Adelges piceae (balsam woolly adelgid)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Risk of Introduction
- Habitat List
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Plant Trade
- Wood Packaging
- Environmental Impact
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Adelges piceae (Ratzeburg)
Preferred Common Name
- balsam woolly adelgid
Other Scientific Names
- Adelges piceae piceae
- Chermes piceae
- Chermes piceae bouvieri
- Dreyfusia piceae (Ratzeburg, 1844)
- Dreyfusia piceae bouvieri
International Common Names
- Spanish: pulgon del abeto
- French: chermés cortical du sapin pectiné
- Russian: korovoi pichtovii kermes
Local Common Names
- Denmark: aedelgranstammelus
- Germany: Europäische Tannenstamm-Rindenlaus
- Italy: afide lanigero dell'abete bianco
- Norway: edelgranlus
- Sweden: silvergranlus
- ADLGPI (Dreyfusia piceae)
Summary of InvasivenessTop of page
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Hemiptera
- Suborder: Sternorrhyncha
- Unknown: Aphidoidea
- Family: Adelgidae
- Genus: Adelges
- Species: Adelges piceae
Notes on Taxonomy and NomenclatureTop of page
Börner (1908, 1930, 1952) provided a generic subdivision of adelgids, which, in the course of the last century, was followed by authors such as Inouye (1953), Börner and Heinze (1957), Steffan (1972), Ghosh (1983) and Pashtshenko (1988). In contrast, other authors, have been following the subdivision of adelgids made by Annand (1928) in only two genera, Adelges and Pineus. This considers Börner's genera, other than Pineus, to be subgenera of Adelges (cf. Carter, 1971; Blackman and Eastop, 1994). This datasheet follows Blackman and Eastop (1994) where the Balsam woolly aphid is referred to as A. piceae as opposed to Dreyfusia piceae.
DescriptionTop of page
The eggs are 0.33 mm long and 0.16 mm wide, and a lengthened, ovoid shape. They are yellow at first and then they become progressively orange. At the end of embryonic development, the black eyes of the larvae are clearly visible through the chorion (Binazzi and Francardi, 2001).
Newly born larvae (pseudosistens/progrediens)
The newly born larvae are 0.30-0.45 mm long and orange-brown. When settled, the first instar larvae have a flattened reddish-brown body with a strongly sclerified dorsal integument. There are dorsal and marginal white wax threads covering the dark cephalothorax and the dorsal abdominal tergites. The identification of the species of Adelges is classically based on the first instar larva of the pseudosistens types (pseudohiemosistens and pseudoaestivosistens). The most suitable structural feature for use in identification is the number of glandular pores within the ten 'middle fields' of the spinal plates of the meso- and meta-thorax and of the first three abdominal segments. The shape of the pores and their relief, the shape of the 'middle fields' and the degree of sclerotization are also useful for identification. Within the bio-ecological patterns, the phenology of the single generations and the symptoms of attack are also useful diagnostic features. In A. piceae, the average number of glandular pores, as stated above, is less than 30 (about 26 in Italy). Other diagnostic morphological features in the neosistens forms are the shallow wax pores that vary in shape from polygonal to roundish with rounded angles, the weakly dorsal sclerotization and the sub-triangular shape of the 'middle fields' (Binazzi and Covassi, 1991). When the third instar larvae start to reach the adult stage they are blackish and become more rounded, developing a rather spherical shape (0.50 x 0.37 mm).
They have a purplish-black, ovoid-shaped body (0.80-1.20 mm x 0.60-1.15 mm) sometimes with dorsal spino-pleural marked prominence. White woolly wax threads cover the body. The adult pseudohiemosistens females are more rounded and sclerified than mature pseudoprogrediens females. The former almost completely lack dorsal wax pores.
Alatae (exulans or exule)
These have small bodies (0.84 x 0.42 mm) and the forewings are 1.28 mm long while the hind ones are 0.75 mm. The body is light green-brown (Binazzi, 2000).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 17 Feb 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Bosnia and Herzegovina||Present||Native|
|Italy||Present, Widespread||Native||Original citation: Binazzi & Covassi, 1991|
|-Newfoundland and Labrador||Present||Introduced||1949||Invasive|
|-Prince Edward Island||Present||Introduced||Invasive|
Risk of IntroductionTop of page
Habitat ListTop of page
Host Plants and Other Plants AffectedTop of page
Growth StagesTop of page
SymptomsTop of page
List of Symptoms/SignsTop of page
|Growing point / discoloration|
|Growing point / distortion|
|Growing point / dwarfing; stunting|
|Growing point / external feeding|
|Growing point / honeydew or sooty mould|
|Growing point / wilt|
|Leaves / abnormal leaf fall|
|Leaves / honeydew or sooty mould|
|Roots / necrotic streaks or lesions|
|Stems / dieback|
|Stems / distortion|
|Stems / honeydew or sooty mould|
|Stems / internal red necrosis|
|Stems / spittle mass|
|Stems / stunting or rosetting|
|Whole plant / discoloration|
|Whole plant / distortion; rosetting|
|Whole plant / plant dead; dieback|
Biology and EcologyTop of page
A. piceae is anholocyclic on Abies sp. and is native to Europe where it colonizes Abies alba and other congeneric species. In North America, A. piceae has become a serious pest on many of the indigenous species of Abies since the beginning of the last century.
In central Europe, A. piceae has one to three generations per year while in North America it may have another partial one (Antonelli, 1992). The adelgid usually overwinters as a first or second instar larva (Balch, 1952; Karafiat and Rudinsky, 1956; Harris, 1978; Covassi and Binazzi, 1981). As a rule, the number of generations per year appears to depend mostly on the altitude and on the summer and winter temperatures. It also depends on the local conditions of the fir stands (Mitchell et al., 1961). In Italy, the newly born larvae reach maturity in about 1.5-2 months in the early spring and in the autumn, while they complete their growth in only 1 month in the summer (Binazzi and Francardi, 2001).
In silver fir woods ranging from 900 to 1200 m, four to five generations per year have been counted, which depend directly on the spring/summer weather. In those habitats, A. piceae normally overwinters as a first or second instar larva and reaches the adult stage at the end of February so that the first egg-laying females occur at the end of March. In fir stands at a low altitude (under 300 m a.s.l.), A. piceae has a life cycle of five generations and overwinters as first, second or third instar larvae. They reach maturity from the end of January onwards. A similar behaviour has been observed in other lower mountain fir woods or on single trees of parks, gardens, hills and lowlands. Here pseudohiemosistens larvae start their development at the end of January and reach the adult stage from the middle of February onwards. In warmer climatic conditions, A. piceae does not have a very long quiescence time contrary to what has been observed by some authors in other geographical areas of the northern hemisphere (Mitchell et al., 1961).
The mature females are able to lay an average of 70-80 eggs per cluster, which gradually hatch within 3-4 days. From the beginning of spring onwards, the young larva develops after three moults, into an apterous adult female (pseudoprogrediens) or after four moults into an alate female (exulans). In mid-July, some of them can change into a more resistant, sclerified form (pseudoaestivosistens) without moulting during the whole of the summer. This form can reach maturity in October or spend the following winter as a pseudohiemosistens. However, all of them are able to survive the hardest climatic conditions. In Italy, the adult females have an egg-laying peak in May and June, when they appear to be most numerous from the middle to the highest levels of the host-tree crown, mainly on young tree canopies. The pseudoprogrediens egg-laying females have been observed until mid-November (Francardi et al., 1999). In central Europe, the mature females start their egg-laying in June with a peak during the summer (August to September). Egg production decreases then ceases during the early autumn (Eichhorn, 1968).
The newly born larvae are mobile. They spread along the leading axis and lateral twigs of white fir trees. They settle on the bark or under the ring of perules at the bud bases, along the trunk or branches, and even on the protruding roots from the ground (Binazzi and Covassi, 1991). Each young larva may cover more than 30 m in the crawler stage (Harris, 1978) and this stage is more susceptible to the hardest environmental conditions and to predators (Eichhorn, 1968). After the moult, individuals of A. piceae can change their feeding sites by moving along the stem. Dispersal of the woolly aphid on the host plant can occur in a relatively short time. All of the different instar larvae of A. piceae tend to gather in small groups of at least 10-15 individuals or settle in scattered positions along the lower surface of the twigs. Here they can find damper and shadier sites, more favourable to their requirements (Balch, 1952).
When the woolly aphids increase in number, all the epigean parts of the plants (i.e. trunk, branches and twigs) can be heavily colonized even when exposed to the sun and the colonies can also be found on the exposed roots. In addition, sooty mould fungi grow plentifully on the adelgid honeydew and this fact, together with the adelgid sap sucking, often leads to the decline or death of the attacked trees. This aggressive behaviour is more common on isolated trees in parks or gardens, in critical physiological conditions. When the attacks persist, they can heavily damage the trees but when the infestation breaks down, the trees can recover. The woolly aphids overwinter inside the bark crevices (trunk and branches) or under the bud perules. In addition, the importance of other vegetative structures as feeding and overwintering sites for A. piceae is known in Europe. These include buds left by larvae of the tortricoid moth, Epinotia nigricana (Francardi and Covassi, 1992, 1994). Individuals of A. piceae have also been found in the bottom of stems mined by larvae of Argyresthia illuminatella, in regeneration areas of Abies alba (Covassi and Francardi, 1994).
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Adalia luteopicta||Predator||Adults/Nymphs||Canada; USA||Abies|
|Adalia tetraspilota bispilota||Predator||Adults/Nymphs||USA||Abies|
|Diomus pumilio||Predator||Adults/Nymphs||Canada; USA||Abies|
|Exochomus lituratus||Predator||Adults/Nymphs||Canada; USA||Abies|
|Exochomus uropygialis||Predator||Adults/Nymphs||Canada; USA||Abies|
|Harmonia breiti||Predator||Adults/Nymphs||Canada; USA||Abies|
|Harmonia eucharis||Predator||Adults/Nymphs||Canada; USA||Abies|
|Laricobius erichsonii||Predator||Adults/Eggs/Larvae/Nymphs/Pupae||British Columbia; Canada; USA||Abies|
|Leucopis obscura||Predator||Adults/Nymphs||Canada; USA||Abies|
Notes on Natural EnemiesTop of page
The orders and families that include important natural enemies of A. piceae are:
- Coleoptera - Coccinellidae (Aphidecta, Pullus, Anatis); Derodontidae (Laricobius).
- Diptera - Syrphidae (Neocnemodon); Chamaemyiidae (Leucopis); Cremifaniidae (Cremifania); Cecidomyiidae (Aphidoletes, Lestodiplosis).
- Neuroptera - Chrysopidae (Chrysopa); Hemerobiidae (Hemerobius).
- Moniliales - Tuberculariaceae (Fusarium).
The control of A. piceae brought about by the antagonists may regulate and sometimes minimize insect attack in small, isolated plantations of fir but it has never appeared effective in forest situations. In Europe, where A. piceae is native, damage by it is found only occasionally. Most of the antagonist species listed in the natural enemy table were introduced into the USA and Canada. Despite the acclimation of some of those enemies during the second half of the last century, many fir forests were injured by A. piceae (Smirnoff, 1971; Harris and Dawson, 1979; Coulson and Witter, 1984; Fowler et al., 1986). Over 20 species of insect antagonists of A. piceae have been introduced into North America from Europe, Asia and Australia during the last 50 years but only six of them have become established. However, neither the introduced nor the native ones have caused any significant reduction in the adelgid populations.
The native insect antagonists or the exotic species that were established in areas of North America infested with A. piceae, did not have any satisfactory effect in reducing the adelgid populations during the second half of the last century (Thomas, 1968; Smirnoff, 1970, 1971; Edimann and Ehnstrom, 1975; Harris and Dawson, 1979; Coulson and Witter, 1984; Fowler et al., 1986; Kevan and Klimaszewsky, 1987; Humble, 1994; Zilahi-Balogh et al., 2002).
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Leaves||larvae||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Seedlings/Micropropagated plants||adults; larvae||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Stems (above ground)/Shoots/Trunks/Branches||adults; larvae||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
Wood PackagingTop of page
|Wood Packaging not known to carry the pest in trade/transport|
|Loose wood packing material|
|Processed or treated wood|
|Solid wood packing material with bark|
|Solid wood packing material without bark|
ImpactTop of page
Environmental ImpactTop of page
The impact of A. piceae attacks can be great and cause changes in the forest environment, i.e. tree growth, productivity and use. The wood physiognomy and the cycling of nutrients are affected, and thus the vegetative cycle of phytocoenosis is influenced by A. piceae attacks. Furthermore, A. piceae attacks not only kill trees but also modify the planned or projected use of the fir forests, such as timber production, grazing, wildlife, and recreation.
Detection and InspectionTop of page
Similarities to Other Species/ConditionsTop of page
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.There are few control techniques for A. piceae available to the land manager.
The native insect antagonists or those species that were established in areas of North America infested with A. piceae, did not have any satisfactory effect in reducing the adelgid populations during the last century (Thomas, 1968; Smirnoff, 1970, 1971; Edimann and Ehnstrom, 1975; Harris and Dawson, 1979; Coulson and Witter, 1984; Fowler et al., 1986; Kevan and Klimaszewsky, 1987; Humble, 1994; Zilahi-Balogh et al., 2002).
Insecticides sprayed from the air over forest stands were ineffective in some experiments as A. piceae females are effectively protected by the copious wax wool covering their bodies (Coulson and Witter, 1984; Hastings et al., 1986; Carter and Winter, 1998). This method is often impractical when large areas need to be sprayed because of the harmful impact this may have on the balance of the biocoenosis. Otherwise, insecticides, used in ground applications to protect Christmas trees, seed orchards and highly valued trees in parks and gardens, can be effective in reducing A. piceae population levels but they are extremely costly.
In land management, restrictions on moving true fir logs out of the infested areas or importing true fir nursery stock or ornamentals have been useful methods of control. Other effective attempts to control A. piceae have included harvesting true firs from an infested area to protect the uninfested nearby stands, shortening of the rotation age of true fir stands, or conversion to non susceptible or less susceptible fir species or to other tree species.
ReferencesTop of page
Amman, G. D., Fedde, G. F., 1971. Infestation trends of Balsam woolly aphid in an Abies alba plantation in North Carolina. In: USDA Forest Service Research Note, Southeastern Forest Experiment Station , (No. SE-148) . 6 pp.
Annand PN, 1928. A contribution toward a monograph of the Adelginae (Phylloxeridae) of North America. Stanford University Publications, University Series, 6(1):1-146.
B÷rner C; Heinze K, 1957. Aphidina - Aphidoidea in: SORAUER - Handbuch der Pflanzenkrankheiten, Bd. V, 5(4): Tierische schSdlinge an Nutzpflanzen, 2:1-402.
Balch RE, 1952. Studies on the balsam woolly aphid (Adelges piceae Ratz.) and its effects on balsam fir, Abies balsamea (L.) Mill. Canada Dept. Agric., Publ N. 867:1-76.
Barker M; Miegroet Hvan; Nicholas NS; Creed IF, 2002. Variation in overstory nitrogen uptake in a small, high-elevation southern Appalachian spruce-fir watershed. Canadian Journal of Forest Research, 32(10):1741-1752; many ref.
Bednar, D. M., 2014. Interactions Between the Invasive Balsam Woolly Adelgid (Adelges piceae) and a Native Host, Fraser Fir (Abies fraseri). https://repository.lib.ncsu.edu/bitstream/handle/1840.16/9683/etd.pdf?sequence=2&isAllowed=y
Binazzi A; Covessi M, 1991. Contribution to the knowledge of conifer aphids, XII. The genus Dreyfusia Boerner in Italy with the description of a new species (Homoptera Adelgidae). Redia, 74(1):233-287
Busby RJN, 1962. Species and Forms of the Silver Fir Adelgid in Scotland. Scottish Forestry, 16:243-254.
Börner C, 1908. Eine monographische Studie über die Chermiden. Arb. aus der Kaiserl. Biologische Anstalt für Land- und Forstwirtschaft, 6:81-318.
Börner C, 1930. Beiträge zu einem neuen System der Blattläuse. Arch. Klassif. Phylogenetic Entomology, 1:115-194.
Carter CI, 1971. Conifer Woolly Aphids (Adelgidae) in Britain. Forestry Commission Bulletin, 42:1-51.
Carter CI; Winter T, 1998. Christmas Tree Pests. London, UK: The Stationery Office. Forestry Commission, Field Book 17.
Ceianu I; Teodorescu I, 1975. A little-known pest of Abies alba: Dreyfusia piceae (Hom., Adelgidae). Revista Padurilor Industria Lemnului,Celuloza si Hirtie, Silvicoltura si Exploatarea Padurilor, 90(4):234-237.
Chrystal RN, 1925. The genus Dreyfusia in Britain and its relation to the Silver fir. Philosophical Transactions of the Royal Society & London B, 214:29-61.
Delucchi V, 1956. General notes on the predators of Dreyfusia [Adelges] piceae and their parasites. Bollettino del Laboratorio di Zoologia Generale e Agraria, Portici, 33:283-302.
Eichhorn O, 1968. Problems of the Population Dynamics of the Silver Fir Woolly Aphids, Genus Adelges (=Dreyfusia), Adelgidae. Zeitschrift für Angewande Entomologie, 61:157-214.
Eichhorn O, 1969. Natürliche Verbreitungsareale und Einschleppungsgebiete der Weisstannen-Wolläuse (Gattung Dreyfusia) und die Möglichkeiten ihrer biologischen Bekämpfung. Zeitschrift für Angewande Entomologie, 63:113-131.
Foottit RG; Mackauer M, 1980. Morphometric variation between populations of the balsam woolly aphid, Adelges piceae (Ratzeburg) (Homoptera: Adelgidae), in North America. Canadian Journal of Zoology, 58(8):1494-1503
Foottit, R. G., Mackkauer, M., 1980. Morphometric variation between populations of the balsam woolly aphid, Adelges piceae (Ratzeburg) (Homoptera: Adelgidae), in North America. Canadian Journal of Zoology, 58, 1494-1503. doi: 10.1139/z80-206
Fowler G; Zhang J; Hain FP; Manville JF, 2001. The distribution pattern of two juvenile hormone related compounds in Fraser fir and the induced response to a balsam woolly adelgid infestation. Canadian Journal of Forest Research, 31(10):1701-1707; 36 ref.
Fowler RF; Wilson LF; Paananen DM, 1986. Insect suppression in Eastern Region national forests: 1930-1980. General Technical Report, North Central Forest Experiment Station, USDA Forest Service, No. NC-103:56pp.
Francardi V; Covassi M, 1992. Osservazioni preliminari sull'Epinotia nigricana (H.S.) (Lepidoptera Tortricidae) nociva alla rinnovazione dell'abete bianco nell'Appennino toscano. Atti Convegno "Le Avversità delle Abetine in Italia", Vallombrosa (FI-I) 25-26 giugno 1992. Università degli Studi di Firenze, CNR:227-236.
Gast, S., Beckman, D. P., Livingston, R. L., Gustin, J., 1990. Distribution of the balsam woolly adelgid in Idaho. In: Forest Pest Management Report - Northern Region, USDA Forest Service , (No. 90-5) . Missoula, USA: Northern Region, USDA Forest Service.9 pp.
Harris JWE, 1978. Balsam Woolly Aphid. Forestry Canada, Forest Insect and Disease Survey, Forest Pest Leaflet 1, 1-4.
Harris JWE; Dawson AF, 1979. Predator release program for balsam woolly aphid, Adelges piceae (Homoptera: Adelgidae), in British Columbia, 1960-1969. Journal of the Entomological Society of British Columbia, 76:21-26
Hastings, F. L., Barry, P. J., Ragenovich, I. R., 1979. Laboratory screening and field bioassays of insecticides for controlling the balsam woolly adelgid [Adelges piceae] in Southern Appalachia. In: Research Note, Southeastern Forest Experiment Station, USDA Forest Service , (No. SE-279) . 3 pp.
Hollingsworth, R. G., Hain, F. P., 1992. Balsam woolly adelgid (Homoptera: Adelgidae) effects on wood and bark structure of Fraser fir and silver fir. Environmental Entomology, 21(5), 1103-1109. doi: 10.1093/ee/21.5.1103
Hrinkevich, K. H., Progar, R. A., Shaw, D. C., 2016. A severity rating system for evaluating stand-level balsam woolly adelgid (Hemiptera: Adelgidae) damage in two Abies species in western North America. Forest Science, 62(2), 181-189. http://www.ingentaconnect.com/contentone/saf/fs/2016/00000062/00000002/art00006
Jetton, R. M., Monahan, J. F., Hain, F. P., 2011. Laboratory studies of feeding and oviposition preference, developmental performance, and survival of the predatory beetle, Sasajiscymnus tsugae on diets of the woolly adelgids, Adelges tsugae and Adelges piceae. Journal of Insect Science (Madison), 11, Article 68. doi: 10.1673/031.011.6801
Juronis V, 1996. Aphids (Aphidoidea) - phytophages of introduced firs (Abies Mill.) in green plantations of Lithuania. Lietuvos entomologu darbai (Lietuvos entomologu draugijos 30-meciui)., 217-220; 10 ref.
Karafiat H; Franz J, 1956. Studies of population fluctuations in A. piceae (Hemipt., Adelgidae). Zoologische Jahrbucher (Abteilung fur Systematik, Okologie und Geographie der Tiere), Jena, 84(6):567-504.
Karafiat H; Rudinsky JA, 1956. Studien zur Populationdynamik der Tannenstammlaus Adelges (Dreyfusia) piceae Ratz.(Hemiptera, Adelgidae). Zool. Jb. (Systematik), 84:467-504.
Mester, E. C., Framptom, J., Hain, F. P., 2015. Assessment of Environmental Conditions and Preferences for Rearing the Balsam Woolly Adelgid (Hemiptera: Adelgidae) on Host Material. Journal of Entomological Science, 50(3), 157-167. doi: 10.18474/jes14-32.1
Mitchell RJ; Johnson NE; Rudinsky JA, 1961. Seasonal History of the Balsam Woolly Aphid in the Pacific Northwest. Canadian Entomologist, 93:794-798.
Mitchell, R. G., 1966. Infestation characteristics of the balsam woolly aphid Adelges piceae (Ratz.) on Abies spp.] in the Pacific Northwest [of the United States. In: Res. Pap. Pacif. NW. Forest Range Exp. Stn , (35) : Portland, Ore.18 pp.
Moore, P. T., Miegroet, H. van, Nicholas, N. S., 2008. Examination of forest recovery scenarios in a southern Appalachian Picea-Abies forest. Forestry (Oxford), 81(2), 183-194. doi: 10.1093/forestry/cpn013
Overhulser, D. L., Ragenovich, I. R., McWilliams, M., Willhite, E. A., 2004. Balsam Woolly Adelgid Occurrence on True Fir in Oregon. In: Pest Management Report , USA: Oregon Department of Forestry.1-7. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_025732.pdf
Pasek V, 1954. Aphids attacking Coniferous Trees in Czechoslovakian Forests. Slovak Academy of Science, Bratislava, 1-319.
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Piercey-Normore MD; Bérubé JA, 2000. Artificial inoculation with Armillaria ostoyae in established conifers stressed by defoliation, planting, and thinning in Newfoundland. Canadian Journal of Forest Research, 30(11):1758-1765; 48 ref.
Pschorn-Walcher H, 1964. Comparison of some Dreyfusia (= Adelges) infestations in Eurasia and North America. Delémont, Switzerland: Commonwealth Institute of Biological Control Technical Bulletin N. 4, 1-23.
Pschorn-Walcher H; Zwölfer H, 1956. The Predator Complex of the White-Fir Woolly Aphids (Genus Dreyfusia, Adelgidae). Zeitschrift für Angewande Entomologie, 39:63-75.
Shaposhnikov GKH, 1964. Suborder Aphidinea, Plant lice. In: Bei-Bienko G Ya, ed. Keys to the Insects of the European USSR, 1. Moscow & Leningrad, USSR: Acad. Sci. of USSR, Zool. Inst., 489-616.
Smirnoff WA, 1970. Fungus diseases affecting Adelges piceae in the Fir forest of the Gaspe Peninsula, Quebec. Canadian Entomologist, 102(7):799-805.
Spiegel, L. H., Chadwick, K., Mehmel, C., 2013. Chapter 12. Assessment of Balsam woolly adelgid damage to Eastern Washington and Oregon subalpine fir (Project WC-F-07-01). In: General Technical Report - Southern Research Station, USDA Forest Service,(No.SRS-176) . Asheville, USA: Southern Research Station, USDA Forest Service. 129-134. http://www.srs.fs.fed.us/pubs/gtr/gtr_srs176.pdf
Steffan AW, 1972. Aphidina, Blattläuse. In: Schwenke W, ed. Die Forstschädlinge Europas. Hamburg, Germany: Verlag Paul Parey.
Thomas HA, 1968. Distribution of the Balsam woolly aphid predator, Leucopis obscura, in Maine, 1968. Annals of the Entomological Society of America, 61(5):1344-1346.
Turnquist R; Harris JWE, 1993. Balsam woolly adelgid. Pest Leaflet - Pacific Forest Centre Forestry, Canada Victoria, Canada; Pacific and Yukon Region, Pacific Forestry Centre, Forestry Canada, No. 1:4 pp.
Wood RO, 1968. First occurrence of Balsam woolly aphid in the interior of British Columbia. Journal of the Entomological Society of British Columbia, 65:13-14.
Zilahi-Balogh GMG; Kok LT; Salom SM, 2002. Host specificity of Laricobius nigrinus Fender (Coleoptera: Derodontidae), a potential biological control agent of the hemlock woolly adelgid, Adelges tsugae Annand (Homoptera: Adelgidae). Biological Control, 24(2):192-198; 34 ref.
Zilahi-Balogh GMG; Kok LT; Salom SM, 2005. A predator case history: Laricobius nigrinus, a derodontid beetle introduced against the hemlock woolly adelgid. Second International Symposium on Biological Control of Arthropods, Davos, Switzerland, 12-16 September, 2005. Washington, USA: United States Department of Agriculture, Forest Service, 651-664.
Amman G D, Fedde G F, 1971. Infestation trends of Balsam woolly aphid in an Abies alba plantation in North Carolina. In: USDA Forest Service Research Note, Southeastern Forest Experiment Station, 6 pp.
Barker M, Miegroet H van, Nicholas N S, Creed I F, 2002. Variation in overstory nitrogen uptake in a small, high-elevation southern Appalachian spruce-fir watershed. Canadian Journal of Forest Research. 32 (10), 1741-1752. DOI:10.1139/x02-098
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