Hemlock Woolly Adelgid Heads North
Summary
The hemlock woolly adelgid (HWA, Adelges tsugae) was introduced to Virginia in the 1950s and is now present in much of the Northeast. Cold winter temperatures kill HWA and currently limit its northward spread. Warming winter temperatures reduce this barrier over time, but does local adaptation also play a role? Lombardo and Elkinton tested whether HWA displays local adaptation to cold temperatures by collecting them along a latitudinal gradient (from Kentucky to Massachusetts) and determining the cold hardiness of HWA from different latitudes by supercooling them. They also raised a new generation of the adelgids in a common setting before supercooling them, to distinguish between environmental acclimation versus genetic adaptation. HWA from colder sites froze at lower temperatures, even in a common setting, suggesting that HWA from northern sites had adapted to the colder climate. Both warming winters and selection for cold hardiness may exacerbate the spread of this invasive insect.
Take home points
HWA has quickly adapted to local climate conditions in its invasive range, despite asexual reproduction which can limit adaptive capacity.
Hemlock decline from HWA is likely to be slower in the northern parts of hemlock’s range, but HWA will eventually occupy hemlock’s full range.
Climate change and continued adaptation will hasten HWA’s northward spread.
Management implications
Don’t rely on cold winter temperatures to safeguard northern hemlock populations.
Don’t move potentially infested material. Some states, including Vermont and Maine, have an HWA quarantine that regulates the movement of hemlock plants and wood products.
Slow the spread of HWA by moving bird feeders away from hemlock trees, especially in the spring and summer when HWA is mobile. Birds offer easy and convenient transportation for the HWA.
Keywords
Range Expansion; Shifting Seasons; Adaptation; Cold Hardiness; Hemlock Woolly Adelgid; Invasive Invertebrate; Parthenogenesis; upercooling