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Balsam Woolly Adelgid: A Tiny Bug with a Big Name and a Big Bang
Paul Oester and Dave Shaw, OSU Extension Forestry and Natural Resources
Figure 1. Characteristic “gouting” of branch resulting from balsam woolly adelgid attack. Photo credit: Ohio State University
This tiny sap feeding insect, Adelges piceas, was first introduced into North America from Europe around 1900 and showed up in Oregon in the 1920’s. By the 1960’s large outbreaks had occurred in the Cascades with mortality spreading over thousands of acres. Following these events this insect has continued to expand over much of the true fir type and tree mortality has subsided to lower levels. This adelgid feeds through the bark on stem and branches of true firs, causing branch gouting, dieback, and mortality. Impacts have been significant and include elimination of low elevation populations of grand fir in the Willamette Valley and the disappearance of sub-alpine fir at some high elevation sites.
Hosts in the Northwest
Subalpine fir and Pacific silver fir are frequently attacked by balsam woolly adelgid (BWA) throughout their natural range. Grand fir is particularly susceptible to damage in the lowland valleys of western Oregon and occasionally other sites. Infestations can develop in off-site plantings of noble fir, white fir, and red fir, but these species growing in their native range suffer no damage. Among ornamental trees, Frasier, balsam and cork bark fir are very susceptible to infestation.
Current situation in Oregon
In 2012, aerial surveys detected over 95,000 acres affected by BWA, down from 126,000 acres in 2011. Stand decline and tree mortality over the last decade has been most apparent along the crest of the Cascade Range from Mount Hood south to the Rogue River National Forest and in northeast Oregon in the Umatilla and Wallowa-Whitman National Forests. The primary hosts in these areas are subalpine and Pacific silver firs.
Symptoms and impacts
The two external symptoms of balsam woolly adelgid infestation are gouting (swelling) on the terminal growth and at the ends of lateral branch tips (Figure 1), and main stem infections characterized by white woolly tufts covering the bark (Figure 2). Feeding is by using piercing sucking mouth parts that cause changes in hormonal activity within the tree, resulting in abnormal cell division and differentiation in the bark and newly formed wood. When active BWA feeding produces gouts, the tree’s production of new foliage and cones can be reduced dramatically. Once the gouts are formed they can persist and provide a history of past infestation. Gouting can cause crown decline in the tree which reduces tree growth, and makes the tree susceptible to attacks by other pests.
A more serious type of attack is a mass infestation along the main bole. In this situation, BWA populations can reach densities of 100-200 adelgids per square inch of bark surface. Tree decline symptoms vary somewhat with the species affected, but generally foliage turns yellow, then deep red or brown as the tree is dying. Stem attacks by BWA cause reddish, irregular growth rings, similar to compression wood in the outer ring of sapwood. This abnormal wood conducts water poorly and often leads to tree death within 2-3 years of attack.
In North America, balsam woolly adelgid populations are composed entirely of females; and as a result reproduction is parthenogenetic (reproduction without fertilization). Adults are very small, about 1mm, dark purple to black, and wingless. The crawler is the only mobile stage of this insect and at this stage movement to new locations is facilitated by wind. The adults pierce through the bark and suck nutrients from the phloem. Adelgid numbers can escalate rapidly because the population is composed of all females and there are typically two generations in the mountains of the West. Warmer than average summer temperatures can improve insect survival and increase tree damage.
No parasites of the BWA are known, however many native predacious insects and spiders have been observed. These are general feeders and not reliable as control factors. Several species of insect predators have been introduced into North America from other parts of the world, primarily from Europe. These introductions include three beetles and three flies; but as yet none have provided any detectable level of adelgid population control. Cold weather can reduce BWA survival. In cold winters, only BWA below the snowline will survive temperatures of below 30 degrees F. Stand and host susceptibility appear to be related to environmental factors such as elevation, stand age and site condition. For example, grand fir is susceptible at low elevations but is rarely attacked at high elevations. Subalpine fir experiences heavy infestations at high elevations but Pacific sliver fir is rarely attacked at similar elevations.
Silvicultural management approaches in forests include:
1) harvest infested hosts and plant non-host trees appropriate for the site;
2) remove BWA-infested fir when thinning in mixed species stands; and
3) collect seed (cones) from non-symptomatic trees for breeding resistance.
For landscape situations, replace badly infested ornamental trees with non-host alternatives. Control of BWA with insecticides requires that trees be thoroughly sprayed with high-pressure equipment. Spray should be applied in early spring at or near bud break. Imidacloprid and endosulfan are registered for this use.
Balsam Woolly Adelgid. Forest Health Note, Oregon Department of Forestry. July 2007
Balsam Woolly Adelgid. Forest Insect & Disease Leaflet 118, USDA Forest Service. Revised May 2006