Cooley spruce gall adelgid
- French common name: Puceron de l'épinette de Sitka
- Other common names: Cooley spruce gall aphid
- Scientific name: Adelges cooleyi (Gillette)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hemiptera
- Family: Adelgidae
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Partial list of synonyms:
- Chermes cooleyi Gillette
- Gilletteella cooleyi Gillette
General information and importance
Adelgids are a distinct group of small (adults less than 2 millimetres), soft-bodied, pear-shaped insects related to aphids. They are specialist feeders on conifers in the Pinaceae family (pine [Pinus], spruce [Picea], fir [Abies], hemlock [Tsuga], larch [Larix], and Douglas-fir [Pseudotsuga menziesii]) and inhabit boreal and temperate forests of North America and Eurasia. Feeding adelgids exude a white, waxy flocculent material that covers their bodies and gives them their common name, “woolly adelgids”. Several species have been accidentally introduced from Europe and Asia to North America where they have become forest pests.
Adelgids have complex life histories. There may be a two-year life cycle with a sexual generation on a primary host (usually spruce), followed by dispersal to an alternate, secondary host (a conifer species in a different genus) where they reproduce asexually for the second season before returning to the primary host. Alternatively, adelgids may reproduce asexually for several generations on a secondary host exclusively. This life cycle requires only one year. Newly hatched nymphs are called “crawlers” and often are the only actively mobile stage in the life cycle. Once settled at a feeding site, the adelgid nymph remains in this location for the duration of its life. All stages feed by inserting piercing mouthparts (stylets) into tree tissues, injecting saliva, and sucking up nourishment. On primary hosts, feeding induces formation of galls. On secondary hosts, feeding causes distortion and eventual loss of needles, twigs, and/or stems near the feeding site. In all cases, feeding interferes with healthy function of the tree.
Cooley spruce gall adelgid is native to western North America. It has been introduced to eastern North America, the United Kingdom, and northern Europe (where its North American hosts have been introduced). The adelgid alternates between its primary hosts, western spruce species, and its secondary host, Douglas-fir. On its primary spruce host, feeding Cooley spruce gall adelgids inject salivary compounds near the growing tips that induce formation of galls by the tree. Nymphs feeding within the gall are provided rich nutrients and protection.
Distribution and hosts
This insect is native to western North America, from the eastern slopes of the Rocky Mountains, Alberta in the north, Colorado in the south, and along the Pacific Coast from British Columbia to California. It has been introduced to several locations east of the Rocky Mountains and is generally resident in forests wherever hosts occur. Cooley spruce gall adelgid has also been introduced to the United Kingdom and northern Europe where North American spruce and Douglas-fir hosts have been planted.
Primary native hosts are Sitka spruce (Picea sitchensis), Engelmann spruce (P. engelmannii), Colorado blue spruce (P. pungens), and white spruce (P. glauca). The secondary host is Douglas-fir.
Tree parts affected
Galls form on new growing tips of spruce. Needles become discoloured and distorted on Douglas-fir.
Symptoms and signs
Adelgids are tiny insects, difficult to see with the naked eye. Cone-shaped galls about 5 centimetres long with sharp projections develop on new growth at the branch tips of spruce, the primary host. Galls are green initially, and eventually turn reddish purple. As galls desiccate, mouth-shaped openings appear on the gall surface. These characteristic, dried galls can remain on the tree for several years. On Douglas-fir, the white “woolly” flocculence marks the feeding site of the adelgid. Damaged needles of Douglas-fir are discoloured.
Life cycle
When both spruce and Douglas-fir hosts are present, Cooley spruce gall adelgid has a two-year life cycle. Winged females migrate from the secondary host, Douglas-fir, to a suitable primary host spruce in late-summer. These migrants lay eggs on the primary spruce host and give rise to a sexual generation of male and female wingless adults. The adults move inwards on the tree, mate, and the fertilized female lays a single, large egg. The female hatching from this egg is called a fundatrix or “founding mother”. She settles near a bud and overwinters. The fundatrix matures the following spring and lays a large clutch of eggs as new buds are forming. Her feeding causes the formation of galls on the developing spruce shoot tip. These galls act as protected feeding chambers for hundreds of nymphs. The reddish-brown nymphs pass through four stages inside the gall. As the gall dries, the chambers open, allowing the nymphs to move to foliage and complete their development to winged, parthenogenetic adult females. These migrate to the secondary host, Douglas-fir, settle on the lower surface of current-year needles, lay their eggs, and die.
Nymphs hatching from these eggs on Douglas-fir settle immediately and enter a physiologically dormant state (diapause) for the winter. The following spring, they mature and reproduce asexually (parthenogenesis) for a few generations before producing winged, parthenogenetic females. Females migrate back to spruce later in the summer to complete the two-year life cycle. In areas where only the primary spruce host is present, such as the Prairie Provinces in Canada, Cooley spruce gall adelgid reproduces asexually, producing only wingless, parthenogenetic females, which continue to produce their own kind.
Damage
Cooley spruce gall adelgid is not considered a major pest in forest settings. During severe infestations, both the yield in spruce seed orchards and the aesthetics and growth of ornamental spruce can be diminished.
Prevention and management
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
Pruning out and destroying galls while they are still green and before adelgid adults emerge is an effective practice in nurseries and for smaller ornamental trees. Active management has usually been considered only in seed orchard or nursery settings. The protective nature of the gall and waxy coatings associated with the various feeding stages precludes most conventional pesticide treatments. Insecticidal soaps broadcast on fundatrices in early spring can reduce populations on a small scale.
Pesticides registered for use against Cooley spruce gall adelgid under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.
Photos
Cooley spruce gall adelgid nymphs feeding at the base of spruce needles. This feeding induces the formation of elongated galls.
Robert Gagnon
Eggs deposited on spruce in late summer by winged female Cooley spruce gall adelgids and covered by white, woolly secretions.
Robert Gagnon
Cooley spruce gall adelgid nymphs feeding at the base of spruce needles. This feeding induces the formation of elongated galls.
Robert Gagnon
Old galls (dead and reddish brown) and new galls (bluish green) caused by Cooley spruce gall adelgid on Colorado spruce.
Etats-Unis
White "woolly" flocculence and discoloured needles on Douglas-fir caused by Cooley spruce gall adelgid.
Selected references
Ahern, R.G.; Hawthorne, D.J.; Raupp, M.J. 2009. Phylogeography of a specialist insect, Adelges cooleyi: historical and contemporary processes shape the distribution of population genetic variation. Molecular Ecology 18(2): 343–356. https://doi.org/10.1111/j.1365-294X.2008.04034.x
Ahern, R.G.; Hawthorne, D.J.; Raupp, M.J. 2009. Founder effects and phenotypical variation in Adelges cooleyi, an insect pest introduced to the eastern United States. Biological Invasions 11: 959‒971. https://doi.org/10.1007/s10530-008-9308-0
Cumming, M.E.P. 1959. The biology of Adelges cooleyi (Gill.) (Homoptera: Phylloxeridae). The Canadian Entomologist 91(10): 601–617. https://doi.org/10.4039/Ent91601-10
Duncan, R.W. 1996. Common woolly aphids and adelgids of conifers. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 19. 8 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/9d25485b-a4d9-4b1b-bdf5-2a12441a63ed?fromSearchPage=true
Havill, N.P.; Foottit, R.G. 2007. Biology and evolution of Adelgidae. Annual Review of Entomology 52: 325–349. https://doi.org/10.1146/annurev.ento.52.110405.091303
Havill, N.P.; Foottit, R.G.; von Dohlen, C.D. 2007. Evolution of host specialization in the Adelgidae (Insecta: Hemiptera) inferred from molecular phylogenetics. Molecular Phylogenetics and Evolution 44(1): 357–370. https://doi.org/10.1016/j.ympev.2006.11.008
Strong, W.B.; Bennett, R.G. 2010. Sampling plan for Adelges cooleyi (Hemiptera: Adelgidae) in spruce seed orchards. The Canadian Entomologist 142(1): 14–23. https://doi.org/10.4039/n09-013