The biology and management of bark beetles in old growth pine forests of Itasca State Park
| Executive summary part VII: Effects of winter temperatures on bark beetles |
| The ability to survive annual temperature minima can be a critical determinant of insect abundance. If winter conditions are a
significant source of mortality for Ips, then it would be possible to predict population abundance in the upcoming summer as a
function of winter temperatures and snow cover, and such models could have high utility for anticipating and mitigating risks of tree
mortality from bark beetles. This component of our research was designed to measure the lower lethal temperature of Ips spp., identify
their overwintering sites, and develop a model to predict winter mortality of Ips. The lower lethal temperature for adults of I. pini, I.
grandicollis, and I. perroti averaged -12.6, -15.2, and -14.4 °C , respectively (Table 4.1). Larvae and pupae are less cold tolerant than
adults and are obliged to remain within the phloem of their host trees (where they are poorly buffered from temperature extremes).
Apparently, the only life stages that can survive most winters are the adults that reach overwintering sites in the forest litter.
We collected microclimatic data to develop a mathematical model that estimates temperatures within the overwintering habitat of Ips adults (Eq. 4.1). This allowed us to identify winter conditions that can kill bark beetles (Fig. 4.2). For example, during a cold snap where minimum air temperatures reach -35 °C, with no snow cover, we expect about 65% mortality in Ips adults (Fig. 4.3). Such conditions are less common at Itasca than we had expected. Based on climate records from nearby Bemidji, MN, winter mortality of Ips adults reached a maximum of only 33% from 1947 - 1992 (compare to a maximum of 71% in west central Wisconsin during the same years; Fig. 4.4). From the perspective of Ips bark beetles, Itasca has relatively benign winters because snowfall usually insulates the upper soil well before the coldest annual air temperatures (Figs. 4.6-4.7). Fig. 4.3 provides a tool for predicting Ips mortality under any combination of winter air temperature and snow cover. |
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