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Cooley spruce gall adelgid, Douglas fir adelgidIdentification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:On spruce trees (Picea) Adelges cooleyi is most commonly identified using the characteristics of its elongated gall: it is more than 1.5 times as long as it is wide, and is often curved with long needles protruding. Adelges cooleyi galls are highly variable in colour: yellow-green, pink, red or even deep purple (see first picture below).
First image: courtesy of Whitney Cranshaw, Colorado State University / copyright Bugwood.org under the Creative Commons Attribution 3.0 United States license. Second image, copyright Claude Pilon/ Les Hemipteres du Quebec - pucerons all rights reserved.
Winged females of Adelges cooleyi are reddish brown to purplish black (see second picture above). They are somewhat larger when originating on spruce (length 1.7-2.5 mm) than on Douglas fir (length 1.2-1.7 mm).
The cooley spruce gall adelgid typically has a two year life cycle alternating between spruce (Picea sitchensis, Picea pungens, Picea engelmanii) and Douglas fir (Pseudotsuga menziesii, Pseudotsuga macrocarpa). Host alternation is sometimes lost and the species remains on one host all year round. For example a form in Canada remains on Picea glauca) and forms in Europe and California remain on Pseudotsuga. Adelges cooleyi is native to western North America, but is now found throughout Europe and North America.
Biology & Ecology:
The life history of the cooley spruce adelgid is described by Cumming (1959). Winged individuals called sexuparae migrate from the secondary host (Douglas fir) to the primary host (spruce) in June. There they die with their roof-like wings remaining to shelter the eggs. The eggs hatch to give males and females. They disperse toward the center of the tree where they mate. After mating, a single, relatively large egg (an unusual life-history strategy!) is laid that becomes the wingless fundatrix. The young fundatrix settles on or near a bud where she overwinters. Fay & Whitham (1990) found that Adelges cooleyi fundatrices prefer to settle on lower branches. Those that settle on the uppermost branches produce more than twice as many progeny, but suffer much higher winter mortality.
In spring the Adelges cooleyi fundatrix feeds on sap and matures, inducing a swelling of the bud into a gall. Shortly before bud-break she reaches maturity and produces a cluster of eggs inside a mass of filaments. These eggs hatch to give brownish nymphs with long legs and antennae (crawlers) which crawl into the developing gall. The gallicolae feed inside the gall chambers (see picture below), where they pass through four nymphal instars.
Image courtesy of Whitney Cranshaw, Colorado State University / copyright Bugwood.org under the Creative Commons Attribution 3.0 United States license.
The size of Adelges cooleyi galls is positively correlated with the number of insects inside, and has therefore been used to estimate fundatrix fitness (Sopow & Quiring, 2001 ). In mid-summer the gall dries and the chambers to open. The gallicolae emerge, undergo a final moult into winged adults, and migrate to Douglas fir tree. There they lay eggs and die within hours of settling.
These eggs on Douglas Fir hatch to give wingless, parthenogenetic females known as exules. Some of these exules (known as progredientes) develop without interruption and on reaching maturity produce more exules parthenogenetically. Others (known as sistentes) have a period of diapause during the first instar, and overwinter on the underside of Douglas fir needles (see first picture below).
Early the next spring these sistentes begin to feed and produce wax - initially as a fringe around the body and then around the whole body. At maturity they deposit eggs which hatch at budburst. The second picture above shows an adult sistens plus egg mass on Douglas fir in April.
The eggs laid by the sistentes hatch and develop to give both wingless and winged forms. The picture below shows the developing nymphs in late May. Nymphs crawl into the opening buds and feed on the elongating needles. Chlorotic spots and bending of the needles result from the nymphs feeding. Subsequent generations of wingless forms occur on Douglas-fir through the season.
The winged forms complete the cycle of host-alternation by flying to spruce, where their offspring develop into males and females. Although the two host life cycle is typical, some Adelges cooleyi populations persist on one host for many cycles. This may occur either on spruce or on Douglas fir. An excellent review of the biology and evolution of the Adelgidae is given by Havill & Foottit (2007).
Large numbers of predators may aggregate in areas where there are large numbers of Adelges cooleyi, such as the heavy infestation shown below in late May on Douglas fir.
We have often found adult hoverflies (syrphidae) feeding on the abundant honeydew produced by Adelges cooleyi colonies, although we have yet to find any evidence of oviposition and subsequent predation by syrphid larvae.
Damage & Control:
The most severe damage is caused by heavy Adelges cooleyi infestations on young Douglas fir trees. On the basis that most damage is caused by the first spring generation, control is often directed at killing the overwintering sistentes before they can lay eggs. Insecticidal control with carbaryl or permethrin has been effective, either in autumn or early spring. Horticultural oils have also been effective, although care must be taken in their application of the needles may be discoloured. Insecticidal soaps are used widely to control this adelgid on Douglas fir.
On spruce the only damage is aesthetic damage caused by the galls.
Courtesy Ies, licensed under the Creative Commons Attribution 3.0 United States license.
The curvature is especially apparent in this picture (above) of an old hardened vacated gall.