Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Cooley spruce gall adelgid, Douglas fir adelgidOn this page: Identification & Distribution Biology & Ecology Life cycle Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution:The Adelges cooleyi gall (see first picture below) is elongated, more than 1.5 times as long as wide, and often curved with the needles slightly shorter than usual (cf. Adelges laricis & Adelges abietis which have globular or ellipsoidal galls, less than 1.5 times as long as wide, and with needles much shorter than usual). Adelges cooleyi galls are highly variable in colour: yellow-green, pink, red or even deep purple. The gall is initiated in spring by the fundatrix which feeds on sap and matures, inducing a swelling of the bud into a gall. At maturity she produces a cluster of eggs which hatch to give brownish nymphs which crawl into the developing gall. The galls release winged females, the gallicolae (see second picture below) which migrate to Douglas fir. The gallicolae of Adelges cooleyi have reddish brown to purplish black abdomens and reddish brown wing veins , and are somewhat larger when originating on spruce (length 1.7-2.5 mm) than on Douglas fir (length 1.2-1.7 mm). Antennal segments III and IV are carrot shaped, and have slit-like rhinaria. The rhinaria extend slightly more than half way round, but do not occupy more than 25% of the length of the segment (cf. Adelges nordmannianae where the rhinarium on III is almost half the length of the segment and extends more than half way round it).
The eggs of the gallicolae hatch out to give apterous exules, which overwinter on fir. These develop the following spring, when they become covered in white woolly wax (see third picture above). The offspring of mature exules develop to give both wingless and winged forms, and the winged forms migrate back to spruce. Their offspring develop to males and females, and the mated females lay eggs. These hatch to give young fundatrices which overwinter on the spruce buds.
The cooley spruce gall adelgid typically has a two year life cycle alternating between spruce (Picea sitchensis, Picea pungens, Picea engelmannii) 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). In spring the Adelges cooleyi fundatrix feeds on sap of spruce 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.
Images: 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.
Image, copyright Claude Pilon/ Les Hemipteres du Quebec - pucerons all rights reserved.
The gallicolae emerge, undergo a final moult into winged adults, and migrate to Douglas fir tree where they lay eggs.
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.
Early the next spring these sistentes begin to feed and produce wax - initially as a fringe around the body (see first picture above) and then around the whole body. At maturity they deposit eggs (see second picture above). The eggs laid by the sistentes hatch at budburst and develop to give both wingless and winged forms. Nymphs crawl into the opening buds and feed on the elongating needles.
The nymphs feeding often cause chlorotic spots and bending of the needles.
Subsequent generations of heavily waxed apterous forms occur on Douglas-fir through the season.
From June onwards, winged individuals called sexuparae migrate back 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 (see picture above). Note the characteristic feeding discoloration caused by the sexuparae before they die. 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. 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.
Other aphids on same host:
Adelges cooleyi has been recorded from 5 Picea species (Picea engelmannii, Picea glauca, Picea mariana, Picea pungens, Picea sitchensis).
Blackman & Eastop list about 170 species of aphids as feeding on spruces (Picea) worldwide, and provide formal identification keys.