Biological Control of Pests in Forests of Eastern United States



(Fiorinia externa Ferris)

Roy Van Driesche and Kris Abell,

University of Massachusetts, Amherst,

Range in North America

Fiorinia externa, the elongate hemlock scale (EHS), is a diaspidid scale native to Japan (McClure 1978a), which invaded the eastern United States about 1907 (Davidson and McComb, 1958). It is now found on hemlock (Tsuga canadensis) from Massachusetts to Georgia,  and west to Ohio (Drooz, 1985). 


Extremely high densities of this pest (30 or more scales per 100 needles) occur in natural hemlock stands in Connecticut hemlock stands (McClure, 1977a,b). Planted hemlocks can experience even higher scale densities, in part due to increased scale survival as a response to fertilization (McClure, 1977c). The pattern of EHS over the regional landscape is still only partially understood, but surveys have shown that Massachusetts is near the scale's northern limit. However, both scale density and range are increasing northward [Preisser et al., unpub]). Scale densities in natural stands are highest in Connecticut, followed by infestations in Pennsylvania.  At other locations, infestations in natural forests are lower but isolated pockets of high density do occur and densities on ornamental hemlocks may be extremely high anywhere.

Fig. 2. Hemlock foliage with heavy

infestation of elongate hemlock scale

Resident Natural Enemies

McClure studied the population dynamics of EHS in Connecticut and compared what he found there to observations made on planted hemlocks at low elevation (100-200 m) at Kyoto, Japan (35 °NL) (McClure, 1977d, 1978b, 1986). At both locations, McClure found Encarsia citrina to be the dominant natural enemy of EHS. This species is reported as attacking numerous species of armored scales. Krombein et al. (1979) lists 25 hosts and a CAB search turned up 30 armored scale hosts. But Krombein et al. (1979) indicate that E. citrina is likely a composite of several sibling species .

Biological Control Efforts Against the Pest

The feasiblity and desirablity of implementing a classical biological control program against this pest is been investigated. The basic issue is whether the high densities of scale in parts of the United States are due to escape from specialized natural enemies or other factors. One such other factor might be poor host/parasitoid synchrony. McClure hypothesized that the high EHS densities in Connecticut might be caused by host scarcity during E. citrina’s second adult generation in August. During this period in Conneticut there are few live second instar scales, the only stage attackable by E. citrina. Host scarcity is predicted to areas in areas like Connecticut occurred where the scale was univoltine but not where the scale is bivoltine and a second crawler flush occurs in August and September. 

     Alternatively, key natural enemies of EHS in Japan may simply be missing in the United States.  While McClure found E. citrina to be the dominate parasitoid in Japan, EHS populations are low there even in mountainous areas where the scale in certainly univoltine. We now suspect the Japanese E. citrina is in fact a different species from that in the eastern United States. 

Up to the MInute Status

Current research is comparing EHS populations in univoltine and bivoltine areas in the United States to see if McClure's hypothesis is supported or not.  Other work is examing the natural enemies attacking EHS in Japan and comparing their DNA to that of species found in the United States to see if they are the same or different


Upper left: close up of elongate hemlock scale (photo credit: Sven-Erik Spichiger Pennsylvania Department of Conservation and Natural Resources,

Upper right: hemlock foliage densely infested with elongate hemlock scale (photo credit: Eric R. Day, Virginia Tech, Department of Entomology;

References Cited

Davidson, J. A. and C. W. McComb. 1958. Notes on the biology and control of Fiorinia externa Ferris. J. Econ. Ent. 51: 405-606.

Drooz, A. T. (ed.). 1985. Insects of Eastern Forests. United States Department of Agriculture, Forest Service, Miscellaneous Pub. 1426, Washington, D.C.

Krombein, K. V., P. D. Hurd, Jr., D. R. Smith, and B. D. Burks. 1979. Catalog of Hymenoptera in America North of Mexico. Smithsonian Institution Press, Washington, D.C.

McClure, M. S. 1977a. Ecology and control of Fiorinia externa Ferris (Homoptera: Diaspididae) on eastern hemlock. NY Ent. Soc. 85: 187-188.

McClure, M. S. 1977b. Resurgence of the scale Fiorinia externa (Homoptera: Diaspididae) on hemlock following insecticide application. Environ. Entomol. 6: 480-484.

McClure, M. S. 1977c. Dispersal of the scale Fiorinia externa (Homoptera: Diaspididae) and effects of edaphic factors on its establishment on hemlock. Environ. Entomol. 6: 539-544.

McClure, M. S. 1977d. Parasitism of the scale insect Fiorinia externa (Homoptera: Diaspididae), by Aspidiotiphagus citrinus (Hymenoptera: Eulophidae. In a hemlock forest: density dependence. Environ. Entomol. 6: 551-555.

McClure, M.S. 1978a. Two parasitic wasps have potential for controlling hemlock scales. Frontiers of Plant Science 30(2): 2-3.

McClure, M. S. 1978b. Seasonal development of Fiorinia externa, Tsugaspidiotus tsugae (Homoptera: Diaspididae), and their parasite, Aspidiotiphagus citrinus (Hymenoptera: Diaspididae): Importance of parasite-host synchonism to the population dynamics of two scale pests of hemlock. Environmental Entomology 7: 863-870.

McClure, M. S. 1986. Population dynamics of Japanese hemlock scales: a comparison of endemic and exotic communities. Ecology 67: 1411-1421.

Preisser, E. L., A. G. Lodge, D. A. Orwig, and J.S. Elkinton. Range expansion and population dynamics of co-occurring invasive herbivores. Biological Invasions in review.