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Hemlock Woolly Adelgid

Evaluation of Temperature Effects on Feeding by Adelgid Biological Controls

Research Issue

The response of insects to cold temperatures is not a static process characterized simply by lethality in laboratory trials.  An ecophysiological approach is necessary to evaluate factors that delineate the potential geographic range limits imposed by temperature. For instance, seasonal behavior through which insects seek refuge from temperature extremes can extend their range into areas otherwise considered climatically uninhabitable. Likewise, food taken into the gut when winter-active insects feed during periodic warming may diminish cold tolerance during a time of sometimes widely fluctuating winter temperatures. Efforts to model the potential climatic range of hemlock woolly adelgid (HWA) and its introduced predators require research to elaborate on factors influencing cold tolerance.

Three predators released into the eastern U.S. hemlock forests, S. tsugae, S. sinuanodulus, and L. nigrinus, all show promise for biological control of HWA. Laboratory data on cold tolerance of S. tsugae and L. nigrinus suggest more susceptibility to cold than found with HWA, which would limit their range relative to HWA. Little is known about the cold tolerance of S. sinuanodulus. L. nigrinus are winter active whereas S. tsugae and S. sinuanodulus appear to hibernate in, as yet, undetermined locations and resume feeding in late winter when temperatures can drop rapidly. A better understanding of the interplay between low temperature feeding, behavior to ameliorate cold stress, and physiological tolerance to cold will substantially improve efforts to model the range of these predators and lay a foundation for assessing the potential of future biological control candidates.

Our Research

  • Determine the low temperature feeding threshold of cold-acclimated adults of S. tsugae, S. sinuanodulus, and L. nigrinus
  • Characterize fluctuations in supercooling point, as indicative of minimum lethal temperature for freeze-intolerant insects, in response to feeding status of cold-acclimated adults of S. tsugae, S. sinuanodulus, and L. nigrinus
  • Using laboratory trials, assess behavioral response of cold-acclimated adults of S. tsugae, S. sinuanodulus, and L. nigrinus under conditions of decreasing temperature

Expected Outcomes

This project will facilitate U.S. Forest Service modeling efforts to delineate the potential range of HWA and three predators currently released for its biological control, i.e., Sasajiscymnus tsugae, Scymnus sinuanodulus, and Laricobious nigrinus. The research will increase our understanding of predator feeding and behavior at low temperatures and the influences on predator cold tolerance.

Research Results

Three beetle species, Scynmnus sinuanodulus, Sasajiscymnus tsugae, and Laricobius nigrinus, are under examination to identify relationships between temperature and feeding behavior.  Using electronic “bug condos” developed in 2008, these three winter/spring feeding species are being studied to examine the impact of temperature on feeding behavior, to identify the conditions under which feeding will take place, and those conditions, such as post-feeding freeze that may place the beetles at risk.  These exotic predators being released to suppress adelgid populations may be slightly less cold tolerant than the adelgid, but the cold tolerance does not appear to be affected by feeding. These predators are able to feed at near 0°C temperatures. In areas with relatively cold climates, the low winter temperature and the three predators tested could work together to regulate HWA populations.  This information is being integrated with V1 of the Adelgid and Biological Control life history simulator (ABCs), in combination with landscape analyses of weather to identify regions of the landscape with high or low suitability for the biological control agents.

Research Participants

Principal Investigator

  • Scott Costa, Research Assistant Professor, University of Vermont, Department of Plant and Soil Science

Research Partners

Last Modified: 01/06/2017