Relationships Between Site Factors and Bark Beetle Attack on Pine Trees

Dendroctonus adjunctus Blandford is the main bark beetle attacking high elevation (>2600 m) conifer forests in Mexico. In a natural Pinus hartwegii L. forest at Cerro Tlaloc, Texcoco, Estado de Mexico, we found high pine tree mortality caused by D. adjunctus. Silvicultural practices have been insufficient to date to control beetle populations. An adequate control strategy needs knowledge of the factors that affect the presence and insect behavior. Accordingly, the objective of this study was to determine possible relationships among a series of site factors and D. adjunctus attack to pine trees. To achieve this goal, data were collected at 40 sampling sites randomly distributed around the study area. In each sampling site the following variables were measured: elevation, air temperature, relative humidity, aspect, slope, soil fertility, soil moisture and internal pedregosity, depth of the A-horizon, number of D. adjunctus-infested pine trees, number of clods per site, number of death trees including pine stumps per site and breast height diameter of all trees within the site. After organizing the data, they were processed using multiple regression analysis by means of the SAS Ver. 9.3 STEPWISE procedure. Four models that relate site factors with variables indicating bark beetle attack were obtained. It was concluded that there is a relationship between some site factors and the attack of D. adjunctus at the natural forest studied. The most influencing site factors were: terrain aspect, soil cation exchange capacity, tree basal area per hectare, soil N, P and K concentrations, terrain slope and soil internal pedregosity.