The pineapple gall adelgid (Adelges abietis) is a species of conifer-feeding insect that forms pineapple-shaped plant galls on its host species, commonly Norway and Sitka spruce. The adelgids (genus Adelges) are pear-shaped, soft-bodied green insects with long antennae, closely related to the aphid. Adelges lays up to one hundred eggs at a time, one on each needle. Adelges abietis (Linnaeus, 1758) is one of the most common species; synonyms are A. gallarum-abietis, Chermes abietis and Sacciphantes abietis.
The pineapple or pseudocone gall is a type of insect-formed gall, or abnormal outgrowth of plant tissue, that develops as a chemically induced distortion of needles, observed mostly on Norway spruce and Sitka spruce.
Terminology and distribution
Prominent in appearance, the pineapple or pseudocone gall is often confused with the actual cone of the tree, due to their superficial resemblance. Pineapple galls are distinguishable by their small size, position on the base of budding twigs, and habit of the branch growing on and extending past the gall.
The pineapple gall adelgid is endemic to Europe, belonging to the 'woolly adelgid' group; it is also widely distributed in the north-eastern United States. Spruce gall aphid, eastern spruce pineapple gall adelges and eastern spruce gall aphid are alternative names. This insect belongs to the superfamily Aphidoidea, and family Adelgidae. It is the primary pest of Norway Spruce. Another similar species, the Cooley spruce gall adelgid, is indigenous to North America. This adelgid usually affects Colorado Blue, Sitka, Englemann, and Oriental spruces.
The eastern spruce gall adelgid (Adelges abietis Linnaeus) is an introduced species that feeds only on spruce. At least in 1985, the species was found in Canada from Ontario eastward and in adjacent parts of the United States.
Physical appearance of the pineapple gall
The gall is formed from the swelling of the bases of the needles punctured by the feeding activity and activated by the saliva of the insect. The unilarval chambers are set in a woody core. The stem and needles of the host can continue growing beyond the position of the gall.
Sitka and Norway spruce are the main hosts, but A. abietis galls can be found on Colorado blue, white, and red spruces.
The yellowish green galls pass through pink and then reddish-brown colour phases. On average, they measure 1.5 to 3.0 cm in length.
Similar galls are produced by other aphids: on Sitka spruce, by the Cooley spruce gall adelgid, Gilletteela cooleyi, some generations of which migrate to Douglas fir (Pseudotsuga taxifolia); and on spruce by Cnaphalodes sp., which is heteroecious with larch. The Cooley spruce gall adelgids form a larger gall, long, which usually covers the entire tip of infested new growth.
Life-cycle
Only females of the pineapple gall adelgid are known. In spring, the newly hatched nymphs formed from over-wintered eggs feed at the bases of the growing needles; the induced swellings eventually coalesce to form the pseudocone structure, each cell of which contains about twelve nymphs. The nymphs overwinter under wax threads at the base of buds.
In late summer (July–September) the fully developed nymphs emerge from the galls and crawl out onto the needles, where they molt and develop wings. These adult adelgids may fly to other susceptible conifers, such as larch (Larix), pine (Pinus) and silver fir (Abies alba) or may continue the cycle on the same tree, as some are poor flyers. They die shortly afterwards, leaving the eggs, which resemble white, cottony twigs, protected beneath their bodies. Several generations are raised on these secondary hosts, however no galls are formed. In July of the following year winged adelgid appear and fly to the spruces to breed there again, completing the life cycle.
Gall forming insects
Some herbivorous insects create their own microhabitats by forming a highly distinctive plant structure called a gall, made up of plant tissue, but controlled by the insect. Galls act as both the habitat, and food sources for the maker of the gall. The interior of a bedeguar gall is formed from the bud, and is composed of edible nutritious and structural tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts. Galls may also provide the insect with some physical protection from predators.