Pineapple gall adelgid
The pineapple or pseudocone gall[1] develops as a chemically induced distortion of needles, mostly on Norway Spruce and Sitka Spruce.
The aphid-like insect Adelges lays up to one hundred eggs, 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.
Terminology and distribution
Prominent in appearance, the pineapple or pseudocone gall is often confused with the actual cone of the tree, despite its small size, position and the habit of the branch growing on past the gall.[1] Spruce gall aphid, eastern spruce pineapple gall adelges and eastern spruce gall aphid are alternative names.[2] This insect belongs to the superfamily Aphidoidea, and family Adelgidae.
The pineapple gall adelgid, is a native of Europe, belonging to the 'Woolly Aphid' group; it is also widely distributed in the north-eastern United States. It is the primary pest of Norway spruce. Another species, the Cooley spruce gall adelgid is indigenous to North America. This adelgid usually affects Colorado blue, Sitka, Englemann, and Oriental spruces.[3]
The 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 aphid.[1] The unilarval chambers are set in a woody core. As stated, the stem and needles of the host can continue growing beyond the position of the gall.[4]
As stated, 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. They measure 1.5 to 3.0 cm in length.[4]
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.[1] The Cooley spruce gall adelgids form a larger gall, 1 to 3 inches (25 to 76 mm) long, which usually covers the entire tip of infested new growth.[3]
Life-cycle
Only females of the pineapple gall adelgid are known.[3] In spring the newly hatched aphids 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 had overwintered under wax threads at the base of buds.[5] They died shortly afterward, leaving the eggs protected beneath their bodies, which resemble a white, cottony twigs to overwinter.[6]
In June to July the hatched adelgids mature and crawl out onto the needles, where they moult and develop wings. These aphids may fly to other conifers, such as Larch (Larix), Pine (Pinus) and Silver Fir (Abies alba) or may continue the cycle on the same tree as they are poor flyers.[7] Several generations are raised on these secondary hosts, however no galls are formed. In July of the following year winged aphids appear and fly to the spruces to breed there again, completing the life-cycle.[1]
The Adelgid are pear-shaped, soft-bodied green insects with long antennae and are not typical aphids.[7]
Gall forming insects
Some herbivorous insects therefore create their own microhabitats by forming, in this case, 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.[8] Galls may also provide the insect with some physical protection from predators.[9]
Infestations of pineapple galls
Galls interfere with the natural formation of twigs and cause curling, stunting and the eventual death of new growth. Heavy infestations give the trees a ragged appearance and destroy their beauty.[3] Thus galls prevent twig growth and, if they are abundant, may affect entire trees. Individual trees vary in susceptibility to attack; some appear immune, others suffer growth reduction, and some are eventually killed. Trees growing on good sites are generally less susceptible to attack than those on poor sites.[6]
Control
Although the old galls can be unsightly this pest has little lasting impact. On small trees galled shoot tips can be cut off but generally this is a pest that can be tolerated.[10]
Biological control
There are no known effective parasites or predators of Adelges abietis.[3] Specimen trees in gardens may be afforded a degree of protection by hanging up fat balls to encourage tits which will also feed on the adelgid gall formers.[11]
Pesticides
If control measures are required, spray with bifenthrin on a dry, mild day in late February to control the overwintering generation.[10]
See also
References
- ^ a b c d e Darlington, Arnold (1975) The Pocket Encyclopaedia of Plant Galls in Colour. Pub. Blandford Press. Poole. ISBN 0-7137-0748-8. P. 114.
- ^ Forestry compendium
- ^ a b c d e Cornell Co-operative Extension - Spruce Gall Adelgids.
- ^ a b Stubbs, F. B. Edit. (1986) Provisional Keys to British Plant Galls. Pub. Brit Plant Gall Soc. ISBN 0-9511582-0-1. P. 38.
- ^ The eastern spruce gall adelgid
- ^ a b Forest Pests.
- ^ a b Eastern Spruce Gall Adelgid - Ohio University.
- ^ Larson, K. C., and T. G. Whitham. 1991. Manipulation of food resources by a gall-forming aphid: the physiology of sink-source interactions. Oecologia 88, P.15 – 21.
- ^ Weis, A. E., and A. Kapelinski. 1994. Variable selection on Eurosta’s gall size. II. A path analysis of the ecological factors behind selection. Evolution 48, P.734 – 745.
- ^ a b Royal Horticulture Society
- ^ The Gardeners Magazine. April 2008.
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