Eastern Larch Beetle Outbreak Just Keeps Going When Winter’s Not So Cold
By Jess Hartshorn, Ph.D.
For many in the northeastern United States, Great Lakes region, and Canada, tamarack (Larix laricina, also known as eastern larch) is a legendary tree, forming vast expanses of critical habitat for birds and wildlife and being the only native conifer in the Northeast to lose its needles and provide fall colors.
Until recently, eastern larch beetle (Dendroctonus simplex) has been a part of the tree’s complex ecology, occasionally reaching outbreak levels at small scales for a few years and quickly retreating back to endemic levels. Historical outbreaks have been associated with stress factors like severe drought or defoliation events by insect pests like larch sawfly (Pristiphora erichsonii) or larch casebearer (Coelophora laricella), an invasive defoliator introduced to North America in the mid-19th century.
However, the most recent outbreak of eastern larch beetle, currently concentrated in northern Minnesota, is going into its 18th year and does not appear to be associated with defoliation or drought events. As of 2017, the Minnesota Department of Natural Resources Forest Health Unit reported that more than 440,000 acres of tamarack were in some stage of infestation by the eastern larch beetle. At least a quarter of those acres have already been killed and, because the beetle feeds on phloem (nutrient-conducting tissue) in the trunk of the tree, there are not high hopes for the remaining trees.
This unprecedented event led Brian Aukema, Ph.D., at the University of Minnesota to hire Fraser McKee, Ph.D., a student at the time and currently the Forest Health Officer for the Lower Athabasca Region in Alberta, to investigate the factors behind the current eastern larch beetle outbreak.
Historically, outbreaks never proceeded much further than a few years because the asynchronous beetle development and emergence brought on by a cooler climate halted any opportunity to produce explosive populations. Eastern larch beetles have a development pattern similar to other Dendroctonus species in that reproductively mature adults can produce what are called “sister broods.” Parent adults emerge in the spring and, upon finding a new host, mate and lay eggs under the bark of a suitable host, which is often a stressed or dying tree. Eggs soon hatch and larvae begin developing by feeding on the phloem under the bark.
While these larvae are developing, parent females re-emerge and search out another new host to make a final deposit of eggs. This second clutch is known as a “sister brood” or “sibling brood” and begins developing later in the season compared to initial broods. Females can produce two to three sibling broods in a single season. Due to this late-season development, sibling broods are unable to reach maturity by the time cold temperatures trigger overwintering diapause, and thus they require additional time for maturation the following spring. This extra time allows the sibling broods to develop proper flight muscles and be capable of reproduction. It was thought that perhaps eastern larch beetles required this overwintering diapause.
Aukema and McKee completed several lab experiments to examine development of beetles in different temperatures. What they found surprised them both. These sibling broods were able to reach maturity earlier than they expected, and in the absence of diapause, indicating that at least some beetles are able to reach maturity without requiring an overwintering period.
The absence of an obligatory overwintering period, combined with longer growing seasons brought by warming temperatures, may allow for multiple generations per year on a consistent basis. This switch in life history results in faster spread and increased tree mortality. Warmer winters are also presumably causing less winter mortality for overwintering beetles. In addition to the exploding populations of beetles, warmer winters mean less access for loggers to manage tamarack stands, which typically require frozen ground to operate machinery.
All of these variables combine to create a situation in which eastern larch beetle is killing tamarack faster than we can manage for it. As of now, we don’t have an answer, but we hope that future research will yield management strategies to mitigate the spread and impact of this insect throughout the range of eastern larch.
Jess Hartshorn, Ph.D., is currently a Forest Health Specialist with the Minnesota Department of Natural Resources and will be starting as an Assistant Professor of Forest Health at Clemson University this fall. She also runs the blog Feminist Forester with fellow forest entomologist Molly Darr, Ph.D. Email: email@example.com