An Invasive Species Helps Another Attack Hemlocks

Instead of one invasive insect out-competing the other, as researchers expected, hemlock woolly adelgid appears to be helping the elongate hemlock scale infest trees in the northeastern United States. Both insects, originally from Asia, have recently expanded their range in New England, where they feed on and kill eastern hemlocks.

The insects attack eastern hemlock by sucking on tree needles. High numbers of adelgids weaken and eventually kill trees within 4 to 15 years, while the scale is less lethal. The pair continue to infest new stands and pose a dire threat to eastern hemlock forests.

Although the latter arrival in eastern North America, hemlock woolly adelgids have spread more quickly from their 1950s introduction in Virginia, now reaching northern Massachusetts. Between 1998 and 2005 in Connecticut, more forests have become infested and a higher proportion of hemlocks killed. In some southern CT stands, three-quarters of hemlocks have died. In comparison, at the northernmost front of the insects' expansion, mortality averages 12%.

The initially slower-moving elongate hemlock scale stayed confined to the New York area for the first 70 years after its 1908 arrival. The insects' population has since amassed the numbers that enable it to rapidly expand its range. It followed the adelgid from central CT in 1998 to northern MA in 2005.

Researchers expected that the adelgid's life history characteristics, such as starting to feed a month earlier in spring, would give it enough of an advantage to deter the scale. Instead they found in 2005 surveys that both species were attacking the same trees and scales were occurring more densely in stands harbouring adelgids.

Rather than out-competing, adelgids may be facilitating scales. By weakening hemlock trees, the adelgids could be opening the door for more acute scale infestations. Under high adelgid densities, though, the two species may end up competing for a limited food supply of hemlock needles.


Evan L. Preisser, Alexandra G. Lodge, David A. Orwig and Joseph S. Elkinton. 2008. Range expansion and population dynamics of co-occurring invasive herbivores. Biological Invasions. 10(2): 201-213.

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