This European insect was first introduced to the Pacific Coast of North America. By 1927, it was established throughout the coastal Sitka spruce (Picea sitchensis (Bong.) Carr.) forests of the region (Lynch, 2004). A series of defoliation events on Sitka spruce can cause death of branches or the entire tree (Dave Overhulser, pers. comm. November 2004). The first mention of significant impacts from this aphid came in the early 1950s, when F.P. Keen (1952) reported that the spruce aphid had killed millions of board feet of Sitka spruce along the tidelands of the Oregon and Washington Coast and the Columbia River. In 1953, an aerial survey found 20,500 acres of defoliation in Oregon and 2,100 acres in Washington (Dave Overhulser, pers. comm. November 2004).
Since Sitka spruce is not favored for timber and thus is not replanted after harvesting, the acreage of forests containing significant Sitka spruce has gradually decreased. Most damage occurs on open-grown trees - such as those on golf courses or in yards - or on the edge of stands. In Oregon young spruce trees and those in the interior of stands rarely have noticeable defoliation (Dave Overhulser, pers. comm. November 2004).
To date, the aphid has not attacked Engelmann spruce (Picea engelmannii) growing in high-elevation forests in Oregon and Washington (Dave Overhulser, pers. comm. November 2004).
The spruce aphid was introduced to the Southwest in the 1970s (Lynch, 2003); the first outbreak detected in wildlands was on the Fort Apache Indian Reservation in the White Mountains of Arizona in 1988. Since 1995, there have been four significant outbreaks; two defoliated more then 150,000 acres (Lynch, 2004). The insect is now known from five mountain ranges in the region (Lynch 2004), White Mountains, Pinaleño Mountains (including the habitat for the federally endangered Mt. Graham squirrel), San Francisco Peaks in Arizona and the: Mogollon Mountains and Sacramento Mountains in New Mexico (Lynch, 2004).
In the Southwest, Engelmann spruce is the most suitable host, while Colorado blue spruce (P. pungens) receives minor to moderate damage (Lynch, 2004). The vulnerable Engelmann spruce is widespread in mountains of the Southwest and in the Rocky Mountains (Little Jr., E.L. 1971. Atlas of United States Trees, Volume 1. Conifers and Important Hardwoods. USDA Forest Service Miscellaneous Publication No. 1146).
The aphid's impact has been much more severe in the Southwest than along the Pacific coast. Mortality of Engelmann spruce can reach 24-40% within 5 years in stands heavily defoliated by a single aphid outbreak; mortality approaches 70% among spruce that experience both severe defoliation by spruce aphid and severe infection by dwarf mistletoe (Lynch, 2004). While all age and size classes are defoliated, smaller trees within the stand are the most severely defoliated. Site and stand factors do not appear to greatly influence risk (Lynch, 2004). Defoliation is heaviest in lower portions of tree crowns and in understory trees (Lynch, 2004). As a consequence, the extent of defoliation is not reliably detectable by aircraft surveys that occur in summer after new foliage has expanded (Lynch, 2004).
These results indicate that spruce aphid will alter the character of the future forest, given the frequency of outbreaks and severity of impact. Research on weather patterns and population dynamics should provide an idea of how much of the interior Engelmann spruce forest is at risk (Lynch, 2004).
Behavior of the aphid in the Southwest differs significantly from that along the Pacific Coast. In the Pacific Northwest, larvae are produced from late winter to early summer and again in fall (Lynch, 2004). Reproduction is by parthenogenesis.
In the Southwest, by contrast, aphid numbers begin increasing in August, and peak in November or December. Damaging populations may persist over relatively warm winters through April or May. (Ann M Lynch, pers. comm. September 07, 2004). These aphids are surviving much colder winters than in the Northwest maritimes or in Europe - damaging populations can persist even when temperatures fall to -13C, whereas -5C usually devastates maritime populations. Some aphids in the Southwest have survived -30C -- but not at outbreak levels.
Lynch (Ann M Lynch, pers. comm. September 07, 2004) hypothesizes that the insect survives the coldest winters as cold-hardy eggs, although such eggs have not yet been found. Lynch believes the autumnal population increases result from the weeks during which the spruce are dormant due to cold nights and the risk of very cold events, but daytime temperatures allow aphid feeding and reproduction. As to why the insect is more cold-hardy in the Southwest than in maritime climates, Lynch suggests a combination of four factors might play a role:
Keen, F.P. 1952. Insect Enemies of Western Forests, USDA Miscellaneous Publication No. 273, Washington, D.C. Government Printing Office.
Little Jr., E.L. 1971. Atlas of United States Trees, Volume 1. Conifers and Important Hardwoods. USDA Forest Service Miscellaneous Publication No. 1146.
Lynch, A.M. 2004. Fate and Characteristics of Picea Damaged by Elatobium abietinum (Walker) (Homkoptera: Aphididae) in the White Mountains of Arizona. Western North American Naturalist: 64(1), 2004, pp. 7-17.
Lynch, A.M. 2003. Spruce Aphid in High Elevation Habitats in the Southwest U.S. USDA Forest Service, Rocky Mountain Research Station, Proceedings: 2002 U.S. Department of Agriculture Interagency Research Forum GTR-NE-300.
Overhulser, D. Oregon Department of Forestry.