Have you been a little chilly lately? Cold enough to drop dead? If you answered yes to that question, you may be a hemlock woolly adelgid. Inspired, no doubt, by the recent cold snaps all over the East Coast, there have been several news stories spreading the word that extreme cold temperatures lead to widespread hemlock woolly adelgid (HWA) mortality. Indeed, much like the polar vortex events of the winters of 2014-15, the frigid temperatures this past weekend have put HWA at risk, to the benefit of the beautiful hemlocks that they have infested. But these cold-induced mortality events are only one part of the story, and it is the surviving adelgids that will write a new narrative in our forests.
The mortality question is not simple. Here at the New York State Hemlock Initiative, forest entomologist Mark Whitmore has been studying the effects of extreme cold on HWA since 2013. Samples taken at study sites like Minekill State Park have shown that cold events can result in up to 90% mortality in an HWA population. Yet, even following the freezing temperatures of past winters, the remaining individuals have continued to build their numbers and spread into new regions in New York. Cold events, while they do lower populations of this invasive forest pest in the short term, are unable to completely do away with the threat of HWA, so it is dangerous to become complacent in hemlock conservation efforts as a result of cold winter temperatures.
The problem arises when we consider the fate of surviving HWA populations. Since HWA reproduces asexually, it is likely that individuals that are able to survive extreme cold events, assuming that cold tolerance is genetically linked, will pass along that ability to their offspring. The result is a pest population that is better adapted for the next cold snap. While the cold will help reduce HWA populations for one season, we cannot rely on just climatic conditions to hamper the spread of HWA. Today, the NYS Hemlock Initiative continues to study the ability of HWA to survive the cold. Using observational data from the field, as well as experimental data from artificial cold events in the lab, we can better understand how HWA populations respond to cold and how remaining populations recover after a cold event.
The benefit of HWA mortality from extreme cold is that it buys us time to implement management strategies on recovering HWA populations. It is important to remember that since HWA reproduces asexually, completes two generations per year, and has no natural predators, management techniques such as chemical control and biological control are better tools for our forests than just relying on cold temperatures alone. While we welcome the winter when it comes to helping us manage HWA populations, it is up to us to continue our hemlock conservation efforts in all seasons.