research

The impact of competing invasive herbivores on eastern hemlocks

The invasive hemlock woolly adelgid Adelges tsugae (HWA) is a major threat to eastern hemlock (Tsuga canadensis) in eastern North America. A high-density HWA infestation can kill even mature trees in four years (McClure 1991) and most trees usually die within 10-15 years (Orwig et al. 2002). As a result, substantial hemlock mortality has occurred in stands from Virginia to Connecticut (Orwig and Foster 1998). Despite initial fears that HWA would rapidly remove hemlocks from the southern New England landscape, however, stand-level mortality in this area appears to be occurring much more slowly than predicted. Overwintering mortality of HWA due to cold weather (Parker et al. 1999; Parker et al. 1997) undoubtedly plays a role in reducing hemlock mortality, and it has been shown that stressed hemlocks die more quickly than unstressed trees (Young and Morton 2002). A third potential (but non-exclusive) explanation involves the recent rapid range expansion of another introduced homopteran, the elongate hemlock scale Fiorinia externa (EHS). Although highdensity EHS infestations can kill stressed trees (McClure 1980), this insect is not considered as harmful as HWA. The gloomiest predictions of hemlock mortality were made in the mid-1990s; since that time, the invaded range and stand-level density of EHS has increased dramatically in southern New England.

The effects of both HWA and EHS on hemlocks have been studied; however, the interactions between these two invasive herbivores, and the effect of their combined presence on hemlocks, remain essentially unknown (McClure 2002). Although the only published work onthese two species, a survey of 153 trees in a single hemlock stand, documented a positive correlation between HWA density and EHS density (Danoff-Burg and Bird 2002), resource competition is known to be common and often strong between sedentary sap-feeders such as HWA and EHS (Denno et al. 1995), and the fact that both species tend to occur in dense aggregations further increases the likelihood of competition.

Our work in this area addresses three fundamental questions:

  1. How does the co-occurrence of HWA and EHS on eastern hemlock affect each species’ density and overwintering survival?
  2. What is the effect of HWA alone, EHS alone, and both species together on eastern hemlock growth at the branch and whole-tree level?
  3. Do the answers to (1) and (2) accord with landscape-level patterns of both insects’ abundance and the health of eastern hemlock?
  • Danoff-Burg, J., and S. Bird. 2002. Hemlock woolly adelgid and elongate hemlock scale: Partners in crime?, Pages 254-268 in B. Onken, R. Reardon, and J. Lashomb, eds. Hemlock woolly adelgid in the eastern United States symposium. East Brunswick, NJ, US Forest Service.
  • Denno, R., M. McClure, and J. Ott. 1995. Interspecific interactions in phytophagous nsects: competition reexamined and resurrected. Annual Review of Entomology 40:297-331.
  • Lagalante, A., and M. Montgomery. 2003. Analysis of terpenoids from hemlock (Tsuga) species by solid-phase microextraction/gas chromatography/ion-trap mass spectrometry. Journal of Agricultural & Food Chemistry 51:2115-2120.
  • McClure, M. 1980. Competition between exotic species: scale insects on hemlock. Ecology 61:1391-1401.
  • McClure, M. 1991. Density-dependent feedback and population cycles in Adelges tsugae(Homoptera: Adelgidae) on Tsuga canadensis. Environmental Entomology 20:258-264.
  • McClure, M. 2002. The elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae): a new look at an old nemesis., Pages 248-253 in B. Onken, R. Reardon, and J. Lashomb, eds. Hemlock Woolly Adelgid in the Eastern United States Symposium. East Brunswick, NJ, US Forest Service.
  • McClure, M., and C. Cheah. 2002. Important mortality factors in the life cycle of hemlock woolly adelgid Adelges tsugae in the northeastern United States. Pages 13-22 in R. Reardon, B. Onken, and J. Lashomb, editors. Symposium on the hemlock woolly adelgid in eastern North America. New Jersey Agricultural Experiment Station, New Brunswick, NJ.
  • Orwig, D., and D. Foster. 1998. Forest response to the introduced hemlock woolly adelgid in southern New England, USA. Journal of the Torrey Botanical Society 125:60-73.
  • Orwig, D., D. Foster, and D. Mausel. 2002. Landscape patterns of hemlock decline in New England due to the introduced hemlock woolly adelgid. Journal of Biogeography 29:1475-1488.
  • Parker, B. L., M. Skinner, S. Gouli, T. Ashikaga, and B. H. Teillon. 1997. Survival of hemlock woolly adelgid (Homoptera: Adelgidae) at low temperatures. Forest Science 44:414-420.
  • Parker, B., M. Skinner, S. Gouli, T. Ashikaga, and H. B. Teillon. 1999. Low lethal temperature for hemlock woolly adelgid (Homoptera: Adelgidae). Environmental Entomology 28:1085-1091.
  • Young, J., and D. Morton. 2002, Modeling landscape-level impacts of HWA in Shenandoah National Park. R. Reardon, B. Onken, and J. Lashomb, eds. Symposium on the hemlock woolly adelgid in eastern North America:73-85.