Host selection by the eastern larch beetle (Dendroctonus simplex LeConte) in relation to host quality during a large-scale outbreak in the Great Lakes Region of North America

The eastern larch beetle (Dendroctonus simplex LeConte) is found throughout the range of its host tree, the eastern larch (tamarack), and is considered a semi-aggressive bark beetle that typically attacks recently dead or stressed tamaracks.  Infestations of eastern larch beetles are usually localized, short-lived (i.e., 2 – 4 years), and decline as the stressed host supply dwindles and healthy tamaracks are attacked.  While rare, widespread defoliation of tamarack can initiate landscape-level outbreaks of eastern larch beetles.  Since 2000, a sustained outbreak of eastern larch beetles in Minnesota, U.S.A. has killed over 86,500 ha of apparently healthy tamarack.  The sustained utilization of healthy tamaracks by eastern larch beetles has not been observed in previous outbreaks.  This outbreak provides a rare opportunity to study the interactions between eastern larch beetle and healthy tamaracks during outbreak conditions.  We examined the effect tamarack diameter, phloem thickness, age, growth rate, phloem resin pocket density, basal area of adjacent trees, and history of previous unsuccessful beetle attacks on the host selection of eastern larch beetles.  Host selection was recorded for five consecutive periods of beetle attack between 2011 and 2013 within three tamarack stands containing epicenters of beetle infestation.  Eastern larch beetles preferentially attacked the largest available tamaracks early in the infestation and attacked the remaining smaller-diameter tamaracks at random late in the infestation.  Tamarack diameter correlated with phloem thickness and growth rate meaning eastern larch beetles initially attacked and killed the largest and most vigorous tamaracks.  This is the first study to demonstrate that ELBs under outbreak conditions can exhibit tree-killing behavior indicative of more aggressive and economically important Dendroctonus species and has important implications for understanding the population dynamics of this insect and its potential to cause widespread forest mortality.