Rob Venette, Lindsey Christianson and Mark Abrahamson
- Emerald ash borer (EAB) causes problems when it becomes very abundant in an area. Populations grow slowly until they reach a "tipping point" after which they can grow very rapidly - killing many trees in a short time (1-3 years).
- We have found that some EAB larvae begin to freeze and die at around -20 F and that survival is very unlikely when temperatures reach below -30 F.
- In areas where the coldest winter temperature is generally warmer than -20°F, cold mortality is unlikely to have much or any impact on the population increase of EAB.
- In areas where the coldest winter temperature is generally colder than -30F, cold mortality may have a major impact on population increase of EAB - perhaps to the point of constraining populations below tree-killing levels. We cannot confirm this right now, but we are working to answer this question.
- We speculate that temperatures within known EAB-infested areas in Minnesota have been cold enough in recent weeks to cause a moderate to high level of larval mortality. This winter mortality should slow EAB population growth in these areas but it is probably not enough to justify changing management plans. EAB populations will likely recover and should still be expected to grow to tree-killing levels.
Recent media reports have described the potential impact of extreme cold weather on emerald ash borer. Data collected by us from the winters of 2009-2012 indicate that a substantial fraction of emerald ash borer larvae may die as temperatures fall below -20°F. At that temperature, mortality should be about 50%. Mortality rates increase quickly to about 90% as temperatures approach -30°F. Recent cold temperatures were unlikely to eliminate emerald ash borer populations. In most cases, the cold has simply set the populations back. Without additional, severe periods of cold, emerald ash borer populations would be expected to rebound to current densities in a generation or less (<1 to 2 years); however, this brief population setback provides additional time for communities to develop or implement plans for ash borer management.
EAB is a problem because of their potential for rapid population growth and resulting tree mortality. When EAB invades an area there is a characteristic lag period where populations are too small to kill trees and typically too small to detect. However, EAB populations eventually grow to a "tipping point" where population growth accelerates and tree mortality occurs rapidly. Winter mortality of larvae would help to slow population growth and consequently the rate of tree mortality. However, unless winter mortality is consistently very high, EAB populations should still be expected to reach tree-killing levels, albeit more slowly than in areas where no winter mortality occurs.
For these reasons, the authors are not advising any short-term changes to the implementation of municipal or state plans to manage emerald ash borer. Forecasts of emerald ash borer mortality need to be confirmed with independent observations.