Biology and population dynamics of the eastern larch beetle, Dendroctonus simplex LeConte, and its interactions with eastern larch (tamarack), Larix laricina.

Abstract

The range of the eastern larch beetle, Dendroctonus simplex LeConte (Coleoptera: Scolytinae), is concomitant with its primary host, eastern larch (tamarack), Larix laricina (Du Roi) K. Koch, throughout the North American boreal forest. Since 2000, an ongoing outbreak of eastern larch beetles in the south-central part of tamarack’s range throughout the Great Lakes region has caused extensive mortality to mature tamaracks, affecting over 86,500 hectares of tamarack forest in Minnesota. Extended outbreaks in live trees are atypical of this insect, so the eastern larch beetle’s biology and ecology were studied under laboratory and field conditions in Minnesota from 2011 – 2014 to decipher the factors contributing to this ongoing outbreak. In the laboratory, the minimum and optimal developmental temperatures for eastern larch beetles were determined to be 7.5 and 27.9°C, respectively. Some progeny were able to reproduce in the absence of an overwintering period, suggesting that a reproductive diapause may not be obligate in all individuals. This was confirmed by field studies, which found that a second generation of eastern larch beetles successfully completed development during the summer and fall of 2012. Confirmation of two generations instead of three sibling broods established by re-emerging parents in one year was established by detailed phenological and physiological methods. As beetle infestations progressed through tamarack stands, beetles initially preferred to attack the largest tamaracks before killing smaller hosts at random in successive years. Reproductive success of females increased in larger and older tamaracks, and those “challenged” by unsuccessful attacks in the recent past. Higher concentrations of resin pockets within the phloem consistently reduced beetle reproduction. The size of male and female beetle offspring, as well as the total lipid content of female offspring, increased with tamarack size and phloem thickness. Development within “challenged” tamaracks reduced both the total and proportional lipid contents of all beetle offspring. New understandings of the population dynamics of eastern larch beetles are discussed. Expanding growing seasons, for example, may facilitate fractional voltinism, or, two generations in one year, among a portion of the population. Synchronous beetle emergence the following spring – shown in phenological studies – would enhance host procurement, especially of the largest and most preferred hosts that produce the most vigorous offspring, thus exacerbating the outbreak.