility of using poisoned trap-trees for protecting timber or live stands. To date, several pyrethroids with sufficient remanence have been provisonally selected for use against Ips typographus and Tvypodendron lineatum. Lying and standing trap trees (insecticide-treated and bearing a pheromone dispenser) have been tested, particularly against I. typographus. The s atial distribution of insects landing around the pheromone dispensers and subsequently killed was anayysed and hence, the total numbers caught were estimated. These results were compared to the catches of artificial traps, showing that trap trees catch 2-14 times as many I. typogvaphus as traps. To establish a measure of the impact of trap trees on bark beetles, the size of overwintering I. typographus populations was estimated by sampling both the bark of trees attacked late in the summer 1990, and the litter around these trees. In June-July, each trap tree caught on average slightly less than the overwintering population from one tree attacked in late 1990. Guidelines for trap tree establishment could probably be derived from these assessments. Trap tree impact on natural enemies has been analysed in one site. Numbers of parasites and predators represented 2.5% of the number of I . typographus caught. Presently, removal of infested material and trap tree deployment are standard methods used by the Regional Forest Service in Wallonie.

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.