Native to Asia, the hemlock woolly adelgid, or HWA, is an invasive, aphid-like insect that attacks North American hemlocks. Juvenile HWA, known as crawlers, search for suitable sites on the host tree, usually at the base of the needles.
HWA remain in the same spot for the rest of their lives, continually feeding and developing into adults. Their feeding severely damages the canopy of the host tree by disrupting the flow of nutrients to its twigs and needles. Tree health declines and mortality usually occurs within 4 to 10 years.
DEC’s Invasive Species program, in partnership with the NYS Hemlock Initiative, has been monitoring the numbers of HWA crawlers in many parts of New York State to see if cold winter temperatures affect HWA mortality.
Surveys this past winter were done in the eastern and western parts of New York to count how many HWA crawlers were found alive vs. dead after the several cold snaps we had last winter. Based on early counts, it appears that the cold weather caused 70-90% mortality of HWA crawlers.
This is encouraging, as it means that natural environmental factors do play a part in slowing the spread of this invasive species in the northeast. However, climate change could be a concern if we are to expect increasingly warm temperatures in the future. This level of mortality is not enough to eradicate HWA in infested areas, but it can, along with biocontrol efforts, help preserve the life of New York’s hemlocks.
Photo of white fuzzy masses at the base of hemlock needles from an HWA infestation.