Silver fir woolly adelgid
- Latin: Dreyfusia nordmannianae (Börner)
- English: Silver fir woolly adelgid
- French: Chermès des rameaux du sapin pectiné
- Synonym(s): Dreyfusia nuslinii , Adelges nordmannianae , Chermes nuslinii
This species is not found in Canada
Damage, symptoms and biology
In Europe, there are many insects that can damage fir trees, but the silver fir woolly adelgid is one of the most destructive. It is an aphid-like sapsucking insect in the family Adelgidae. In its native region, the Silver fir woolly adelgid has a two-year life cycle which involves alternation between two hosts: a primary host, Picea orientalis (oriental spruce), and an alternate host, typically Abies nordmanniana (Nordmann fir). In Central Europe, where Dreyfusia nordmannianae was introduced on imported Nordmann fir trees, the insect tends to colonize Picea pectinata (silver fir), on which it reproduces parthenogenetically.
Like all adelgids, the silver fir woolly adelgid has an extremely complex life cycle, which comprises five stages corresponding to an equal number of generations. One of these generations comprises males and females, which reproduce sexually, by mating, on the primary host. The other four generations consist solely of females which reproduce parthenogenetically, on the primary host and then on the alternate host. The silver fir woolly adelgid requires the presence of its primary host, Picea orientalis, in order to complete its life cycle. The eggs are laid on Picea orientalis and the first two generations, specifically the fundatrices and the gallicolae (larvae), develop on this tree species. Through their feeding activity, these two generations contribute to the formation of pineapple-shaped galls that develop from the buds. The second generation (gallicolae) gives rise to a generation consisting solely of winged female migrants. After developing on Picea orientalis, these winged individuals fly off to colonize the alternate host, on which they will reproduce parthenogenetically. The eggs laid by this generation can develop into two types of insects: a generation of wingless (apterous) females, which continuously reproduce by parthenogenesis on the alternate host in the absence of the primary host, and a winged generation, the sexuparae, which are able to fly and therefore migrate to the primary host, on which the species’ life cycle began.
Not only do the larvae induce the deformation of spruce buds, the adult insects feed on the sap of silver fir trees, thereby weakening them. Needles attacked by the young larvae in the spring curve downwards. Repeated attacks cause distortion and drying of leaders, which in fir trees leads to the growth of numerous replacement branches producing the characteristic bushy appearance. Attacks by these woolly adelgids can kill fir trees, especially when the damage is compounded by the activity of various parasites and diseases.
Although trees growing in the understory are the most frequently attacked, isolated fir trees sustain the worst damage. In fact, the insect preferentially colonizes sun exposed trees.
The insect was introduced into Central Europe on imported specimens of Nordmann fir, a species native to the Caucasus region. It quickly spread to stands of silver fir, the dominant conifer species in mountainous areas of Central Europe. Silver fir is grown in Christmas tree plantations, which are therefore at risk of Dreyfusia nordmannianae infestations.
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