Felled trap trees as the traditional method for bark beetle control: Can the trapping performance be increased?

Abstract

Felled trap trees have been traditionally used to control bark beetles in central Europe. There is, however, little research on the method and on factors affecting trap tree performance. We therefore evaluated the effect of felling date, tree diameter, site shading, and weather conditions on the pattern of trap tree infestation by bark beetles. An experiment was conducted at two localities with contrasting forest damage rates and bark beetle outbreak phases in the Czech Republic during three periods: December to April of 2004–2005, 2005–2006, and 2006–2007. For each of the three periods, eight dates of trap tree felling were equally spaced. On each date and at each locality, 5–7 trees located at the open forest edge were felled (295 trap trees in total). The number of Ips typographus (Linnaeus, 1758) entry holes dm−2 counted in May each year was used as the response variable. Neural network-based regression models were used to identify the variables that most affected trapping performance.The performance of the regression models in terms of Pearson’s correlation between observed and predicted entry hole densities ranged from 0.15 to 0.93. The effect of predictor variables on the recorded entry hole density was much lower in the forest with a culminating bark beetle outbreak than in the forest with outbreak in retrogradation phase. For trap trees felled before winter, trapping performance was positively associated with the length of the period that the trap trees were covered by snow. The trap trees exposed to higher heat sums tended to show lower entry hole density than trap trees exposed to lower heat sums.The results indicate that, in forests with high beetle densities, there are few options for increasing trap tree performance based on selection of specific sites, tree characteristics, or felling dates. The identified sensitivity of trap trees to weather indicates that trees should be felled just before beetles emerge if conditions have been relatively warm and snowless but can be felled far in advance under cooler conditions with regular snow. Finally, the results indicate that foresters should continue to use large-diameter trees located on non-shaded sites as felled trap trees.