The effect of overwintering cold exposure on the spring emergence behavior and reproductive success of the eastern larch beetle, Dendroctonus simplex, (Coleoptera: Curculionidae)

The eastern larch beetle, Dendroctonus simplex, occasionally erupts and kills large numbers of mature tamarack, Larix laricinia.  In Minnesota, sustained epidemic behavior of the beetle has been noted since 2000, which deviates from historic patterns in this southern extent of its range.  Little is known regarding the insect’s overwintering and spring emergence habits, although shorter and warmer winters are suspected of contributing to this outbreak.  We brought naturally infested tamaracks into the laboratory at regular time intervals throughout the winter to rear overwintering eastern larch beetles to examine the effect of cold exposure on spring emergence and insect reproduction.  Cooling degree days (CDD) were calculated using a 5 °C threshold.  Insect emergence did not occur until 168 CDD were accumulated.  Increasing cold exposure to 1054 CDD reduced the number of days until beetles initiated emergence from logs, and days needed to complete emergence.  When emergent beetles were introduced to uninfested tamarack logs at 23 °C, we found that insects were reproductively viable following exposure to 168 – 1054 CDD.  Female beetles with greater cold exposure excavated longer egg galleries and produced higher numbers of brood.  The number of gallery starts, and brood production per centimeter of gallery, were unaffected by adult cold exposure.  Our results suggest that eastern larch beetles would be able to take advantage of earlier spring phenologies in the southern part of its range to emerge and reproduce, and may explain a component of its sustained eruptive behavior to date.